- UW Department of Geology and Geophysics
The type Ferris Formation of south-central Wyoming is thick, comparatively undeformed, and relatively fossiliferous. We documented more than 100 vertebrate-bearing, stratigraphically superposed fossil localities that span roughly 3,000 ft (c. 900 m) of continental strata of Lancian (latest Cretaceous) and Puercan (earliest Paleocene) age. Fossil mammals were recovered from 39 of the localities, 32 or 33 of which represent Puercan time. The mammalian fossils allowed a detailed biostratigraphic zonation of the Puercan section, which is thicker, by nearly an order of magnitude, than any other known of that age. Preserved in a 1,763 ft- (537 m-) thick section are mammalian assemblages that represent all three Puercan Interval-zones (i. e., Pu1–Pu3), originally defined elsewhere from principally non-superposed strata. The local strata underwent only minor deformation, and that occurred late in the regional Laramide orogeny, not before the late Paleocene. On the basis of mammalian faunas, we place the Lancian-Puercan boundary at approximately 2,050 ft (625 m) above the base of the type Ferris Formation; remains of dinosaurs occur to just above that level, in absence of Puercan mammals. The lowest stratigraphic occurrence of Protungulatum donnae, a placental mammal diagnostic elsewhere of the earliest Puercan, exists at the 2,075 ft (632 m) level. Taxonomic composition of palynological samples is compatible with our placement of the Lancian-Puercan boundary.
Previous workers assumed that advent of locally derived clasts in the Hanna Formation could be used to distinguish its outcrops from those of the underlying Ferris Formation. However, diverse pebbles from local sources also occur in the type Ferris Formation, even within its dinosaur-bearing parts. We have been unable to determine any combination of lithologic criteria that can be used reliably in the field to distinguish between outcrops of Ferris and Hanna Formations. We summarize important variations in depositional regime within Lancian-Puercan parts of the type Ferris Formation.
We provide systematic description and discussion of multituberculate and peradectian components of the mammalian fauna. All reported taxa represent new records for the Hanna Basin and southern Wyoming in general, and the faunas help fill distributional gaps between species known to the north and south of central Wyoming. At least one species of multituberculate is recognized as new. Geographic range extensions include: (1) most southerly records of Cimolodon nitidus, Alphadon lulli, Mesodma ambigua, M. hensleighi, M. sp. cf. M. garfieldensis, and Catopsalis joyneri; and (2) most northerly records of Ptilodus sp. cf. P. tsosiensis and Taeniolabis taoensis. Within the Hanna Basin, no genera of multituberculates or peradectians from the Ferris Formation have been documented in strata both of Lancian and Puercan age; several examples of pseudoextinction, however, may exist through taxonomic artifact. Temporal range extensions include first: (1) Puercan records of Mesodma hensleighi and Ectypodus spp.; (2) records within Puercan Interval-zone Pu3 of Ptilodus sp. cf. P. tsosiensis; and (3) record in Puercan Interval-zone Pu2 of Catopsalis joyneri. In general, the Lancian multituberculate and peradectian faunas of the type Ferris Formation are similar to, although not nearly so diverse as, those from the type Lance Formation; the lower diversity almost certainly is an artifact of paucity of specimens available for study.
- Cretaceous-Tertiary boundary
- Ferris Formation
- Hanna Basin
This paper is the first of a series to deal with mammalian fossils of Lancian (latest Cretaceous) and Puercan (earliest Paleocene) age as they occur in geological context of the continental Ferris Formation, western Hanna Basin of south-central Wyoming.
Among the deepest structural depressions in North America's Rocky Mountain region, the Hanna Basin contains more than eight miles (c. 13 km) of marine and nonmarine strata, most of which are Late Cretaceous and early Tertiary in age (Lillegraven and Snoke, 1996, fig. 5). The Ferris Formation preserves an extraordinarily thick, rather consistently fossiliferous section of principally fluvial deposits spanning late Lancian through Puercan time. Upper parts of the Ferris Formation are thicker, by nearly an order of magnitude, than any other known packages of Puercan strata. Preserved in a 1,763 ft- (537 m-) thick section are unequivocally superposed mammalian assemblages that represent all three Puercan Interval-zones (i.e., Pu1–Pu3), originally defined elsewhere principally from non-superposed strata (Archibald et al., 1987). As we cannot recognize a Pu0 Interval-Zone as distinct from Pu1 in the Hanna Basin, we refer to the stratigraphically lowest, early Puercan mammalian assemblages as Pu1.
Very little research on fossil vertebrates had been conducted in the Hanna Basin since John Bell Hatcher collected dinosaurian material in 1896. Fieldwork for this project, involving teams from The University of Wyoming, extended from 1990 into 1997. We have documented more than 100 vertebrate-bearing localities, spanning roughly 3,000 ft (c. 900 m) of stratigraphic section, across southern parts of T. 23 N., R. 84 W, Carbon County, Wyoming. Fossil mammals have been recovered from 39 of these localities, at least 32 of which represent Puercan time.
This research contributes to the paleobiological literature developed since advent of the idea of catastrophic termination of the Mesozoic, as originally presented by Alvarez et al. (1980). Our work provides part of the documentation, utilizing a stratigraphically superposed fossil record, necessary to document changes in mammalian diversity through this transition.
From a more geological point of view, our research provides a detailed biostratigraphic zonation for an unusually thick accumulation of wholly non-marine strata. Through this process, the earliest Cenozoic section of the Hanna Basin now becomes, from a biostratigraphic point of view, among the most closely constrained nonmarine depocenters in the entire Rocky Mountain region, or for that matter, the world.
Finally, our paper represents the first paleontological documentation of strata of Puercan age from the Hanna Basin. In terms of documentation based upon fossil vertebrates, our work also provides the only available data for strata of Puercan age between Wyoming's Bighorn Basin and Colorado's Denver Basin. Following a brief introduction to local geography, stratigraphy of the Ferris Formation, and the general paleoecological setting, we provide systematic description and discussion of multituberculate and marsupial (“peradectian,” of present taxonomy) components of the total mammalian fauna. Placement of the boundaries between sequential Puercan Interval-zones within the local stratigraphic column was determined through analysis of the entire mammalian fauna (see Eberle and Lillegraven, 1998), not just available fossils of multituberculates and peradectians.
The terms Lancian and Puercan are “ages” of a provincial time scale, unique to North American nonmarine strata, based upon occurrences of mammalian fossils (i.e., North American Land Mammal “Ages” or NALMA; Wood et al., 1941; Lillegraven and McKenna, 1986; and Woodburne, 1987). Commonly, at least when used as a formalized term, “Age” is enclosed within quotation marks to differentiate NALMAs from geochronologic ages as defined in the Stratigraphic Code of the North American Commission on Stratigraphic Nomenclature (NACSN, 1983). The Puercan part of the section represents approximately the first million years of Cenozoic time (Swisher et al., 1993).
Advent of the Puercan age is defined by appearance of the arctocyonid “condylarth” Protungulatum. Termination of the Puercan is defined by appearance of the periptychid “condylarth” Periptychus (see Archibald et al., 1987), exclusive of its subgenus Carsioptychus (see Van Valen, 1978 and Williamson, 1996). Archibald et al. (1987) subdivided the Puercan age into three Interval-zones (Pu1–Pu3, from oldest to youngest, respectively), based upon successive first appearances of mammalian taxa. Existence of a fourth Interval-zone, Pu0 (preceding Pu1) was suggested later by Archibald and Lofgren (1990). Where it was defined originally in the upper Hell Creek Formation of northeastern Montana, Pu0 lacks index taxa; it is distinguished from Pu1 by absence of taxa typically found at Pu1 sites. Pu0 and Pu1 faunas, therefore, can be virtually identical (Lofgren, 1995). Until further research allows clearer faunal distinction of Pu0 and Pu1, we contend that recognition of Pu0 is ambiguous, and suggest that it be restricted in use to its type area in northeastern Montana. Throughout the text of this paper we equate the faunally based designations Pu1, Pu2, and Pu3 with early, middle, and late Puercan time, respectively.
Directly preceding the Puercan, the Lancian age is defined by the mammalian fauna from the type Lance Formation in east-central Wyoming (see Lillegraven and McKenna, 1986). The Lancian-Puercan (L–P) boundary is defined here on the basis of mammalian taxa, specifically the first appearance of “condylarths.” In other terrestrial sections, the L–P boundary has been equated by some to the terrestrial Cretaceous-Tertiary (K–T) boundary or Cretaceous-Paleogene boundary (e.g., Swisher et al., 1993; Johnson, 1992; Archibald and Bryant, 1990). Within the geochronologic resolution currently attainable, the Lancian-Puercan boundary does appear to coincide at least approximately with the European, marine-based Cretaceous-Tertiary boundary; both are dated at 65 Ma (Swisher et al., 1992; 1993). Nevertheless, we recommend against assuming that the Cretaceous-Tertiary and Lancian-Puercan boundaries were exactly synchronous within timescales relevant to biological and evolutionary considerations.
We emphasize use of the provincial time scale for two principal reasons: (1) to circumvent uncertainties inherent in biostratigraphic correlation to the various marine-based Cretaceous-Tertiary boundary sections of the Old World; and (2) to reduce concomitant circularity in scientific reasoning when comparing local geological/biological events that occurred within documentable, faunally defined interval-zones in the Hanna Basin with other events that occurred elsewhere in the world at the end of Mesozoic time.
Uniquely among the NALMAs, we consistently enclose the term “Edmontonian” (which temporally preceded the Lancian) in quotation marks, because of its presently inadequate paleontological definition (see Lillegraven and McKenna, 1986). It could be argued that the “Edmontonian” problem has unfortunate, and practical significance to our research. Specifically, because of uncertainties in recognizing the beginning of Lancian time on the basis of fossil vertebrates, nowhere has the “Edmontonian”-Lancian boundary been adequately defined stratigraphically. No mammalian fossils collected from the Ferris Formation, however, suggest pre-Lancian affinities. Furthermore, the Mesozoic fossils under study here were derived from parts of the local section that are stratigraphically very high within the extraordinarily thick, known Cretaceous levels (see Lillegraven and Snoke, 1996, fig. 5). We would expect the “Edmontonian”-Lancian boundary to be situated much lower in the local section, probably within lower reaches of the Medicine Bow Formation (which underlies the Ferris Formation), or perhaps even lower. Following that logic, we assume that the fossils considered in the present study represent late Lancian time, and are so-considered throughout the remaining text.
GEOGRAPHIC AND GEOLOGIC SETTINGS
Laramide Tectonic Setting
The Hanna Basin is bounded on all sides by major uplifts (shaded areas, Fig. 1) that developed during the Laramide orogeny. In the general Rocky Mountain region, orogenesis occurred principally from Late Cretaceous through early Eocene time (Snoke, 1993). The most important deformation in the Hanna Basin itself, however, occurred late in the orogeny (Lillegraven and Snoke, 1996).
Although strata in the northern Hanna Basin were severely faulted, extensively folded, and locally overturned during late parts of the Laramide orogeny, our area of research in west-central parts of the basin experienced relatively little deformation. Strata in the immediate area of study consistently dip 14°–15° eastward, toward the basin's center. No major faults or folds complicate interpretation of the section. Strike of the strata is consistent, varying from 3°–12° west of north.
Location and General Nature of Type Ferris Formation
Strata under study originally were included as part of the Upper Laramie Group by Veatch (1907). The Ferris Formation was named and defined by Bowen (1918, p. 230) for outcrops on “the old Ferris ranch” along the North Platte River. Gill et al. (1970, p. 46) located the base of the section in “… sec. 33, T. 23 N., R. 84 W ….” These vague definitions make it impossible to know just where the type section originally was intended. Thus our research involves either: (1) lower reaches of the true type Ferris Formation (or parts thereof); or (2) the next set of exposures to the north (i. e., across the river) from lower elements of the intended type section. Exposed rock types within our area of research, in either case, reflect characteristics of the type section intended by Bowen within his description. According to Bowen, the greatest thickness of the Ferris Formation is 6,500 ft (1,970 m). He interpreted the formation as entirely nonmarine in origin.
In the Hanna Basin, the Ferris Formation rests conformably upon the Medicine Bow Formation (Bowen, 1918). The latter is predominantly a continental rock unit with marine, deltaic, and estuarine fingers in its basal strata (J. E. Fox, 1971). Lower strata of the Medicine Bow Formation represent the last local occurrences of the Western Interior Cretaceous Seaway as its western shoreline retreated eastward toward the mid-continent (Lillegraven and Ostresh, 1990). The combination of the entire Medicine Bow Formation plus Lancian parts of the Ferris Formation is approximately equivalent in age to the type Lance Formation (see Clemens, 1964) in the Powder River Basin of east-central Wyoming. By convention, the Ferris Formation has been differentiated from the underlying, lithologically similar Medicine Bow Formation principally by the former's locally coarser-grained nature (i. e., presence of lenses of pebbly sandstone; Bowen, 1918).
Conglomeratic sandstone is volumetrically minor in the type area of the Ferris Formation. Clasts in such sandstone are dominated by multicolored chert that commonly bear invertebrate fossils (especially bryozoans and fusulinids) of Paleozoic age. Other contributory clasts include white siliceous shale, multiply colored quartzite, vein quartz, porphyry, grey and reddish granite and granitic fragments, amphibolite, phyllite, water-worn petrified wood, and fragmentary, commonly water-worn bones and teeth (Bowen, 1918; Ryan, 1977; personal observations). Thin-sections of locally pebbly sandstone show them to be arenites (Ryan, 1977) with only modest cementation, principally by hematite. Although rhyolitic grains were reported by Bowen (1918), examination of recently prepared thin-sections of local sandstone showed no recognizable volcanic elements other than possible metavolcanic grains of Precambrian origin (Lisa M. Amati and Arthur W. Snoke, personal communications, 1997). Feldspar grains (especially microcline) are common in the sandstone, but not in such abundance that it would be classified as arkosic (Ryan, 1977). Fragments derived from carbonate rocks are common in certain sandstone layers, but are essentially absent as pebbles. The distribution of common heavy minerals in sandstone of the Ferris Formation was reported by Ryan (1977).
Bowen (1918, p. 230) emphasized the idea that most of the pebbles of the Ferris Formation are highly resistant, “… seem foreign to this region …”, and must have derived from source areas well outside vicinity of the Hanna Basin. For purposes of simplicity, we refer to this diverse assemblage of resistant pebble types in discussions below as “chert/rock fragment assemblage.” We have recognized lithologically identical assemblages of pebbly sandstone on the west flank of the Rawlins uplift (easternmost Green River Basin), in dinosaur-bearing strata identified as Lance Formation.
Dobbin et al. (1929) divided the Ferris Formation into two informal parts. The lower Ferris, composed dominantly of mudstone and shale, also contains irregularly occurring beds of sandstone and lenses of presumably distantly derived pebbles. Dobbin et al. (1929) interpreted the lower part of the section as Late Cretaceous in age, based upon fossils of ceratopsian dinosaurs. A characteristic Late Cretaceous assemblage of palynomorphs also occurs in lower parts of the Ferris Formation (Gill et al., 1970; new data presented below).
Upper parts of the Ferris Formation as defined by Dobbin et al. (1929) consist of variably colored sandstone, alternating in the stratigraphically highest reaches with numerous beds of minable coal. The upper unit of the Ferris Formation was determined by Bowen (1918) to be Paleocene in age, based upon presence of leaves and shells of freshwater molluscs characteristic of the Fort Union Formation elsewhere in various Rocky Mountain basins. A few samples of pollen characteristic of Paleocene strata also were identified from upper parts of the Ferris Formation (Gill et al., 1970; this report).
Traditional division of the Ferris Formation into two parts, therefore, was based upon presence of Cretaceous or Tertiary fossils. For purposes of convenience and simplicity of discussion, we follow that tradition where appropriate (i. e., “lower” ∼ Lancian, and “upper” ∼ Puercan and younger parts of the formation). We recognize the Lancian-Puercan boundary at approximately 2,050 ft (625 m) above the local base of the Ferris Formation. That level is near the base of a 23 ft (7 m) thick zone of uncertainty. The lower limit of this zone (at 2,053 ft; 626 m) is based upon occurrence at that level of taxonomically diverse dinosaurian teeth in combination with lack of mammalian fossils characteristic of the Puercan. The upper limit of the zone of uncertainty (2,075 ft; 632 m) is based upon presence at that level of Protungulatum donnae, a placental mammal diagnostic of the earliest Puercan, and the virtual absence of dinosaurian remains.
The uppermost 2,000 ft (c. 600 m) of the Ferris Formation are located east of Seminoe Reservoir, stratigraphically higher than our study area. Much of the original landscape there has been disturbed by commercial coal mining, and the very top of the formation has comparatively poor exposures. According to existing geologic maps, the contact between the Ferris and overlying Hanna Formations exists to the east of the present coal mines (Dobbin et al., 1929; Love and Christiansen, 1985). On the basis of mammalian biostratigraphy, the highest localities within our study area are late Puercan in age. The top of the Ferris Formation east of the reservoir, therefore, must be younger than late Puercan.
Age and Nature of the Hanna Formation
The Ferris Formation is overlain by the Hanna Formation, named by Bowen (1918, p. 231) “… because the formation is well exposed to the west and north of the town of Hanna …”, Wyoming. He defined no type section for the Hanna Formation, and none has been specified subsequently. The Hanna Formation is an entirely continental, syntectonic depositional unit consisting principally of carbonaceous shale and mudstone. Also present, however, are subsidiary amounts of sandstone, thick beds of coal, extensive lake-beds, and true conglomerates (Gill et al., 1970; Lillegraven and Snoke, 1996). In the northeastern corner of the Hanna Basin, where the best exposed and most complete section in the basin is preserved, the Hanna Formation is over 11,600 ft (3,550 m) thick. Detailed age of the base of the Hanna Formation in the Hanna Basin remains uncertain. The age of its base in northern parts of the adjacent Carbon Basin (Fig. 1), however, is either latest Torrejonian or earliest Tiffanian (Secord, 1996). Strata of the Hanna Formation there rest with minor angular unconformity upon strata mapped as Ferris Formation. Similarly, Bowen (1918) considered the type Ferris Formation of the Hanna Basin to be unconformably overlain by the type Hanna Formation.
The equid Hyracotherium grangeri, restricted elsewhere to strata of early Wasatchian age, was recovered from near the stratigraphic top of the Hanna Formation of the northeastern Hanna Basin (Lillegraven, 1994). This suggests a latest Paleocene or earliest Eocene age for the formation's presently highest level. Because the existing top of the Hanna Formation is a Holocene erosional surface, however, the original total thickness of the formation is not determinable.
Some of the included points in Bowen's (1918, p. 231) criteria for distinguishing the Hanna Formation from the underlying Ferris Formation will become important in light of new lithologic data presented below. Therefore, we reproduce his original statement verbatim:
“This formation differs from the Ferris in being highly feldspathic, in being conglomeratic throughout, and in the fact that the conglomerate contains an abundance of local material, notably granite, Mowry shale, and Cloverly conglomerate.”
Lithologic criteria used to distinguish among the Medicine Bow, Ferris, and Hanna Formations are reviewed below.
Genetic Relationship Between Medicine Bow and Ferris Formations
Although one can gain distinct impressions to the contrary from available literature, the Ferris Formation in vicinity of its type section is not conglomeratic. Indeed, with exception of discontinuous lenses of intraformationally derived, mudstone-dominated rip-up debris, true clast-supported conglomerate is virtually non-existent. Even the volume of pebbly sandstone of the type Ferris Formation is relatively small, and individual beds of such composition generally have highly limited lateral distributions. Indeed, almost all strata within the area of our study are composed of sandstone and finer-grained clastic rocks. Nevertheless, examination of the relatively coarse-grained fractions of the local section provide important new information about the Lancian-Puercan geologic history of what today is known as the western Hanna Basin.
Bowen (1918) pointed out that, from a lithologic compositional point of view, clastic elements of the type Ferris Formation are basically the same as those seen in upper parts of the underlying Medicine Bow Formation. Bowen originally recognized that diverse rock components of what we defined above as the “chert/rock fragment assemblage” for the Ferris Formation exist only as sand- or finergrained particles within upper parts of the Medicine Bow Formation. Distinction between the Medicine Bow and Ferris Formations, therefore, revolves principally around differing capacities of ancient streams to transport pebbly materials, at least intermittently. Principal source-areas for the clastic materials themselves, however, probably did not change appreciably through the history of deposition of the upper Medicine Bow and lower Ferris Formations. In these various points, we agree with the sense of original descriptions of the type Ferris Formation as provided by Bowen.
Distinguishing Lithologically Between Ferris and Hanna Formations
In contrast to agreements mentioned immediately above, however, our field observations have weakened the utility of specific criteria by which Bowen (1918) distinguished his newly-named Ferris and Hanna Formations. Our disagreement is based on three specific observations. First, while it is true that most sandstone in the Hanna Formation has feldspathic components, essentially the same is true for the underlying Ferris Formation, throughout its extent. Even in the upper third of the Medicine Bow Formation, feldspar becomes a common component of the sandstone. Secondly, although there certainly exist genuinely conglomeratic strata within different parts of the Hanna Formation, far-and-away the greatest volume of the formation, across virtually all areas of its outcrop, is composed of clastic rocks finer-grained than siltstone. It is incorrect to think of the Hanna Formation as “… being conglomeratic throughout…” (see Bowen, 1918, p. 231).
Finally, even lower reaches of the Ferris Formation in vicinity of its type area contain clasts of granite, Mowry Shale (also recognized by Ryan, 1977), and quartzite. All these features are in agreement with the usual situation in the Hanna Formation, and the quartzite in the Ferris Formation may well have derived in part from the Cloverly Formation. It is true that relative abundance of locally derived materials (such as red granite, Mowry Shale, and quartzite from the Cloverly Formation) is greater in the Hanna Formation of the eastern Hanna Basin than in the Ferris Formation near its type area. However, an enormously thick, steeply dipping to overturned, locally thrusted, relatively coarse-grained section that bears these same, potentially locally derived lithologic components occurs at the north-central margin of the Hanna Basin. Lower parts of that section are dinosaur-bearing, and thus correlate with Cretaceous parts of the type Ferris Formation. Such clasts occur in considerable relative abundance within strata of that northern section, almost as low as the Medicine Bow Formation. In that same section, components of the “chert/rock fragment assemblage” characteristic of the type Ferris Formation continue to occur far above the stratigraphic level at which previous workers have recognized an arbitrary boundary between Ferris and Hanna Formations (e. g., McElhaney, 1988).
Two main consequences derive from the above observations. First, the mere presence of particular kinds of clasts in the local section that have potential source-areas in close proximity to the Hanna Basin is not reliable for distinguishing outcrops representing the Ferris Formation from those of the Hanna Formation. Secondly, because we have found that the relative abundances of various species of clasts, representing approximately equivalent stratigraphic levels, differ so markedly from one part of the basin to another, we have been unable to determine any combination of lithologic criteria that can be used reliably in the field to distinguish outcrops of the Ferris Formation from those of the Hanna Formation.
Indeed, so far as we have been able to determine, only an historical tradition, as expressed through geologic maps – in which stratigraphically superposed rocks mapped as Hanna Formation unquestionably overlie those mapped as Ferris Formation – can be used in the field to distinguish the two units. Relative to generally accepted conventions of stratigraphic practice and nomenclature (e. g., NACSN, 1983), this represents a highly unsatisfactory situation.
Use of the two formational names is deeply entrenched, however, within existing literature and local geologic tradition. Also, it may be impractical to suggest combining the Ferris and Hanna Formations into a single formation. Relative to stratigraphic tradition within the Rocky Mountain region, the resulting formation would be prodigiously thick, and would have been unusually long-lived in its history of deposition (very late Cretaceous to earliest Eocene). Combining the two formations would, however, simplify the process of mapping, and serve to reduce levels of confusion engendered through attempted discussion of stratigraphic units that lack suitable lithologic criteria for their distinction in the field. The simplification would be especially welcome in areas in which Laramide faulting was profound, such as along the northeastern margin of the Hanna Basin. In any case, the present concept of two formations derives principally from historical artifact, initially based upon incorrect lithologic assumptions.
Prior to our fieldwork in the western Hanna Basin, only estimates of thicknesses of specific parts of the Ferris Formation were available. To provide a quantitative framework for our research, we measured and described two key stratigraphic sections (A–A′ and B–B′ of Fig. 2) within the area of study. We refer to transect A–A′ as the “main section,” and B–B′ as the “Windy Mudstone section” (named after a key paleontological site, V-91004, the “Windy Mud-stone Locality”). Both sections were measured in feet, using a five-foot Jacob's staff and Brunton compass. Geographic placements of these stratigraphic sections, and University of Wyoming mammal-bearing localities, are shown in Figure 3.
Our measured sections are restricted to parts of the Ferris Formation found north and west of the North Platte River (the flooded Platte arm of Seminoe Reservoir); uppermost reaches of the formation, therefore, are not yet documented. The base of the main section (A–A′) is at the contact between the Medicine Bow and Ferris Formations as shown to occur in Section 29, T. 23 N., R. 84 W. on Dobbin et al.'s (1929, p1. 27) geologic map of the Hanna Basin. The basal 935 ft (285 m) of our main section are obscured by vegetation (Fig. 4). We estimated thickness of this covered zone by assuming constant strike and dip of the strata of N 6° W., 15° NE, and correcting for the roughly 100 ft (30.5 m) increase in topographic elevation from the formational base. Unless specified otherwise, references to specific stratigraphic levels in the remainder of the text relate to positions above the base of the Ferris Formation as illustrated in Figures 2 and 3.
Detailed lithologic notes were gathered in the field, and written on standard stratigraphic strip-charts. To aid in analysis of the enormous amount of raw data gathered by this means for the main section, we devised a computerized retrieval system. The procedure uses, as its basis, three-digit descriptive codes, unique for commonly observed rock types (following general concepts provided by Treworgy, 1992). We entered the codes into spreadsheet form, expressing lithologies characteristic of one-foot (30.5 cm) intervals. The resulting, lithologically descriptive database was then interrogated through a spreadsheet program (entitled “Finley1”) that we designed. Finley 1 is capable of extracting from an appropriately configured database, at any user-specified stratigraphic interval, any descriptive lithologic information that can be expressed through a unique three-digit code (or through a search string using two or more three-digit codes). From these data were derived the following summaries of lithologic trends through measured section A–A′.
SPECIFIC NATURE OF LOCAL FERRIS FORMATION
Almost all strata exposed along section A–A′ are composed of sandstone and finer-grained, clastic rocks (Figs. 5 and 6). Only a minuscule part of the section is represented by pebbly sandstone; clast-supported conglomerate is virtually nonexistent. Conglomeratic sandstone typically occurs in beds less than two feet (60 cm) thick and, while not lenticular, usually they are not laterally extensive. Except for intraformational mudstone rip-ups, cobble-or larger-sized clasts do not exist within the area of study. The great majority of conglomeratic particles throughout the section occur as small (< 4 cm) to tiny pebbles (< 1 cm). The pebbles occur both as intraformational, mudstone rip-up clasts and as more competent, extraformational rocks (including those of the “chert/rock fragment assemblage,” as defined above). Mudstone rip-ups are common throughout the entire section, and the sandy matrix surrounding the clasts often contains vertebrate fossils. Within Puercan parts of the section, pebbles of competent rocks generally are less than one centimeter in greatest diameter.
The section along section A–A′ is devoid of substantial beds of nonclastic evaporitic or carbonate strata, although carbonate-derived rock fragments are observable in thin-sections of certain sandstones. Coaly beds, while commonly present, are thin (< 1 ft); generally they are discontinuous laterally. One unusually widespread, two ft- (60 cm-) thick coal-bed does occur, however, at about the 2,965 ft (904 m) level.
Comparison of Figures 5 and 6 shows that sandstone, within strata of Puercan age, gains in relative abundance at the expense of finger-grained clastic rocks. While that is true along section A–A′ itself, the proportion of finer-grained strata of Puercan age just to the north (within the map-area of Fig. 3) appears much the same as in Lancian parts of section A–A′. As discussed below, the extensive sand of Puercan age along section A–A′ reflects a temporally persistent, narrow, and geographically stable system of river-channels. The transition from domination by mudstone and shale to domination by sandstone, even along the section, does not coincide with the faunally recognized Lancian-Puercan boundary.
Dark-colored mudstone and grey to brown carbonaceous shale characterize most of the fine-grained strata of section A–A′. Macroscopic evidence of fossil plants (i. e., leaves, wood, wood impressions, seeds, etc.) is common within these strata. As summarized below, palynomorphs also represent common elements of highly carbonaceous parts of the fine-grained section. Scarce, maroon-colored mudstone, containing vertebrate fossils in situ, occur as interbeds within sandstone bodies in basal exposed parts of the section. In general, however, we were more successful in finding vertebrate fossils in mudstone of Puercan age than from mudstone in Lancian parts of the section.
Sandstone beds in the Ferris Formation are poorly sorted, rich with muddy matrix, and composed of subangular to subrounded grains, occasionally frosted. Muscovite and carbon-aceous debris is common in virtually all sandstone of the section. Leaf impressions and carbonized leaves and wood usually are present within sandstone throughout section A–A′. Rounded concretions, typically calcareous, are common locally within virtually any channel sandstone body.
As emphasized in Figure 6, most of the sandstone throughout the section is medium- to fine-grained. Coarse-grained sandstone is common, however, and usually these strata bear the pebbly lenses. Grey- and reddish-colored granitic grains contribute to coarse-grained sandstone throughout the section. The granitic fragments become particularly obvious as larger grains and tiny pebbles within coarser sand-bodies found stratigraphically above the 2,000 ft (610 m) level. Contributions from the “chert/rock fragment assemblage,” because of grain-size relationships, are most obvious in Lancian parts of the section. The rich diversity of rock types that defines the “chert/rock fragment assemblage” however, occurs sporadically throughout the Puercan strata, nearly to the top of our measured section. A particularly good example of this chert-rich, tiny pebble assemblage occurs at about the 3,190 ft (972 m) level.
The Mowry Shale, a relatively resistant, easily recognizable Cretaceous marine formation, is widespread across most of Wyoming and major parts of adjacent states (see McGookey et al., 1972). The formation also is well-exposed around margins of the Hanna Basin. Clasts of the Mowry Shale add to the lithologic diversity of pebbly sandstone along section A–A′, in strata both of Lancian and Puercan age. Typical clasts vary from tiny pebbles to about three centimeters in greatest length. Especially well-developed examples can be seen at the following levels above the base of the Ferris Formation: 1,578 ft (481 m); 2,031 ft (619 m); and 3,215 ft (980 m).
As pointed out by Bowen (1918), clasts of Mowry Shale can occur in abundance within the local Hanna Formation, strongly suggesting local derivation. In the northeastern corner of the Hanna Basin, for example, localized conglomeratic beds commonly are dominated by packed masses of fragments of Mowry Shale (Lillegraven and Snoke, 1996). Generally smaller chips of Mowry Shale occur in lesser relative abundance along section A–A′, within Bowen's original concept of the Ferris Formation. Plates of Mowry Shale, associated with dinosaurian remains, also occur at multiple levels along the northern margin of the Hanna Basin in strata mapped as Ferris Formation (Love and Christiansen, 1985). It is clear, therefore, that the presence of detritus of Mowry Shale is not dependable as a lithologic criterion distinctive of the Hanna Formation.
Depositional Features and Environments
The following represents an interpretive summary of paleoenvironmental settings based upon description of lithologic changes that occur through the Ferris Formation from bottom to top through section A–A′. Exposed strata along section A–A′ begin at about the 935 ft (285 m) level.
Large sets of light-colored, planar-crossbedded strata, cropping out between 935 ft (285 m) and 1,123 ft (342 m), probably reflect deposition within a Platte-type braided river system (sensu Miall, 1977). These strata contain poorly sorted, alternating fine and coarser sandstone laminae, as well as angular, maroon-colored, mud rip-up clasts containing vertebrate fossils. Some of these mud clasts are up to two feet (0.6 m) in greatest length, although most are cobble-sized or smaller. Probably the fossiliferous mud rip-ups were deposited within channels of the ancient river system, as a result of undercutting and subsequent collapse of parts of the principally sandy channel walls. It appears that most vertebrate fossils present in the light-colored sandstone actually derived from erosion and partial destruction of the reworked maroon mudstone. Limited outcrops of the fossil-bearing mudstone occur in the immediate area. Planar-and trough-crossbedded strata, scoured at their bases, occur in this part of the section. Secondarily developed, highly regular sets of joints occur in addition to the primary sedimentological features related to cross-bedding and limited soft-sediment deformation.
A sharp lithologic change occurs above the 1,123 ft (342 m) level along section A–A′. Large sets of crossbedded sandstone are replaced by the first substantial, well-exposed layers of mudstone and shale. The individual bed thickness generally decreases toward the Lancian-Puercan boundary. Waterworn and intact pieces of petrified wood, as well as leaf imprints, are common throughout this finer-grained section. The first regularly occurring, highly carbonaceous and/or lignitic shale appears at approximately the 1,830 ft (558 m) level.
Strata above the 1,123 ft level of the section are consistent with interpretation of deposition in a relatively low-energy, meandering river system. These conditions persist to the north of section A–A′ well beyond the faunally recognized Lancian-Puercan boundary. Classic fining-upward sets of strata characteristic of meandering river systems occur throughout Lancian parts of the section, stratigraphically above the braided stream deposits. Thin but sometimes laterally extensive lenses of coarse-grained, pebbly sandstone are overlain by layers of more massive or laminated sandstone, which in turn usually are overlain by thick beds of grey, moderately carbonaceous mudstone or shale. Individual mudstone units typically become progressively more carbonaceous upward. The finest-grained strata were cut by channels as new cycles began, and typically were capped either by sandstone or conglomeratic sandstone. Heavily sideritized zones, sometimes concretionary, abound throughout this part of the section.
Although most sandstone in this second part of the section is relatively massive, sedimentary structures such as trough and planar cross-stratification, horizontal lamination, and oscillation ripples are preserved at various levels. The coaly shale, first appearing approximately at the 1,830 ft (558 m) level, may well represent deposition within heavily vegetated flood basins, oxbows, or levees. An unusual abundance of algal cysts, combined with the paucity or absence of other palynomorphs, suggests that these swamps formed occasional boghead coals that were compositionally different from most Lancian swamp deposits recognized elsewhere (see section entitled “Palynology”). Fragmentary remains of aquatic or semiaquatic vertebrates (including chondrichthyans, gars, amiids, amphibians, turtles, champsosaurs, and crocodilians) are common fossils throughout the section. Leaf impressions, including intact palm fronds and other angiosperms (see Gill et al., 1970; Johnson, 1996), as well as dinosaur bones and teeth also locally abound through this section. Fossil mammals, although far less common, do occur. Teeth of typical Lancian sharks and rays occur in association with mammal teeth, gas-tropods, and coprolites in dark mudstone roughly 45 ft (14 m) below the Lancian-Puercan boundary (UW locality V-92010). Partially articulated postcranial remains of a ceratopsian dinosaur occur at the 1,985 ft (605 m) level (UW locality V-91033).
A third, and final, characteristic lithologic complex shows initial phases of development along section A–A′ roughly 200 ft (60 m) below the Lancian-Puercan boundary. However, the new complex becomes fully expressed along the section a few hundred feet above the Lancian-Puercan boundary and continues nearly to the top of section A–A′. Individual beds of mudstone and shale dramatically decrease in thickness and number above this stratigraphic level, and comprise only a small percent of the strata between the 2,100 ft (640 m) and 4,200 ft (1,280 m) levels (Fig. 5). Thick beds of generally yellow-colored, inconspicuously cross-bedded, fine-to medium-grained, muddy sandstone, commonly grading to coarse-grained, comprise most of the strata above the Lancian-Puercan boundary (Fig. 6). Some of these individual sandstone beds exceed 100 ft (30.5 m) in thickness. The sandstone generally is only moderately cemented, and thus weathers rapidly into rounded masses.
The stratigraphically highest depositional setting within section A–A′ represents the persistence, in a remarkably stable geographic position, of an enormous channel system. Over-steepened foreset beds and soft-sediment deformation as paleo-slumps are common features. The depositional nature of the system, however, remains uncertain in detail. The volume of sand contained within its roughly 2,000 ft (610 m) stratigraphic extent speaks to prodigious supplies of clastic input available to the Hanna Basin during Puercan time.
Vertebrate fossils are preserved commonly within these channels, both as isolated occurrences of individual bones or teeth and as concentrated fossiliferous lenses. The strata preserve intact, thin-shelled tortoises and fragmentary crocodilian skulls encased in calcareous concretions. Nowhere, however, have we discovered vertebrate skeletons in articulation. The entire channel sequence is remarkable in its consistently fossiliferous nature, nearly to the stratigraphic top of section A–A′. Additional to the common occurrences of piscine, reptilian, and mammalian bones and teeth, petrified wood abounds, including occasional complete logs in fallen position, sometimes with roots.
Conglomeratic sandstone is relatively inconspicuous and highly localized in the third depositional complex of section A–A′. Approximately one meter of friable, clay-pebble and muddy sandstone conglomerate, containing a diverse, late Puercan mammalian assemblage, crops out at approximately the 3,585 ft (1093 m) level. Although the conglomeratic bed as a whole remains essentially unlithified, most of the individual, intraformational clay-pebbles were lightly cemented prior to their deposition; the muddy matrix lacks consequential cementation.
Quite high within this sandstone-dominated unit of the Ferris Formation is an 85 ft (26 m) thick lacustrine (or possibly lacustrine-deltaic) sequence. At its base, at about the 3,433 ft (1,046 m) level, is an indistinctly ripple-marked sandstone. The overlying, dark-colored lacustrine mudstone bears fossilized scales and teeth of gars, and grades upward into yellow, fine-grained muddy sandstone. The thinly and regularly laminated sandstone yields abundant leaf impressions that preserve tertiary venation. These represent the only beds related to lake or lake-margins recognized within section A–A′.
At about the base of the lacustrine series, there exists abundant evidence in outcrop along section A–A′ for initiation of a pattern of deposition involving regular cutting-and-filling. Stratigraphically below this point, the pattern of deposition was much more regularly aggradational, with only minor evidence of intraformational scouring. The new cut-and-fill regime continues upward beyond the top of our section A–A′, at least into the levels of minable coal seen east of Seminoe Reservoir. The resulting complex stratigraphy, which intimately involves the coal-beds themselves, is magnificently exposed along the eastern shoreline of the reservoir (i. e., west flank of Pats Bottom), east of section A–A′.
The cut-and-fill regime that begins near the 3,433 ft (1,046 m) level probably heralds changing conditions of subsidence that led ultimately to deposition of the massive coal-beds that dominate uppermost reaches of the formation. In terms of individual thicknesses and lateral extent, the coal layers high in the Ferris Formation dwarf those lower in the section.
During the 1992 field season, Dr. Douglas J. Nichols (U.S. Geological Survey, Denver) sampled seven stratigraphic levels for palynomorphs. The samples were taken along stratigraphic sections A–A and B–B′ (Figs. 2 and 3). Results of Nichols' palynological analyses follow, using USGS sampling numbers (Nichols, personal communication, September, 1994, November, 1996, and January, 1997).
The first five samples were collected in stratigraphic order along section A–A′. The lowest samples, D8105-A to -C, were in a coal and fine-grained deposits immediately below and above the coal, at about the 1,925 ft (587 m) level. Sample D8105-A, from below the coal, yielded only cysts of no biostratigraphic utility. The sample from the coal (D8105-B) is an unusual deposit, dominated by cysts of fossil algae, constituting a boghead coal. Sample D8105-D, taken from a coaly mudstone at the 1,970 ft (601 m) level, yielded 16 genera of palynomorphs. The sample is inferred to be Late Cretaceous in age, based upon presence of three indicator taxa (Gunnera microreticulata, Liliacidites complexus, and Proteacidites spp.).
Sample D8106, taken from a grey mudstone about the 2,010 ft (613 m) level, also was inferred to be Late Cretaceous in age, based upon occurrences of the Cretaceous indicator, Proteacidites. Six of the seven taxa recovered from D8106 are known to occur elsewhere both below and above the Cretaceous-Tertiary boundary. Sample D8107, taken from a grey-brown mudstone at about the 2,030 ft (619 m) level, was barren of palynomorphs.
Sample D8108 (supplemented by D8176, collected by Eberle in 1994), taken from a grey mudstone at about the 2,099 ft (640 m) level, contains 12 species-level taxa of palynomorphs, characteristic of both Cretaceous and Paleocene strata. A single specimen of Proteacidites sp. and several specimens of Gunnera microreticulata, both usually considered as Cretaceous indicators, were recovered from sample D8108. Taxa usually characteristic of the early Paleocene include: Momipites wyomingensis, M. inaequalis, Corollina sp., Cupuliferoidaepollenites sp., Cyathidites diaphana, Deltoidospora sp., Laevigatosporites ovatus, Nyssapollenites sp., and Reticuloidosporites pseudomurii. Based upon identification of mammalian fossils, the strata from which sample D8108 was taken are Puercan in age. The lowest, undoubted Puercan locality (at which the typically Puercan “condylarth” Protungulatum donnae occurs) is 24 ft (7.3 m) stratigraphically below sample D8108. Puercan mammals also have been recovered from an anthill that occurs six feet stratigraphically above sample D8108. It is possible, therefore, that specimens of Gunnera microreticulata and Proteacidites sp. in D8108 were reworked from Cretaceous strata. Nichols also noted that Gunnera microreticulata sometimes occurs in basal Paleocene rocks in the region.
A sixth sample was taken along section B–B′. Sample D8109 (supplemented by D8177, collected by Eberle in 1994), taken from carbonaceous shale at the Windy Mudstone Locality (V-91004) at about the 2,170 ft (661 m) level, contains only three palynomorph taxa. Two species recovered from sample D8109 (Kurtzipites circularis and Ulmipollenites krempii) are known to have crossed the Cretaceous-Tertiary boundary outside vicinity of the Hanna Basin; algal cysts (Schizophactus) are without biostratigraphic utility. The high relative abundance of Kurtzipites, however, suggests a Paleocene age for sample D8109. The palynologically determined age at locality D8109 is compatible with conclusions that we draw from identification of co-occurring mammals.
Sample D8110, derived from a coal located well within dinosaur-bearing parts of the formation, west of section B–B′ and down-section from sample D8109, contained only two kinds of palynomorphs, algal cysts and Ulmipollenites krempii.
Localities bearing fossil vertebrates and plants were plotted on the 7.5-minute Pats Bottom and Ferris Lake topographic quadrangle maps. Detailed data pertinent to specific vertebrate-bearing localities from this research are available to qualified researchers (contact: Collections Manager, Departmental Scientific Collections, Department of Geology and Geophysics, The University of Wyoming). Specimens of fossil mammals were collected from six Lancian and 32 Puercan localities, as shown in Figure 3. A single mammal tooth was recovered from UW locality V-91032, a site that is questionably Puercan in age. Stratigraphic correlations of the fossil-bearing localities to sections A–A′ or B–B′ were determined through detailed field mapping, combined with use of enlarged color aerial photographs. Plant macrofossils were collected, and are under study, by Drs. Kirk R. Johnson (Denver Museum of Natural History) and Steven R. Manchester (Florida State Museum, The University of Florida).
Vertebrate localities fell into four categories for purposes of sampling: (1) those containing finegrained matrix that could be disaggregated through techniques of standard underwater screen-washing (see McKenna, 1962; we used screens having openings between wires of 0.75 mm); (2) those containing a sandy, often heavily cemented, matrix more suited to hand quarrying; (3) anthills, which were dry-screened on-site and sorted elsewhere; and (4) localities that combined two or more of the above categories. Anthills alone account for only two localities.
Vertebrate localities suited for screen-washing were dealt with in the following manner. Three large (40–50 lb) test bags from each wash site were transported to Laramie to be screen-washed in an indoor UW laboratory facility. If any mammalian and/or dinosaurian teeth were found during test-washing, several more bags were collected from the locality and were washed and sorted during the academic year. In 1994, due to temporary loss of our indoor washing facility, matrix was screen-washed in a relatively wind-protected bay of Seminoe Reservoir.
Matrix was initially screen-washed under water. Mud matrix subsequently was broken down by a variety of solvents, including diesel fuel (a procedure now prohibited on campus), Polychem “Quik-Sqeez”™ (a citrus-derived, commercially available, non-toxic solvent), hydrogen peroxide, laundry detergent, and water-softener. Several laboratory procedures were pioneered by Eberle to separate the teeth and bones from matrix that could not be broken down by water and/or common solvents. These techniques, to be more fully described separately, included use of an electromagnetic separator and a nontoxic heavy liquid (sodium polytungstate). These techniques reduced some samples by one-half to three-quarters of the original volume, thereby dramatically decreasing time spent in recovery of fossils under a microscope.
Certain individual localities were quarried extensively, in some cases during different field seasons. Weathered debris at the base of the outcrop, and finer matrix removed through quarrying, commonly were dry-screened on-site. Fossils contained within hard matrix sometimes were plaster-jacketed, and/or removed by use of a portable, gasoline-powered rock-saw. All fossils were prepared at The University of Wyoming.
Several localities involved two, or even all three, of the above-listed categories. An example (UW V-91031) includes a fossiliferous anthill perched atop a quarry site. In this instance, we assumed that fossils contained within the anthill originally weathered from the adjacent outcrop; thus the anthill was included within the same locality as the quarry site. However, some less than ideal situations occurred, in which anthills containing fossils were relatively close to several outcrops, or isolated in grassy areas. Locality UW V-91014 is an example of an ant-hill situated a good distance away from any outcrop. As observed by Lavigne (1969) through excavation of their burrows, harvester ants are known to bring particles to the surface from depths as great as seven feet (2.1 m).
Special sampling techniques were applied to the Windy Mudstone Locality (UW V-91004, early Puercan), a crevasse splay deposit, because of its unusual richness and proximity to the Lancian-Puercan boundary. In 1992, Eberle mapped out an eight m2 grid system, into one meter squares, covering areas known to have yielded fossils. One full gunny-sac of mudstone was taken from each square, and screen-washed/sorted at The University of Wyoming. Of the eight samples, the most productive were the three southernmost, adjacent to the sandstone that caps the site. These three quadrats were sampled in greater volumes during the 1993 and 1994 field seasons.
Paleontological locality- and specimen-records were entered into the FileMaker Pro® relational database system used universally for our Departmental Scientific Collections.
Only mammalian dental remains are included in this study. Mammalian fossils in the western Hanna Basin consist mostly of isolated teeth. A single jaw fragment was collected from Lancian strata of the Ferris Formation, while several jaw fragments (some nearly complete) and a partial skull were collected from Puercan strata. All dental measurements are reported in millimeters. Isolated teeth and small jaw fragments were measured to the nearest one-thousandth of a millimeter on an Ehrenreich Photo-Optical Industries Shopscope™. Although such precision in reporting measurements does not imply accuracy to that level, the last significant digit follows standard scientific procedure in representing an estimate. As documented by Lillegraven and Bieber (1985), repeatable accuracy of dental measurements to 0.01 mm can be expected through routine use of the Shopscope™. Larger specimens, such as dentaries, were measured with a dial micrometer or metric rule.
Terminology and most measurements used to describe multituberculate dentitions follow Clemens (1964), Jepsen (1940), and Krause (1982). Measurements specific to upper and lower fourth premolars of multituberculates follow conventions established by Novacek and Clemens (1977) and Sloan (1981). With regard to specific measurements from p4s of multituberculates, “L1” follows Archibald's (1982) definition, and is the distance, measured horizontally parallel to the line used to measure the length, from the tooth's anterior margin to the point at which the highest serration occurs. Orientations of standard measurements are illustrated in Figure 7.
- The American Museum of Natural History, New York NY
- Denver Museum of Natural History, Denver CO
- Florida State Museum, The University of Florida, Gainesville FL
- New Mexico Museum of Natural History, Albuquerque NM
- St. Paul Science Museum, St. Paul MN
- Laboratory of Paleontology, The University of Alberta, Edmonton ALTA
- University of Arizona Laboratory of Paleontology, Tucson AZ
- Museum, The University of Colorado, Boulder CO
- Museum of Paleontology, University of California, Berkeley CA
- Museum of Paleontology, The University of Minnesota, Minneapolis MN
- U.S. Geological Survey, Denver CO
- Collection of Fossil Vertebrates, Departmental Scientific Collections, Department of Geology and Geophysics, The University of Wyoming, Laramie WY
- Peabody Museum of Natural History, Yale University, New Haven CT
Dental Terminology and Measurements
Lower-case letters (e. g., m1) designate teeth from lower jaws
Upper-case letters (e. g., M1) designate teeth from upper jaws
- Numbers of cusps of multituberculate upper molars in external, medial, and internal rows, respectively (observed variability indicated by hyphens)
- Numbers of cusps of multituberculate lower molars in external and internal rows, respectively (observed variability indicated by hyphens)
- Left tooth (e. g., Lm1)
- Right tooth (e. g., Rm1)
- Anteroposterior length
- Length of trigonid
- Length of talonid
- Width of trigonid
- Width of talonid
- Height of first serration (see Sloan, 1981)
- Horizontal length to highest serration
- Estimated measurement, required by fracture or excessive wear
- Locus of tooth within dental series is uncertain (e. g., Rmx)
Class MAMMALIA Linnaeus, 1758
Subclass ALLOTHERIA (Marsh, 1880)
Order MULTITUBERCULATA Cope, 1884
We document occurrence of at least seven genera of multituberculates in the Ferris Formation. Species of Cimolodon and Meniscoessus have been recovered there only from Lancian strata, while species of Mesodma, Ectypodus, Ptilodus, Catopsalis, and Taeniolabis have been recovered locally only from Puercan strata. Large specimens of Ectypodus and Ptilodus collected from middle and late Puercan strata of the Ferris Formation probably represent new species, although we do not propose new names. At present, no species of multituberculate are known both from Lancian and Puercan strata of the Ferris Formation, although Mesodma formosa and M. hensleighi are common representatives of the Lancian elsewhere.
Twenty-two simple, peg-like incisors from early Puercan locality V-91004 and a single incisor from middle Puercan locality V-91005 are identified simply as Multituberculata. Most of the incisors probably derived from neoplagiaulacids, as we identify all multituberculates in V-91004 on the basis of other teeth as Neoplagiaulacidae. With few exceptions (including Catopsalis and Stygimys), multituberculate incisors appear to be taxonomically undiagnostic.
We identify a single tooth (UW 26567, P3) from Lancian locality V-92027 as Multituberculata indeterminate.
CIMOLODONTIDAE Marsh, 1889a
Cimolodon Marsh, 1889a
Cimolodon nitidus Marsh, 1889a
Cimolodon nitidus Marsh, 1889a, p. 84.
Holotype.–YPM 11776, Lm1.
Type locality.–Mammal Locality no. 1 of Lull (1915), UCMP locality V-5003, type Lance Formation, Wyoming.
Referred specimens.–UW 26213, Lp4 from UW locality V-92027; UW 26508, Rm1; UW 26509, LM1 fragment (both from UW locality V-92010); and UW 26570, Rm1 from UW locality V-92036.
Localities.–UW localities V-92010, V-92036, and V-92027, lower Ferris Formation, western Hanna Basin, Wyoming (late Lancian).
Known distribution.–Lance and lower Ferris Formations, Wyoming; Hell Creek Formation, Montana; Scollard Formation (upper Edmonton Formation), Alberta (all Lancian); and St. Mary River Formation, Alberta (“Edmontonian”).
Diagnosis.–See Clemens, 1964, p. 56.
Description and discussion.–The four specimens identified as Cimolodon nitidus agree with Clemens' (1964) description of type material.
UW 26213, a slightly damaged, worn Lp4, falls within size ranges of Cimolodon nitidus collected from the Lance and Hell Creek Formations. Clemens (1964) and Archibald (1982) provided descriptions of p4s of C. nitidus; only additions to their descriptions, and information specific to UW 26213, are given here. The posterior margin of UW 26213 is damaged, and serrations posterior to the highest (i. e., the fourth) are worn, allowing only a minimum count of 12. UW 26213 bears eleven external (labial) ridges, with the most posterior being discontinuous and the weakest. Eleven lingual ridges also are present. Three ridges extend from the first serration. These include a labial and a lingual ridge, as well as a medial ridge that extends down the anterior margin of the tooth. The medial ridge does not bifurcate, and is the weakest of the three. A well-developed accessory root is present between the two main roots. Measurements of UW 26213 are: length, 5.552; width, 2.452; height, 3.157; and L1, 3.064.
Isolated molars of Cimolodon nitidus and Cimolomys gracilis are morphologically similar, and their size ranges overlap. However, most molars of C. gracilis tend to be larger than those of C. nitidus, and molar cusps of the former tend to be more strongly crescentic (Clemens, 1964). UW 26508 and 26570 are smaller than mls characteristic of Cimolomys gracilis. Both specimens have an external to internal cusp formula of 6:4, and fit the description of mls from C. nitidus given by Clemens (1964). As in lower molars of C. nitidus from the type Lance Formation (see Clemens, 1964), on UW 26508, short, vertical grooves occur on the medial sides of all cusps, and on the labial sides of the external cusps. Weak vertical grooves are preserved on the lingual side of the most anterior internal cusp of UW 26508. Although no vertical grooves are preserved on UW 26570, wear has nearly removed the external cusp row, and internal cusps possess well-developed, circular wear facets. Both UW specimens 26508 and 26570 have a rectangular outline as seen in occlusal view.
The relative sizes of cusps can be determined on UW 26508, a smaller, unworn version of UW 26570. On the internal row, the most posterior cusp is the largest, and the most anterior cusp is the smallest. Of the six cusps making up the external row, the first cusp is smallest, and cusps three and four are largest.
Measurements of UW specimens 26508 and 26570 are provided in Table 1.
We identify UW 26509, an incomplete LM1, as Cimolodon nitidus on the basis of a strong similarity to casts of this species from the Scollard Formation, and to the description and figures of C. nitidus provided by Clemens (1964). Due to incompleteness, the cusp formula of UW 26509 cannot be determined. Like Mls of C. nitidus from the type Lance Formation (see Clemens, 1964), strong, vertical grooves are preserved on the lingual side of the external row of cusps, both sides of the medial row, and on the labial side of the internal row.
Cimolodon sp. cf. C. nitidus Marsh, 1889a
Referred specimen.–UW 26568, incomplete P1.
Locality.–UW locality V-93002, lower Ferris Formation, western Hanna Basin, Wyoming (late Lancian).
Description and discussion.–We tentatively identify UW 26568 as Cimolodon nitidus on the basis of its large size and morphologic similarity to P1s of C. nitidus described and figured by Clemens (1964). As in P1s of C. nitidus from the Lance Formation, UW 26568 possesses three cusps, all of which have short, vertical grooves on their sides.
In general, anterior isolated premolars of multituberculates cannot be identified below the level of family. As this species is reported on the basis of other teeth from two other Lancian localities within the Ferris Formation, however, its occurrence at UW locality V-93002 might be expected.
We provide no measurements of UW 26568 because of incompleteness, and uncertainties of orientation in the jaw.
Halodon formosus Marsh, 1889b, p. 179.
Holotype.–YPM 11812, Lp4 (Marsh, 1889b, pl. VIII, figs. 36–39).
Type locality.–Peterson's quarry of Lull (1915), Lance Formation, Wyoming (Lancian).
Referred specimens.–UW 26025, Rm1; UW 26019 and 26024, RP4s; and UW 26011, incomplete LM1.
Locality.–UW locality V-91004, upper Ferris Formation, western Hanna Basin, Wyoming (early Puercan).
Known distribution.–Lance Formation, Wyoming; Scollard Formation, Alberta; Hell Creek Formation, Montana (all Lancian); upper Ferris Formation, Wyoming; Ravenscrag Formation, Saskatchewan; Tullock and Bear Formations, Montana; and Nacimiento Formation, New Mexico (last four occurrences Puercan).
Revised diagnosis.–Clemens, 1964, p. 31.
Description and discussion.–A detailed description and revised diagnosis of Mesodma formosa was given by Clemens (1964). Mesodma originally was diagnosed by Jepsen (1940), based upon the type species, M. ambigua. Size ranges of teeth of various species of Mesodma, including M. formosa, were provided by Archibald (1982) and Novacek and Clemens (1977).
UW 26025, a Rm1, falls within the size range characteristic of mls of Mesodma formosa given by Archibald (1982). The anterior margin of UW 26025 is narrower than the rest of the tooth, a feature commonly occurring in mls of M. garfieldensis from the Tullock Formation in northeastern Montana (Archibald, 1982). UW specimens 26019 and 26024, both RP4s, fit well within the size range, and agree with the descriptions of M. formosa given by Archibald (1982) and Clemens (1964).
Measurements of UW specimens from the Ferris Formation referred to Mesodma formosa are given in Table 2.
Mesodma ambigua Jepsen, 1940
Mesodma ambigua Jepsen, 1940, p. 268.
Holotype.–Princeton no. 14414, left dentary with i, p3–4, and ml.
Type locality.–Mantua Lentil, Fort Union Formation (Polecat Bench Formation), Park County, Wyoming (early Puercan).
Referred specimens.–UW 26007, RM1; UW 26009 and 26018, RP4s; UW 26089 and 26550, LM1 fragments; UW 26098, LP4 fragment (all preceding specimens from UW locality V-91004); and UW 26189, LM2 (from UW locality V-91031).
Localities.–UW localities V-91004 and V-91031, upper Ferris Formation, western Hanna Basin, Wyoming (early and middle Puercan).
Known distribution.–Ferris and Fort Union Formations, Wyoming; and Tullock Formation, Montana (early and middle Puercan).
Diagnosis.–Jepsen, 1940, p. 268.
Description and discussion.–Although upper dentitions for most species of Mesodma have been documented, that of M. ambigua has not. Van Valen and Sloan (1965) noted occurrence of two specimens of M. sp. cf. M. ambigua from the Paleocene Purgatory Hill local fauna of northeastern Montana, but made no mention of which positions the specimens occupied in the tooth-row. We identify UW specimens 26007, 26009, 26018, and 26189 as M. ambigua based upon their large size, and a morphology similar to other species of Mesodma. UW 26007, a RM1, is larger than specimens typical of Mesodma thompsoni and M. garfieldensis, and has the same morphology as the majority of Mls identified as Mesodma sp. by Clemens (1964, p. 47, fig. 17). UW 26189, a worn LM2, is larger than, though morphologically similar to, a single M2 of M. thompsoni reported by Archibald (1982) from Lancian strata of the Hell Creek Formation. UW 26189 has a triangular outline as seen in occlusal view, a concave anterior margin, and a cusp formula of 1:3:3–4. Due to wear in UW 26189, the number of cusps in the internal row can only be estimated.
Both UW 26009 and 26018 are larger than, although morphologically most similar to, P4s of Mesodma thompsoni. UW 26009 falls within upper reaches of the range of A–P lengths for M. garfieldensis given by Archibald (1982), but is wider than any documented specimen of that species. UW 26018 is noticeably longer and wider than P4s characteristic of M. garfieldensis.
In order to estimate the expected length of P4s of Mesodma ambigua, we calculated the length of P4 relative to p4 of several species of Mesodma in which both upper and lower dentitions have been documented (M. formosa, M. thompsoni, and M. garfieldensis; original data from Clemens, 1964, Archibald, 1982, and Johnston and Fox, 1984). Based upon this technique, the range of relative lengths of P4 to p4 is 0.55–0.75. The mean length of p4 for Mesodma ambigua from Mantua lentil, given by Novacek and Clemens (1977), is 4.53. The length of the P4 of M. ambigua would be expected, therefore, to range between 2.49 and 3.39. The mean length of P4 in UW specimens 26009 and 26018 is 3.33 mm, at the higher end of the range expected for M. ambigua.
Based upon the close morphological similarity of their p4s, Clemens (1964) suggested that Mesodma ambigua descended directly from M. thompsoni. The p4 of M. ambigua differs from that of M. thompsoni in being slightly larger, and in having more serrations (Clemens, 1964). Assuming that the upper dentitions of these two species are morphologically comparable, we identified UW 26018 as M. ambigua.
Mesodma hensleighi Lillegraven, 1969, p. 16.
Holotype.–UA 3596, Lp4.
Type locality.–Locality KUA-1, Scollard Formation, Alberta (Lancian).
Referred specimen.–UW 26075, Lm2.
Locality.–UW locality V-91004, upper Ferris Formation, western Hanna Basin, Wyoming (early Puercan).
Known distribution.–Scollard Formation, Alberta; Lance Formation, Wyoming; Hell Creek Formation, Montana (all Lancian); and upper Ferris Formation, Wyoming (early Puercan).
Description and discussion.–A single Lm2 from UW locality V-91004 is identified as Mesodma hensleighi on the basis of its small size. UW 26075 is smaller than m2s characteristic of Mesodma formosa.
Lillegraven (1969) stated that teeth of Mesodma hensleighi are nearly identical in morphology to those of M. formosa, and are distinguished from the latter by their minuscule size. UW 26075 has a cusp formula of 3:2, and is morphologically similar to, though smaller than, m2s from the Ferris Formation here referred to Mesodma sp. indet.
Length and width measurements of UW 26075 are, respectively, 1.078 and 0.885.
Mesodma sp. cf. M. garfieldensis Archibald, 1982
Mesodma garfieldensis Archibald, 1982, p. 46.
Holotype and paratype of Mesodma garfieldensis.–UCMP 116622, right dentary with p3–4, and UCMP 116621, right dentary with p3–4.
Type locality.–UCMP locality V-74111, lowermost Tullock Formation, northeastern Montana (early Puercan).
Referred specimen.–UW 26546, Lp4 from locality V-91014; and UW 27448, Rp4 from UW locality V-91031.
Locality.–UW localities V-91014 and V-91031, upper Ferris Formation, western Hanna Basin, Wyoming (early and middle Puercan, respectively).
Description and discussion.–UW 26546, a Lp4 recovered from an anthill, is most similar in profile and morphology to the paratype of Mesodma garfieldensis from the lower Tullock Formation in northeastern Montana. How-ever, UW 26546 is roughly half a millimeter longer than the largest p4s of M. garfieldensis reported by Archibald (1982). UW specimens 26546 and 27448 are larger than p4s of any previously documented species of Mesodma. The proceeding description of UW 26546 also applies to UW 27448, morphologically very similar to UW 26546. Differences between the two are noted.
The profile of UW 26546, a relatively high, nearly symmetrical arc, is virtually identical to that of the paratype of Mesodma garfieldensis (see Archibald, 1982). Based upon figures of the holotype of M. ambigua provided by Jepsen (1940), its profile is much lower, and the slope of the posterior half of p4 is gentler, than that of UW 26546. Also, the anterior margin of the p4 on the holotype of M. ambigua appears squared off, while the anterior margin of UW 26546 is steep and slightly convex, not square. UW 26546 has 14 serrations, as in one p4 of M. garfieldensis noted by Archibald (1982) and one of the p4s of M. ambigua reported by Jepsen (1940). The most posterior two serrations of UW 26546 are cusplike. The fifth serration is broken off at its apex; either the fifth or the sixth serration is the highest. Although some serrations are broken off, UW 27448 probably had 13.
UW 26546 possesses eleven ridges both on its labial and lingual sides. The first serration has one weak labial ridge and a short, weak, lingual ridge. On UW 27448, 12 labial and 11 lingual ridges are discernible; a weak, labial ridge extends from the first serration. On both specimens, the labial and lingual ridges extending from the second serration merge anteriorly with their counterparts of the third serration.
Although most of the posterolabial shelf is not preserved on UW 26546, the small fragment that remains possesses a tiny cuspule. The worn posterolabial shelf of UW 27448 lacks cuspules. A cuspidate posterolabial shelf is characteristic of Mesodma ambigua (see Jepsen, 1940).
In his diagnosis of Mesodma garfieldensis, Archibald (1982) noted that its p4 has the greatest height to length ratio of any species of Mesodma, except possibly Mesodma senecta. The height to length ratio of UW 26546 is 0.47, and fits well within the range of specimens of M. garfieldensis reported from northeastern Montana.
Measurements of UW 26546 are: length, 5.140; width, 1.893; and height, 2.411.
Mesodma sp. indet.
Referred specimens.–UW 26008, Rm1; UW 26010, Lm1; UW 26013, Rm1; UW 26014, Lm1; UW 26015, Rm1; UW 26017, Rm1; UW 26023, Lm1; UW 26026, Rm1; UW 26027, Lm1; UW 26117, Lm1; UW 26114, Lm2; UW 26105, Lm2; UW 26074, Lm2; UW 26090, worn Rm2; UW 26067, Rm2; UW 26072, incomplete Rm2; UW 26068, Lm2; UW 26071, broken Lm2; UW 26073, incomplete Lm2; UW 26076, Rm2; UW 26065, worn Rm2; UW 26066, Lm2; UW 26064, Lm2; UW 26130, Lm2; UW 26543, Lm2; UW 26544, worn Lm2; UW 26555, Lm2; UW 26107, incomplete RM2; UW 26054–26058, all RM2s; UW 26059, LM2; UW 26060–26062, all RM2s; UW 26063, worn and damaged LM2; and UW 26069 and 26070, LM2s.
Locality.–UW locality V-91004, upper Ferris Formation, western Hanna Basin, Wyoming (early Puercan).
Description and discussion.–Several mls from UW locality V-91004 are morphologically similar to mls of Mesodma sp. described by Clemens (1964) and those of Mesodma garfieldensis described by Archibald (1982). According to Clemens' (1964) revised diagnosis of the genus, the cusp formula for ml is 5–7: 4–6. All but one (UW 26014, 6:3) have cusp formulae that fall within this range. The modal formula is 6:4. Only UW 26023 has a cusp formula of 7:5. Sizes of the UW mls fall within ranges of three described species of Mesodma (M. ambigua, M. thompsoni, and M. garfieldensis). All fall within the size range of mls reported by Clemens (1964) for Mesodma sp. As in mls of M. garfieldensis described by Archibald (1982), the anterior margins vary from blunt and squared off to rounded and tapered. Perhaps significantly, the single ml with the 7:5 cusp formula also is the most tapered of the mls from UW locality V-91004.
For the most part, ml-2s of species of Mesodma are morphologically undiagnostic, and size has been used to determine specific identity. This procedure fails when teeth fall within size ranges characteristic of two or more species, as is the case here.
Perhaps some specimens from the Ferris Formation could be identified as Mesodma ambigua Jepsen (1940). Relative proportions of ml of M. ambigua, based upon Jepsen's (1940) measurements of the holotype, differ from those of other large species of Mesodma. Although the length of ml of M. ambigua falls within ranges characteristic of M. garfieldensis and M. thompsoni, its width is roughly half its length; the tooth is proportionately wider than any mls reported from those species. Based upon measurements of length and width given by Archibald (1982, tables 5 and 6 and appendix 3), the mean width of mls both in M. garfieldensis and M. thompsoni is less than, or equal to, 0.44 of the mean length. With exception of UW 26023 (the ml with a cusp formula of 7:5), all mls from the Ferris Formation have widths that are equal to, or greater than, 0.46 of their lengths (Table 4). It is therefore probable that the larger mls could be identified as M. ambigua. UW 26023 perhaps should be identified as M. garfieldensis, as its dimensions fall well within ranges given for that species by Archibald (1982).
We identify 16 m2s from UW locality V-91004 as Mesodma sp. indet. As for mls, the m2s fall within size ranges characteristic of more than one species of Mesodma. Ten m2s fall within the size range of m2s of Mesodma garfieldensis reported from north-eastern Montana (see Archibald, 1982) and Mesodma formosa from the Scollard Formation (see Lillegraven, 1969). Size ranges of ml, m2, and M2 reported for M. garfieldensis overlap those of M. formosa (see Archibald, 1982).
Five m2s from the Ferris Formation are larger than those of Mesodma garfieldensis and M. formosa, and on that basis could be identified either as M. thompsoni or M. ambigua.
With exception of UW 26543, which has a cusp formula of 4:2, all m2s recovered from the Ferris Formation to date have a formula of 3:2. Measurements of m2s assigned to Mesodma sp. indet. are provided in Table 5.
We identify 13 M2s as Mesodma sp. indet. Four of the teeth fall within size ranges characteristic either of M. formosa or M. garfieldensis. One M2 fits the normal size range of M. thompsoni, and seven M2s fall within size ranges characteristic of M. formosa, M. garfieldensis, and M. thompsoni. One specimen (UW 26107) is incomplete and unmeasurable.
With exception of UW 26070, which has a cusp formula of 0:2:3, all of the M2s have a cusp formula of 1:2–3:3. Measurements of M2s identified as Mesodma sp. indet. are provided in Table 5.
Referred specimen.–UW 26012, Lp4.
Locality.–UW locality V-91004, upper Ferris Formation, western Hanna Basin, Wyoming (early Puercan).
Description and discussion.–UW 26012, a Lp4 (Fig. 8A–B), falls within the size range of Mesodma ambigua reported from Mantua Lentil, and is larger than ranges typical of M. thompsoni and M. garfieldensis. Unlike that of any described species of Mesodma, the anterior margin of UW 26012 as seen in side view is not rounded above the anterobasal concavity. Rather, it is relatively straight, reminiscent of the Aquilan Cimolodon electus and C. similis (see R. C. Fox, 1971). The angle between the anterior margin of the tooth and its baseline is acute. The lateral profile of UW 26012 is a relatively high, asymmetrical arc. The greatest height occurs at serration three, with serration four only slightly lower. Although some serrations are broken, it appears that UW 26012 originally had 12 or 13. UW 26012 bears nine labial and 10 lingual ridges. The anterobasal concavity is relatively deep, with a rounded dorsal margin. A vertical groove on the anterior root accommodates p3.
As is characteristic in p4s of species of Mesodma (see Jepsen, 1940), two ridges extend from the first serration on UW 26012. One ridge descends forward along the crest of the tooth and bifurcates. The more posterior bifurcate is short and weak, and veers labially to extinction. The other bifurcate, somewhat stronger, veers slightly lingually, but continues along a relatively straight path toward the anterior margin of the tooth. The second ridge is short and weak. It extends anterolingually from serration one and ends abruptly above the lingual ridge that extends from the second serration.
Linear, vertical grooves separate the most posterior three serrations on UW 26012. Below and slightly posterior to these grooves is a relatively weak, acuspate posterolabial shelf. An interradicular crest connects the two roots.
We tentatively identify UW 26012 as ? Mesodma sp. The number and pattern of ridges extending anteriorly from the first serration of this p4 is characteristic of Mesodma. However, the anterior margin of the tooth is most similar to Aquilan species of Cimolodon (see R. C. Fox, 1971). The lateral profile, on the other hand, is similar both to Mesodma and Ectypodus, with the front of the tooth higher than the rear. In species of Ectypodus, the fourth serration typically is the highest (Sloan, 1981). Although the third serration is highest on UW 26012, the fourth is only slightly shorter. The relative height of the first serration (Hl/L in Table 6) is approximately one-third the length of the tooth; this value falls within the range of Neoplagiaulax and Ectypodus, and is greater than that characteristic of Mesodma. According to Clemens' (1964) revised diagnosis of Mesodma, the Hl/L ratio typically is less than one-third the length of p4. The number of serrations falls within ranges characteristic of Mesodma, Ectypodus, Neoplagiaulax, and Cimolodon.
Because of the asymmetrical lateral profile of UW 26012, it probably should not be identified as Neoplagiaulax or Cimolodon. Based upon Jepsen's (1940) key for identifying American ptilodontid multituberculates, UW 26012 would be identified as Mesodma. Nevertheless, based upon Clemens' (1964) and Sloan's (1981) descriptions and revised diagnoses, UW 26012 is most similar to Ectypodus.
In light of the above discrepancies, one might argue that UW 26012 could be identified either as Mesodma or Ectypodus. With one possible exception (see Johnston and Fox, 1984), however, Ectypodus has been reported only from post-Puercan strata. In text below, we report occurrence of two varieties of Ectypodus from middle and late Puercan strata. A close phylogenetic relationship between Mesodma and Ectypodus was suggested by Sloan (1981), a concept with which we agree. On the basis of parsimony alone, we tentatively identify UW 26012 as ? Mesodma sp.; the specimen falls within size ranges and normal temporal extent of Mesodma. Regardless of its identification, morphology and stratigraphic occurrence of UW 26012 are consistent with an ancestor-descendant relationship between Mesodma and Ectypodus.
Specimen UW 26564 (RM2) from dinosaur-bearing strata of the Ferris Formation (at locality V-92010) perhaps is identifiable as Mesodma sp.
Ectypodus Matthew and Granger, 1921 Ectypodus sp. A
Referred specimens.–UW specimens 26211 and 26212, both Lp4s.
Locality.–UW locality V-92031, upper Ferris Formation, western Hanna Basin, Wyoming (middle Puercan).
Description and discussion.–Based upon Sloan's (1981) revised diagnosis of Ectypodus, two p4s found by dry-screening at UW locality V-92031 represent a large species of the genus. These specimens represent the first of two morphologically distinguishable varieties of Ectypodus, here designated tentatively as Ectypodus sp. A and Ectypodus sp. B.
UW specimens 26211 and 26212 are larger than any described p4s of species of Ectypodus, and fall within the size ranges characteristic of Neoplagiaulax hunteri (see Simpson, 1936) and N. Kremnus (see Johnston and Fox, 1984). However, the UW specimens have a different lateral profile and fewer serrations than p4s identified as Neoplagiaulax. As discussed by Krause (1977), isolated p4s of Neoplagiaulax and Ectypodus are difficult to differentiate. On p4s of Ectypodus, the fourth serration, rather than the fifth, sixth, or seventh, is the highest serration, and the anterior margin is generally more convex than in p4s of Neoplagiaulax (see Sloan, 1981). On UW 26212, the better preserved of the two UW specimens, the greatest height of the tooth is reached at serration 4, with serration 3 almost as high. Although difficult to discern due to breakage and wear, the highest serration on UW 26211 also appears to be the fourth. On both UW p4s, the greatest height occurs anterior to the midline of the tooth (Table 7). UW specimens 26211 and 26212 possess 11 serrations, a count that is less than on p4s of N. hunteri or N. kremnus. The anterobasal concavity on both UW specimens is shallow and rounded at its dorsal margin, unlike that of Neoplagiaulax hunteri.
UW 26212 possesses ten labial and lingual ridges. UW 26212 has diagonal grooves on its labial side, anterior to the posterolabial shelf and posterior to the last labial ridge. Three ridges extend anteriorly from the first serration. On the lingual side of UW 26212, the ridges descending anterolingually from serrations 1 and 2 merge into a single ridge, which continues ventrally for a short distance and fades out posterior to the ridge that descends from the third serration. This situation also occurs in Ectypodus cf. E. powelli, as discussed by Krause (1977). The ridges descending anterolingually from serrations 3 and 4 join at their most anterior points, posterior to the anterior margin of the tooth.
On the labial side of UW 26212, a second ridge descends ventrally from serration 1 for a short distance, and then bends anteroventrally, continuing for some distance toward the anterior margin of the tooth. The labial ridge descending from serration 2 is short, and fades out just anterior to the bend in the ridge extending from serration 1. The labial ridges extending anteriorly from serrations 1 and 2 do not merge. On UW 26212, a third ridge extends anteriorly from the first serration along the crest of the blade for a short distance, but then veers off to the labial side of the blade and fades out. Although largely obscured by glue, both specimens can be seen to have strong interradicular crests.
Ectypodus sp. B
Referred specimens.–UW specimens 26135, 26136, and 26137, two Lp4s and a heavily worn Rp4, respectively.
Locality.–UW locality V-92025, upper Ferris Formation, western Hanna Basin, Wyoming (late Puercan).
Description and discussion.–We identify three p4s from upper parts of the Ferris Formation as Ectypodus sp. B. These specimens represent the second of two morphologies from our samples that are identifiable as a species of Ectypodus. With exception of their larger size, the UW specimens fit Sloan's (1981) revised diagnosis of Ectypodus, and are morphologically similar to the species of Ectypodus with which they were compared (E. musculus, E. aphronorus, E. szalayi, and E. tarovs). The UW specimens are similar in size to Parectypodus armstrongi and Neoplagiaulax kremnus, but differ in their profile and detailed morphology.
Morphologically, species of Ectypodus and Neoplagiaulax are very similar. Although the size ranges of these two genera overlap slightly, species of Neoplagiaulax generally are larger than those of Ectypodus. On p4s of Ectypodus, the fourth serration is highest, while on p4s of Neoplagiaulax, the highest point occurs between serrations 5 and 7 (Sloan, 1981). The profile of p4 of Ectypodus differs from that of Neoplagiaulax in being asymmetric.
As is characteristic of Ectypodus (see Sloan, 1981), the profile of the UW specimens is an asymmetric arc, and the highest point of the tooth occurs at serration 4. The vertical groove on the anterior margin of the forward root is relatively shallow on all of the UW specimens, but nevertheless suggests presence of a p3. On all relevant UW specimens, the height of the first serration (H1 in Table 8) is greater than one-third, but less than 0.45, of the length of the tooth, as in Ectypodus and unlike Parectypodus (see Sloan, 1981).
UW 26135 (Fig. 8E) has 13 serrations, and UW 26136 has at least 11 serrations. The crown of UW 26137 is heavily and anomalously worn, so the number of serrations is uncertain. However, based upon presence of 12 lingual ridges, UW 26137 probably possessed at least 12 serrations. The number of serrations possessed by the UW specimens falls within ranges characteristic either of Neoplagiaulax or Ectypodus.
As is typical of species of Ectypodus, the anterior margin on UW specimens is subvertical and weakly convex. All three UW specimens have 12 lingual ridges. Both UW 26135 and 26136 have 11 labial ridges; only 10 can be seen on UW 26137, but its posterolabial surface is heavily worn. Two of the UW specimens have three ridges emanating from the first serration. These include a short, weak, labial ridge, a medial ridge that bifurcates into labi-ally- and lingually-directed branches, and a strong lingual ridge that is steeper than the ridges posterior to it. Due to wear, a labial ridge on UW 26137 is not visible.
All UW specimens here identified as Ectypodus sp. B have a strong interradicular crest. UW 26135 and 26136 possess a weak, partially worn, acuspate posterolabial shelf, while that area on UW 26137 is obscured by heavier wear.
Lower dentitions of Parectypodus are extremely similar to those of Ectypodus. Parectypodus differs from Ectypodus in lacking p3, however, and in its greater labial height of enamel at the exodaenodont lobe of p4 (Simpson, 1937; Sloan, 1981).
On the UW specimens, height of the first serration is over one-third, but less than 0.45, of the length of the tooth; in this respect it is most like Ectypodus. However, the UW specimens are larger than all documented species of Ectypodus, and are similar in size to some species of Parectypodus and Neoplagiaulax.
Measurements of the UW specimens, here identified as Ectypodus sp. B, are provided in Table 8.
The principal distinguishing features between specimens here identified as Ectypodus sp. A and E. sp. B are the tendencies toward: (1) larger size in E. sp. B (Tables 7–8); (2) increased serrations (11–13) in E. sp. B (compared to 11 in E. sp. A); and (3) greater number of labial (11) and lingual (12) ridges in E. sp. B (compared to 10 each in E. sp. A). Clearly, however, E. sp. A and B are very closely related forms, and increased collections may well show an uninterrupted continuum between the two morphologies.
Neoplagiaulacidae gen. and sp. indet.
Comments.–We identify 40 isolated, anterior upper premolars and one fragment of a Lp4 recovered from UW locality V-91004 (early Puercan), as well as a worn RM2 (UW 26564; perhaps identifiable as Mesodma sp.) from UW locality V-92010 (late Lancian), as Neoplagiaulacidae gen. and sp. indet. Unless found in association with other teeth, anterior upper premolars appear to be taxonomically undiagnostic for neoplagiaulacids (Clemens, 1964). With exception of UW 26031, all anterior upper premolars from UW locality V-91004 bear three or four cusps; UW 26031 has five. Based upon known presence of at least two species of Mesodma at UW locality V-91004, and morphologic similarity to upper anterior premolars of that genus described by Clemens (1964), most of these anterior upper premolars probably derived from Mesodma.
PTILODONTIDAE Gregory and Simpson, 1926
Ptilodus Cope, 1881
Ptilodus sp. cf. P. tsosiensis Sloan, 1981
Ptilodus tsosiensis Sloan, 1981, p. 150.
Holotype and paratypes of Ptilodus tsosiensis.–Holotype, AMNH 59821, LP4; paratypes, 58464, 58658, 59874, Lp4s, and 59800, Rp4.
Type locality.–“… Hemithlaeus zone of the lower part of the Nacimiento Formation in Betonnie Tsosie Wash, San Juan Basin, New Mexico” (Sloan, 1981, p. 150).
Referred specimens.–UW specimens 26131, Rp4; 26132, Lp4; 26286, Lp4; and 26140, worn Rp4, all from UW locality V-92025; UW 26242, Lp4 from UW locality V-92014; and UW 26301, incomplete Rp4 from UW locality V-92034.
Localities.–UW localities V-92014, V-92025, and V-92034.
Known distribution.–Upper Ferris Formation, western Hanna Basin, Wyoming (middle and late Puercan).
Description and discussion.–Ptilodus tsosiensis is the smallest and geologically oldest documented species of Ptilodus, and represents a plausible ancestor to later species of Ptilodus (see Sloan, 1981). Ptilodus tsosiensis is restricted to the “Hemithlaeus zone” (sensu Sloan, 1981), correlative with Intervalzone Pu2 (of Archibald et al., 1987) in its type area, the Nacimiento Formation of the San Juan Basin. In the Ferris Formation, P. sp. cf. P. tsosiensis is documented from both middle and late Puercan strata.
We identify six p4s from the upper Ferris Formation as Ptilodus sp. cf. P. tsosiensis. No original specimens or casts of P. tsosiensis were available for comparison. Identification of them as P. sp. cf. P. tsosiensis, therefore, was based upon size, similarity to other species of Ptilodus contained within the UW collections, and similarity to Sloan's (1981) and Krause's (1982) diagnoses and figures of P. tsosiensis. The UW specimens are briefly described below; measurements and related data are contained within Tables 9 and 10.
As in p4s of Ptilodus tsosiensis from the San Juan Basin (Krause, 1982), the UW specimens are relatively high and arched, and have slightly asymmetrical outlines. The highest part of the blade occurs ahead of the anteroposterior midpoint, at the fourth serration. The anterior margin of all UW specimens is weakly convex in lateral view, and dips forward at a steep angle. A shallow antcrobasal concavity is present on all UW specimens; the top of this concavity is rounded. The UW specimens have lateral profiles similar to the specimen of P. tsosiensis figured by Krause (1982, p. 83, fig. 14).
The serration count observed among UW specimens ranges from 10 to 13 (mode = 12). The range in number of serrations among specimens of Ptilodus tsosiensis reported by Sloan (1981) is 10 to 12. The range of labial ridges among specimens from the Ferris Formation is 9 to 12 (mode = 11 or 12); the range of lingual ridges is 7 to 11 (mode = 10 or 11). On all pertinent UW specimens, the fifth or sixth ridge is longest, both on labial and lingual sides.
As in p4s of Ptilodus tsosiensis from the San Juan Basin, UW 26131 (Fig. 8F) and 26132 have a median and a lingual ridge extending anteriorly from the first serration. On UW 26131, the median ridge (on the crest of the blade) appears to bifurcate into a labial and a weak medial branch. On UW 26232, the median ridge veers labially on the anterior margin of the tooth, and does not bifurcate. The lingual ridge of the first serration merges with the lingual ridge that extends anteroventrally from the second serration. On UW 26286, a single median ridge extends anteriorly from the first serration. Due to wear, the number of ridges descending from the first serration on UW 26140 and 26242 cannot be determined.
All of the UW specimens here identified as Ptilodus sp. cf. P. tsosiensis have a weak posterolabial ledge that bears no cusps. An interradicular crest is present.
UW specimens 26132 and 26286 are longer than p4s of Ptilodus tsosiensis from the San Juan Basin (measurements presented by Krause, 1982; n = 5). The remaining UW specimens are slightly smaller than p4s of P. tsosiensis.
Although Ptilodus tsosiensis is documented from middle Puercan strata of the Nacimiento Formation, occurrence of P. sp. cf. P. tsosiensis in the Ferris Formation is the first record of Ptilodus in strata of late Puercan age. Specimens from the Ferris Formation, therefore, help fill a previously existing (albeit minor) stratigraphic gap between middle Puercan and Torrejonian records of Ptilodus.
Referred specimen.–UW 26233, Lp4 blade, from UW locality V-92026.
Known distribution.–Upper Ferris Formation, western Hanna Basin, Wyoming (late Puercan).
Description and discussion.–UW 26233 (Fig. 8G–H), a large p4 from the Ferris Formation, is morphologically most similar to species of Ptilodus. UW 26233 possesses characters of both P. tsosiensis and later species of Ptilodus, including P. mediaevus and P. trovessartianus. However, UW 26233 is morphologically distinct from documented species of Ptilodus, and therefore we identify the specimen as Ptilodus sp.
The lateral profile of UW 26233 is a high, asymmetrical arch. However, UW 26233 is neither as high, nor as symmetrical, as p4s of the larger species of Ptilodus (P. mediaevus and P. montanus). The highest part of the tooth is ahead of the anteroposterior midpoint, at the fourth serration. In this feature, UW 26233 is most similar to p4s of P. sp. cf. P. tsosiensis from the Ferris Formation and Ectypodus (see Sloan, 1981). UW 26233 is larger than p4s of P. tsosiensis, and falls within size ranges of p4s characteristic of both P. mediaevus and P. montanus.
Heretofore, p4s of Ptilodus tsosiensis and P. mediaevus were reported to have no more than 12 serrations. UW 26233, however, possesses 15 or 16 serrations. In this feature, UW 26233 is most similar to p4s of P. montanus, which have 13 to 15 serrations. On both its labial and lingual sides, UW 26233 bears 14 ridges. As in p4s of P. tsosiensis, median and lingual ridges extend from the first serration.
As seen in lateral view, the anterior margin of UW 26233 is slightly convex, dips steeply forward, and resembles the anterior margin on p4s of Ptilodus tsosiensis, P. mediaevus, and P. trovessartianus. The anterobasal concavity is well-developed, suggesting presence of a p3. Also present is a vertical groove in the anterior margin of the forward root.
UW 26233 possesses a weak, acuspate posterolabial shelf and a well-developed interradicular crest.
As its morphology and large size clearly foreshadows Torrejonian species of Ptilodus, the presence of UW 26233 in late Puercan strata is not unexpected.
Measurements of UW 26233 are: length, 8.020; width, 2.506; height, 3.075; and L1, 1.220.
Comments.–Catopsalis is a polyphyletic genus (Simmons and Miao, 1986), documented from the Late Cretaceous of east Asia (Kielan-Jaworowska and Sloan, 1979) and Lancian through Tiffanian time in North America (Archibald et al., 1987; Fox, 1989; 1990; Lucas et al., 1997). Nine species of Catopsalis have been documented. Two species derive from Asia, and seven are North American (Simmons and Miao, 1986; Fox, 1989).
We report Catopsalis joyneri from middle Puercan strata of the Ferris Formation. Catopsalis joyneri is the oldest and most primitive known taeniolabidid from North America. Probably it descended from the Asiatic Late Cretaceous species C. catopsaloides (see Kielan-Jaworowska, 1974a, b; Kielan-Jaworowska and Sloan, 1979; Simmons and Miao, 1986).
Catopsalis joyneri Sloan and Van Valen, 1965
Catopsalis joyneri Sloan and Van Valen, 1965, p. 225.
Holotype.–UMVP 1494, right maxillary with palate, M1, roots of P4, and alveoli of P3 and M2.
Type locality.–Bug Creek Anthills, Hell Creek Formation, Montana (early Puercan).
Referred specimen and locality.–UW 26186, RI2 from UW locality V-91031.
Known distribution.–Possibly upper Frenchman and Ravenscrag Formations, Saskatchewan (late Lancian); upper Hell Creek Formation, Montana (early Puercan); upper Ferris Formation, western Hanna Basin, Wyoming (middle Puercan).
Description and discussion.–A single right upper incisor from the Ferris Formation is identical to upper incisors identified as Catopsalis joyneri from the Bug Creek Anthills (BCA) locality in the Hell Creek Formation. UW 26186 was compared with specimens of C. joyneri and the eucosmodontid multituberculate, Stygimys kuszmauli from the BCA locality, contained within research collections of the universities of Wyoming and Alberta.
Both Stygimys kuszmauli and Catopsalis joyneri have mitt-shaped upper incisors. However, upper incisors of C. joyneri are much larger than those of S. kuszmauli, and have a different morphology. On upper incisors of S. kuszmauli, the distance and angle between the “thumb” (i. e., the smaller and more posterior of the two projections) and the rest of the mitt is greater than that of C. joyneri, and the posterior margin of the thumb is not concave as in upper incisors of C. joyneri. Unlike those of C. joyneri, some upper incisors of S. kuszmauli from the BCA locality possess a small cusp posterior to, and at the base of, the thumb of the mitt. Upper incisors of C. joyneri have a crease that separates the thumb from the larger part of the mitt. Upper incisors of S. kuszmauli lack the crease between the thumb and the rest of the mitt; the thumb is not as discrete as that of C. joyneri, and looks as though it was budded off the larger part of the tooth.
Catopsalis joyneri originally was believed to be restricted to the “Bugcreekian age” (sensu Sloan, 1987), correlative with Interval-zone Pu0 of Archibald and Lofgren (1990). Subsequently, C. joyneri was documented from a Pu1 fauna, the Brown-Grey local fauna in the Hell Creek Formation (Lofgren, 1995). Archibald and Lofgren (1990) believed that Interval-zone Pu1 temporally succeeded Pu0. Recovery of UW 26186 from the Ferris Formation extends the range of C. joyneri into middle Puercan time (Pu2), and thus may preclude use of this species as an index for the early Puercan (Pu0 and Pu1).
Taeniolabis Cope, 1882b
Comments.–In terms of body size, Taeniolabis is the largest documented member of the order Multituberculata (see Simmons, 1987). Taeniolabis has a wide latitudinal distribution, known in Puercan strata from Saskatchewan (Johnston and Fox, 1984) to New Mexico (Williamson, 1996).
Cope (1882b) named Taeniolabis sulcatus on the basis of a broken upper incisor. Cope originally believed that the tooth derived from a member of the taeniodonts, an order of extinct placental mammal that appeared in the Puercan. When similar incisors were found in association with molar teeth of Polymastodon taoensis, Cope suggested that T. sulcatus and P. taoensis probably were congeneric (Simmons, 1987; Cope, 1885). Polymastodon therefore has been considered a junior synonym of Taeniolabis (see Cope, 1885; Simpson, 1929).
Taeniolabis taoensis (Cope, 1882c)
Polymastodon taoensis Cope, 1882c, p. 684.
Holotype.–AMNH 3036, right maxillary fragment with M1–2, and skull fragments.
Type locality.–Upper “Taeniolabis” zone of Nacimiento Formation, San Juan Basin, New Mexico.
Referred specimen.–UW 26157, nearly complete RM2.
Locality.–UW locality V-91022, upper Ferris Formation, western Hanna Basin, Wyoming (late Puercan).
Known distribution.–Nacimiento Formation, New Mexico; upper Ferris Formation, Wyoming (late Puercan); possibly North Horn Formation, Utah (middle or late Puercan).
Revised diagnosis.–Simmons, 1987, p. 799.
Description and discussion.–UW 26157 (Fig. 8I) is the only specimen of Taeniolabis taoensis recovered from the Ferris Formation. A small central piece of the tooth is missing, leaving a hole, and part of the posterolabial margin is not preserved. UW 26157 falls within the range of lengths of M2s of Taeniolabis taoensis given by Simmons (1987), but is over a millimeter wider than the eleven M2s documented by her.
The outline of M2s of Taeniolabis taoensis is that of a triangle with rounded corners. The cusp formula of M2s of T. taoensis is 1:4–5:4–6 (Simmons, 1987). In the lingual row of UW 26157, there are five cusps, with the most anterior cusp being the smallest; four cusps occur in the middle row. The most posterior cusp in the middle row is the largest on the tooth. One cusp is present in the labial row. All cusps on UW 26157 are worn down completely, leaving a mazelike enamel-dentine pattern on the crown.
By convention, appearance of Taeniolabis taoensis marks the beginning of late Puercan time (Interval-zone Pu3; Archibald et al., 1987). Simmons (1987) contested the biostratigraphic utility of Taeniolabis as an index fossil for the late Puercan, but maintained its use as an indicator for the Puercan. Taeniolabis taoensis appears to be restricted to late Puercan strata, and coincides with other mammalian faunal changes used to signify late Puercan time in both the Nacimiento (Williamson, 1996) and Ferris Formations. Following Archibald et al. (1987), we continue to think of T. taoensis as a marker for advent of Interval-zone Pu3.
Length and width measurements for UW 26157 are 15.462 and 14.187, respectively.
Suborder incertae sedis
Meniscoessus Cope, 1882d
Comments.–The Late Cretaceous cimolomyid multituberculate Meniscoessus is documented from Aquilan through Lancian strata in the North American Western Interior (Lillegraven, 1987). Geographically, it is known to have extended at least from southern Alberta and Saskatchewan to New Mexico. A revised diagnosis of the genus was given by Clemens (1964).
We report a new species of Meniscoessus from the Ferris Formation. It is most similar to Meniscoessus robustus from the Lance and Hell Creek Formations. Meniscoessus robustus is the largest, youngest, and most derived known species of the genus (Flynn, 1986; Archibald, 1982), and is documented from “Edmontonian strata of the St. Mary River Formation in Alberta (Sloan and Russell, 1974), and from Lancian strata of the Lance and Hell Creek Formations in Wyoming and Montana, respectively (Archibald, 1982).
Meniscoessus greeni is documented only from its type locality, the Red Owl Quarry, in the Fairpoint Member of the Fox Hills Formation in west-central South Dakota (Wilson, 1987). With exception of smaller size of its molars, Meniscoessus greeni fits the morphology of Meniscoessus robustus (see Clemens, 1964). Comparison of specimens identified as M. greeni and M. robustus reveals no morphologic differences of taxonomic consequence. Measurements of P4 and p4 of M. greeni fall within size-ranges characteristic of M. robustus (see Clemens, 1964 and Archibald, 1982). M1s, M2s, and m2s of M. greeni are smaller than those of M. robustus, while larger m1s of M. greeni overlap its size range. Because minor differences in size represent the only known means of differentiating specimens of M. greeni from M. robustus, we believe that material referred to M. greeni simply represents small individuals of M. robustus. We treat M. greeni as a junior synonym of M. robustus.
Meniscoessus seminoensis new species
Holotype and only known specimen.–UW 26150, left dentary fragment with p3-m2.
Type locality.–UW locality V-93006, lower Ferris Formation, western Hanna Basin, Wyoming (late Lancian).
Etymology.–Named after the Seminoe Mountains, which lie along the northern margin of the Hanna Basin; its peaks can be seen from the field area.
Known distribution.–Type locality.
Diagnosis.–Most similar in size and morphology to Meniscoessus robustus; p4 relatively longer than that of M. robustus, with lower profile; p4 with nine serrations; first serration on p4 lower and more exaggerated than that of M. robustus, and forms the apex of an anterior lobe; smooth, elongate, shallow groove on both labial and lingual sides of p4 separates anterior lobe from remainder of tooth; p4 over half a millimeter longer than m2; m1 tapers anteriorly in occlusal view; cusp formulae of m1 and m2 5:4 and 4:2, respectively; m2 over a millimeter shorter than m1; and ratio of lengths of m1 to m2 > 1.
Description and discussion.–Meniscoessus seminoensis is most similar in size and morphology (Fig. 9) to M. robustus from the Lance, Hell Creek, and Fox Hills Formations of the Powder River Basin and High Plains. The principal basis for recognition of the new species is morphology of p4. Nevertheless, relative dimensions of the dentition also serve to distinguish M. seminoensis from other species.
The p4 of Meniscoessus seminoensis falls within the upper ranges of lengths and widths of M. robustus from the type Lance Formation (see Clemens, 1964). However, p4 of M. seminoensis is relatively longer than that of M. robustus. On UW 26150, the ratio of lengths of p4 to m1 equals 0.92. That ratio in M. robustus ranges from 0.68 to 0.86 (Clemens, 1964; Archibald, 1982). Contrasting M. robustus, on UW 26150, p4 is over half a millimeter longer than m2.
Measurements of m1 of UW 26150 fall within the lower size ranges characteristic of Meniscoessus robustus. The same is true for the width of m2 of UW 26150. However, the length of m2 in UW 26150 is considerably less than that characteristic of m2s of M. robustus. The ratio of length of m1 to m2 of UW 26150 is 1.18, and is intermediate between those of earlier species of Meniscoessus, including M. collomensis, M. major, and M. intermedius, which range from 1.3 to 1.4 (Lillegraven, 1987), and some specimens of M. robustus which have an m1 to m2 length ratio of less than one (Clemens, 1964; Archibald, 1982). On UW 26150, m2 is roughly a millimeter wider than m1 (see anterior width in Table 11), and over half a millimeter shorter than m1.
The profile of p4 of Meniscoessus seminoensis is that of a long, low arc, much lower than in p4s characteristic of Meniscoessus robustus (see Fig. 10). The height of p4 on UW 26150 is nearly a millimeter less than that of p4s of M. robustus from the Hell Creek Formation (see Archibald, 1982). The highest point on the p4 of M. seminoensis is only slightly higher than the cusps on its m1. The p4 of M. seminoensis has 9 serrations, as in some specimens of M. robustus. The most posterior three are worn to a level slightly below the highest cusps on m1. As in p4s of M. robustus (see Clemens, 1964), the fourth serration is highest on p4 of M. seminoensis, although the fifth serration is nearly as high.
On p4 of Meniscoessus seminoensis, the first serration, lower than that on p4s of M. robustus, forms the apex of an anterior lobe that overhangs a small, peglike p3. The convex, anterior margin of this lobe is slightly posterior to the anterior margin of p3. The anterior lobe is set apart from the rest of the tooth by an elongate, ridgeless, weakly concave groove on both labial and lingual sides, delimited posteriorly by the anterior extremes of the ridges descending from serrations 2 through 5. As on p4 of M. seminoensis, the first serration on p4s of M. robustus is set apart from the following serration by a distinct notch (Clemens, 1964). Although some specimens of M. robustus have large first serrations, no known specimen shows an exaggerated development such that it defines the apex of a distinct anterior lobe.
The anterior two-thirds of p4 on UW 26150, including the exodaenodont lobe, is relatively wide and inflated as seen in anterior view. That also is the case in Meniscoessus robustus. Seven strong, lingual ridges descend anteroventrally from serrations 2 through 8. A short, weak ridge extends ventrally from the first serration. The most posterior lingual ridge is wavy in outline. A short, subvertical ridge occurs between ventral extremes of the two most posterior lingual ridges.
In labial view, the exodaenodont lobe on p4 of Meniscoessus seminoensis is large and inflated, with a straight anterior margin and a subvertical posterior margin. A single labial ridge extends anteroventrally from the first serration. A smooth, weakly concave groove separates all other labial ridges from the ridge that extends from the first serration. Six labial ridges are preserved posterior of the first serration; the first four are continuous with serrations. The posterior half of the labial side of the tooth appears to be worn into a large, weakly concave facet. No posterolabial shelf is present. The p4 of M. seminoensis has two roots.
As in Meniscoessus robustus (see Lillegraven, 1987), the m1 and m2 of M. seminoensis have cusp formulae of 5:4 and 4:2, respectively. The lingual rows of cusps on m1 and m2 are aligned with the crest of p4. Although more worn than any specimens with which they were compared, m1 and m2 of M. seminoensis have crescentic cusps whose apices are deflected posteriorly, as in M. robustus (see Clemens, 1964; Flynn, 1986). Unlike m1s of M. robustus, m1 on UW 26150 appears to have three well-developed roots, with the anterior root being larger than the subequal medial and posterior roots.
As is characteristic of lower jaws of multituberculates (Miao, 1988), the dentary of Meniscoessus seminoensis has a medially inflected angular process.
Meniscoessus seminoensis appears more closely related to M. robustus than to any other documented species. Meniscoessus robustus appears earlier in the fossil record than M. seminoensis, occurring first in “Edmontonian” strata (Archibald, 1982). Both species are documented from Lancian strata, and both are more derived than earlier, Aquilan through “Edmontonian” species of Meniscoessus. Based upon its unique characteristics, and its later appearance, M. seminoensis would have been an unlikely candidate for ancestry of M. robustus. The reverse also would be improbable, based upon the unique characters possessed by M. robustus. Meniscoessus robustus and M. seminoensis probably shared a close common ancestry.
Measurements of UW 26150 are provided in Table 11.
Cohort ALPHADELPHIA Marshall, Case, and Woodburne, 1990
Order PERADECTIA Marshall, Case, and Woodburne, 1990
Comment.–For peradectians, we follow the general taxonomic hierarchy used by Marshall et al. (1990).
ALPHADONTINAE Marshall, Case, and Woodburne, 1990
Alphadon Simpson, 1927
Alphadon lulli Clemens, 1966
Alphadon lulli Clemens, 1966, p. 8.
Holotype.–UCMP 47047, left maxillary fragment containing M1-M2, alveoli of M3, and parts of the alveoli of P3 and M4.
Type locality.–UCMP locality V-5620, type Lance Formation, Wyoming (Lancian).
Referred specimen and locality.–UW 26209, LM2 from UW locality V-92046, lower Ferris Formation, western Hanna Basin, Wyoming (late Lancian).
Known distribution.–Ferris and type Lance Formations, Wyo-ming; Hell Creek Formation, Montana (all Lancian).
Description and discussion.–UW 26209 is virtually identical in size and morphology to a cast of the M2 of the holotype of Alphadon lulli. UW 26209 bears a small stylar cusp A and a large stylar cusp B. Stylar cusps C and D are worn; C is obliterated. No stylar cusp E is present. As in A. lulli (see Clemens, 1966), the paracone is taller than the meta-cone. Both para- and metaconules are present. UW 26209 fits the diagnosis and description of molars of A. lulli given by Clemens (1966).
Length and width measurements of UW 26209, respectively, are 1.855 and 2.153. These measurements fall within size-ranges characteristic of M2s of Alphadon lulli (see Clemens, 1966).
Alphadon sp. cf. A. lulli Clemens, 1966
Referred specimen and locality.–UW 26566, a worn, incomplete RM2 or M3 from UW locality V-92027, lower Ferris Formation, western Hanna Basin, Wyoming (Lancian).
Description and discussion.–We identified UW 26566 principally on the basis of size. UW 26566 is smaller than upper molars characteristic of Alphadon marshi. Based upon its deep ectoflexus, UW 26566 probably is an M3. All cusps on UW 26566 are considerably worn, so our identification must be considered tentative. Stylar cusp B is much larger than stylar cusp C, located in the middle of the ectoflexus. Probably due to wear, there is no evidence of stylar cusps A or E.
Estimated length and width measurements of UW 26566, respectively, are 1.6 and 2.2.
Peradectes Matthew and Granger, 1921
Peradectes sp. cf. P. pusillus (Matthew and Granger, 1921)
Holotype of Peradectes pusillus.–AMNH 16414, dentary fragment with m1 talonid, complete m2–3, and part of alveolus of m4.
Type locality.–“… upper level of Puerco Formation, near Ojo Alamo, San Juan Basin, New Mexico …” (Matthew and Granger, 1921, p. 2).
Referred specimens.–UW 26078, Rm1; UW 26081, Lmx fragment; UW 26084, Rm1; UW 26080, worn LM1; UW 26553, RM1 or M2 labial fragment; UW 26554, LM3 labial fragment; all from UW locality V-91004 (early Puercan); and UW 26141, RM2 or M3, from UW locality V-92025 (late Puercan).
Known distribution.–Upper Hell Creek (Lancian) and lower Tullock (Puercan) Formations, Montana; upper Ferris Formation, Wyoming (Puercan).
Description and discussion.–We identify seven specimens from the Ferris Formation as Peradectes sp. cf. P. pusillus on the basis of similar size and morphology to specimens so identified from the Tullock Formation by Archibald (1982). No specimens of Peradectes pusillus were available to us for direct comparison.
UW 26080, a worn LM1, fits Archibald's (1982) description of molars of Peradectes sp. cf. P. pusillus from the Tullock Formation. Although heavily worn, the paraconule on UW 26080 is well-developed. The metaconule is worn away. As in Peradectes, the paracone is smaller than the metacone. The stylar cusps are worn nearly to the crown. However, based upon their outlines, stylar cusps B, C, and D were relatively large, with B the largest. A small, anteriorly projecting stylar cusp A is present. Due to wear, the relative heights of stylar cusps on UW 26080 cannot be determined.
UW 26141, a large, relatively unworn RM2 or M3, is longer and considerably wider than UW 26080. The difference in size between these two specimens is primarily due to the position each occupied in the tooth-row. M2 and M3 are more transverse than M1. An anterior piece of the stylar shelf, in the region of stylar cusp B, is damaged; with exception of its base, stylar cusp B is not preserved. However, well-developed stylar cusps C and D are preserved, with C slightly the larger. Based upon the outline of its base, stylar cusp B was larger than stylar cusps C and D. A small, spurlike stylar cusp A projects anteriorly. A cingulum extends lin-gually along the anterior margin of the tooth from stylar cusp A. A well-developed paracrista is preserved. On UW 26141, the meta-cone is slightly larger than the paracone, and the metaconule is larger than the paraconule.
Unlike molars of Alphadon, in which the ectoflexus is relatively deep and pronounced, molars of Peradectes ordinarily possess a shallow ectoflexus, or lack an ectoflexus. Both UW specimens 26141 and 26080 have a weak ectoflexus.
The shallow ectoflexus, shallower than that typical of M3s, and presence of both stylar cusps C and D, suggests that UW 26141 is an M2 (see Archibald, 1982). However, UW 26141 is more transverse than M2s of Peradectes sp. cf. P. pusillus from the Tullock Formation, and is most similar to M3s. Nevertheless, we suspect that UW 26141 is an unusually wide M2.
UW 26553, a labial fragment of RM1 or M2, is unusual in its possession of a tiny cuspule anterior to stylar cusp C, and posterior to the large stylar cusp B. UW 26554 is identified as a fragment of a LM3, due to its relatively strong ectoflexus, and fused, ridgelike stylar cusps C and D.
As is characteristic of upper molars of Peradectes pusillus, teeth from the Ferris Formation lack stylar cusp E.
PERADECTIDAE gen. and sp. indet.
Referred specimen and locality.–UW 26506, Lm1 trigonid from UW locality V-92010, lower Ferris Formation, western Hanna Basin, Wyoming (Lancian).
Description and discussion.–We identify UW 26506 as a fragment of m1 because the protoconid lines up with the gap between the para- and metaconids, and is anterior to the metaconid. A labial cingulid is preserved. UW 26506 is about the size of m1 trigonids characteristic of Alphadon lulli, and is slightly smaller than those of Peradectes pusillus. Lower molars of Peradectes normally are distinguished from those of Alphadon on the basis of orientation of the eristid obliqua. The cristid obliqua is not preserved on UW 26506, however, and thus its generic identification remains impossible.
Our study focuses upon the thick, comparatively undeformed, and relatively consistently fossil-iferous Ferris Formation of south-central Wyoming. The rock unit, in close proximity to its intended type section, provides prime opportunities for diverse paleonto-logical and geological studies. It provides the ability to evaluate changing diversity of North American land vertebrates from latest Mesozoic into earliest Ceno-zoic time. Specifically, the intervals include the late Lancian and all of the Puercan ages. We developed a zonation of local strat through the study of fossil mammals collected from unequivocally superposed localities. Our zonation, in turn, allows evaluation of correlation of local geological/biological happenings with diverse events that may be observed even in geographically distant areas.
Upper parts of the Ferris Formation are unique in being nearly an order of magnitude thicker than any other known unit of superposed strata of Puercan age. Clearly, the Hanna Basin was subsiding extraordinarily rapidly during earliest Cenozoic time. Quantification of local rates in the Hanna Basin is becoming a practical possibility.
Faunal Composition, Including Range-extensions
To the right is a skeletonized taxonomic listing of the multituberculate and peradectian mammals discussed in this paper. The order of presentation follows the sequence of appearances of taxa in the section entitled “Systematic Paleontology.” Abbreviations are: L = Lancian; Pu1–3 = successively younger interval-zones of the Puercan, representing early through late Puercan time. We provide greater detail on stratigraphic occurrences in Figures 2, 3, and 11.
Fossils of multituberculates and peradectians from Lancian and Puercan strata of the Hanna Basin provide no evidence for greatly enhanced levels of taxonomic diversity. Similarly, from paleogeographic or temporal points of view, no great surprises were derived from study of the two orders. It is nevertheless true that each of the listed species-level occurrences does represent a first published record for the greater Hanna Basin, and for southern Wyoming in general. Because the biostratigraphic terms themselves are based upon assemblages of fossil mammals, the list represents the first paleontological documentation of strata of Lancian and Puercan age in the basin.
Heretofore, no strata bearing fossilized mammalian assemblages of Puercan age had been documented between the Bighorn Basin of north-central Wyoming and the Denver Basin of north-central Colorado. Therefore, each Puercan species on the list below also constitutes a first geographic record between the Bighorn and Denver basins.
Within the Hanna Basin itself, representatives of no genera on the above list have been documented unequivocally in strata both of Lancian and Puercan age. Stratigraphic ranges known elsewhere, however, confirm that representatives of Mesodma, which typically were characteristic of the Lancian, did cross the Lancian-Puercan boundary. We believe that because of evolutionary continuity, many taxonomic distinctions in paleontology must be considered artificial and arbitrary, and often lead to “pseudoextinction” (see Archibald and Bryant, 1990). Probable examples relevant to our study include: (1) intrageneric evolution of Mesodma thompsoni (Lancian) into M. ambigua (early Puercan); and (2) intergeneric transitions from Mesodma (Lancian and early Puercan) to Ectypodus (middle Puercan and younger) and Alphadon (Lancian) to Peradectes (Puercan and younger).
Cimolodon nitidus (L)
Cimolodon sp. cf. C. nitidus (L)
Mesodma formosa (Pu1)
M. ambigua (Pu1–2)
M. hensleighi (Pu1)
Mesodma sp. cf. M. garfieldensis (Pu1–2)
Mesodma sp. indet. (Pu1)
?Mesodma sp. (Pu1)
Ectypodus sp. A (Pu2)
Ectypodus sp. B (Pu3)
Neoplagiaulacidae gen. and sp. indet. (L–Pu1)
Ptilodus sp. cf. P. tsosiensis (Pu2–3)
Ptilodus sp. (Pu3)
Catopsalis joyneri (Pu2)
Taeniolabis taoensis (Pu3)
Meniscoessus seminoensis new species (L)
Alphadon lulli (L)
Alphadon sp. cf. A. lulli (L)
Peradectes sp. cf. P. pusillus (Pu1 and Pu3)
Peradectidae gen. and sp. indet. (L)
All stratigraphic occurrences and taxonomic identifications of multituberculate and peradectian species recorded from the Lancian-Puercan section of the Hanna Basin are compatible with current evolutionary theories about histories of associated genera. For example, the large species of Ptilodus and Ectypodus clearly foreshadow species known elsewhere from strata of Torrejonian age. In terms of changes in dental morphology among the multituberculates, however, little occurred until middle Puercan time (for more detail, see Eberle, 1996). Dentitions representing the middle and late Puercan multituberculate faunas of the Hanna Basin are morphologically more derived than those known from Interval-zone Pu1. Indeed, although much less diverse, both the multituberculate and peradectian faunas of the early Puercan (Pu1) in the Hanna Basin may be considered simply as phylogenetic relics from Lancian time. Whole new genera appear upon advent of Interval-zone Pu2.
Although additional, previously unknown species of multituberculates and peradectians may be represented within our collections (e. g., Ectypodus sp. A and B; Ptilodus sp.), we recognized only Meniscoessus seminoensis as new.
The following list summarizes the nature of extensions of geographic and temporal ranges that derived from our research on multituberculates and peradectians of the western Hanna Basin. For practicality, we treated referred species (e. g., Peradectes sp. cf. P. pusillus) as though they were the same paleontological species as their original namesakes:
Geographic Range-extensions –
Most southerly known records of Cimolodon nitidus and Alphadon lulli, both extended from east-central Wyoming (Powder River Basin), and Mesodma ambigua, extended from north-central Wyoming (Bighorn Basin)
Most southerly known records of Mesodma hensleighi, Mesodma sp. cf. M. garfieldensis, and Catopsalis joyneri, all extended from northeastern Montana (Williston Basin)
Most northerly known records of Ptilodus sp. cf. P. tsosiensis, extended from northwestern New Mexico (San Juan Basin), and Taeniolabis taoensis, extended from central Utah (Wasatch Plateau)
Although coeval and probably having shared a common ancestor, Meniscoessus seminoensis is not known to co-occur geographically with M. robustus (including, in our view, M. greeni) of east-central Wyoming (Powder River Basin)
Temporal Range-extensions –
First known Puercan records of Mesodma hensleighi, extended from Lancian, and Ectypodus spp., extended from Torrejonian
First known record in Puercan Interval-zone Pu3 of Ptilodus sp. cf.P. tsosiensis, extended from Pu2
First known record in Puercan Interval-zone Pu2 of Catopsalis joyneri, extended from Pu1
Two special comments should be made relative to the range-extensions listed above. First, the multituberculate Mesodma hensleighi, by way of its extension into Puercan time from the Lancian, could be interpreted as another surviving mammalian species (see Archibald, 1996, table 1) of some form of global cataclysm, often postulated to explain extensive extinctions that define the end of Mesozoic time. A later paper in our series, based upon the total known mammalian fauna from Lancian-Puercan strata of the Hanna Basin, will address such issues specifically. Secondly, the presence of Catopsalis joyneri in northeastern Montana traditionally has been considered as diagnostic of Puercan Interval-zone Pu1 (including Pu0, in our view). Our recognition of C. joyneri in lowest levels of Interval-zone Pu2, therefore, may weaken that species' utility as an index fossil for the earliest Puercan, at least in latitudes as far south as central Wyoming.
In general, the Lancian multituberculate and peradectian faunas of the Ferris Formation are similar to, although not nearly so diverse, as those known from the relatively nearby type Lance Formation (Table 14). The Ferris Formation's low taxonomic diversity can be explained simply; it is merely that we have a paucity of specimens available for study. Only 11 specimens relevant to the present research could be identified reasonably to taxonomic levels lower than family.
Despite the relatively low diversity of multituberculates and peradectians recorded from the Ferris Formation, their distribution and taxonomic status through the section is at least consistent with the prevailing idea of widespread extinction of species at or near the Lancian-Puercan boundary (Table 15; see Archibald, 1996). As is typical for fossiliferous rock units elsewhere that represent Puercan time, we have discovered only one genus of peradectian from upper parts of the Ferris Formation. Available data derived solely from multituberculates and peradectians of the Ferris Formation do not add substance to either side of existing debates about magnitudes or timing of end-Cretaceous extinctions.
Taphonomic Aspects Related to Local Taxonomic Diversity
The low diversity of Lancian mammals from the Ferris Formation is due most importantly to biases in preservation that related to our ability to actually find specimens. Almost all of the vertebrate-bearing localities in the local Lancian section that we have discovered occur within sandstone or pebbly sandstone. The tiny teeth generally characteristic of Lancian mammals are difficult to find within coarse-grained strata, especially under field conditions of prospecting, quarrying, and dry-screening. With exception of UW locality V-92010, we have not been successful in locating highly fossiliferous mud-stone of Lancian age. Local Lancian mudstone typically is quite carbonaceous, usually reflecting acidic, bone-destroying conditions within the ancient swamps. In contrast, however, locality V-92010 is remarkable in its contained diversity of taxa, including specimens representing minuscule animals. The locality has yielded remains of a wide variety of lower vertebrates (including teeth and denticles of sharks and rays), four identifiable mammalian teeth, and even shelly material of gastropods.
In contrast to the local situation for the Lancian, the greatest abundance of mammalian specimens from Puercan parts of the Ferris Formation came from localities composed principally of mudstone (e. g., UW locality V-91004). Mudstone typically is better-suited to collecting techniques involving underwater screen-washing than sandstone is. It remains true, however, that even in the local Puercan section, most of the known vertebrate-bearing localities are contained within sandstone. In future exploration in the western Hanna Basin, great emphasis should be placed upon the discovery of washable, fossiliferous mudstone.
As described above, localized taphonomic features have had strong influence upon our knowledge of distributions of mammalian fossils within Lancian-Puercan strata of the Hanna Basin. Even those strong taphonomic biases, however, are inadequate to explain the overall pattern of evolutionary change observed through the total section. Additionally, close examination on the outcrop, as well as inspection of data used in development of Figures 5 and 6, show little recognizable correlation between local lithologic features and faunal changes recognizable at, or even above, the Lancian-Puercan boundary of the western Hanna Basin. The increase in relative proportions of sandstone documented above the 2,000 ft (610 m) stratigraphic level in Figure 6 is localized to section A–A′. As pointed out earlier, just to the north of that section the dominance of fine-grained clastic facies continues well above the 2,000 ft level. Quite expectedly, the faunal transitions observed throughout the Lancian-Puercan section of the Hanna Basin require explanations beyond the influence of local geologic conditions.
Temporal Calibration of Changes in Local Depositional Regimes
Our biostratigraphic data, derived principally from fossil mammals, allow calibration of local geologic events with particular intervals of Earth history. For example, an original depositional regime in what is now the western Hanna Basin that involved comparatively high-energy, braided-streams shifted rather suddenly to a lower-energy situation. Using reliable biostratigraphic data, we know that the shift occurred during late Lancian time. Following the change, forested, palm-rich, swampy lowlands developed in an environmental setting dominated by meandering streams, extensive flood-plains, and the deposition of generally finer-grained sediments. Although well-watered and rich with plants and diverse terrestrial and aquatic animal life, the local environment did not lead to extensive coals. At least one of the lignites was unusual in being principally of algal origin. The system of relatively low-energy, meandering streams persisted in some areas of the western Hanna Basin into early Puercan time.
Prior to advent of the Lancian-Puercan boundary, a new series of major river channels developed locally on the ancient landscape parallel to our section A–A′. The system of channels persisted in that immediate area until the end of Puercan time. This new fluvial system, in addition to its considerable longevity, maintained a narrow, geographically consistent general course of eastward drainage. Today's resulting pattern of east-dipping outcrops forms a stratigraphically thick, but laterally restricted band of mostly fine- to medium-grained, moderately indurated bodies of sandstone. Although the sandstone usually is not conglomeratic, occasional levels of tiny pebbles show that the same lithologic composition was inherited from sources typical for lower reaches of the Ferris Formation.
As an aside, this ribbon-like outcrop pattern of sandstone, because of its usually greater induration than the finer-grained, laterally equivalent overbank strata, aided in diverting the North Platte River to an east-northeasterly course during Quaternary time, as the river gradually exhumed the western Hanna Basin. Once attaining a stratigraphic level above this ancient river system, however, the modern North Platte River resumes its normal northward course toward the Seminoe Mountains.
The important faunal changes that locally define the Lancian-Puercan boundary have no recognizable temporal links with alterations of depositional regimes identified in the western Hanna Basin. Nevertheless, the mammalian assemblages collected along section A–A′ reflect a rapidly evolving fauna within a basin undergoing prodigious rates of subsidence. The subsidence was combined with massive input of clastic debris, with study of encased fossils allowing close temporal calibration of important local geologic changes, even within subdivisions of Puercan time.
As an example, the first obvious sign of a shift from a consistently aggradational regime to a dramatically developed cut-and-fill setting is observable along section A–A′, in outcrops representing Puercan Interval-zone Pu3. This transition is stratigraphically associated with the only lacustrine strata recognized in the area of study, and the cuts-and-fills constitute prominent aspects of the remainder of the section. Immediately above the stratigraphic top of section A–A′ are the extensive commercial coal beds of the upper Ferris Formation. It appears, therefore, that early alterations in the balance between rates of subsidence and deposition that led eventually to formation of the coals had their origins during Puercan time. Specifically, initiation of the alterations occurred during the time represented by Interval-zone Pu3. Furthermore, the coals of the upper Ferris Formation most probably were formed during Torrejonian time.
Ferris Formation as a New Biostratigraphic Standard for the Puercan
Superpositionally controlled, systematic paleontology has contributed to development of a biostratigraphic zonation for a major part of the Ferris Formation in vicinity of its type section. The zonation is based upon the entire known assemblage of fossil mammals, only part of which is discussed in the present paper. This biostratigraphy will be useful in temporally calibrating a wide variety of geological and biological discoveries, present and future, in the Ferris Formation of the Hanna Basin. Indeed, the high degree of biostratigraphic refinement now available for most of the formation suggests that the western Hanna Basin could become a geological standard against which other nonmarine sections of Puercan age might be compared.
The future for stratigraphically controlled paleontological and paleoecological research in the Ferris Formation looks bright. For example, Nichols' preliminary results, summarized above, lead to deserved optimism for more detailed work on the composition and distribution of palynomorphs. Other, already existing collections of fossil leaves from our sections await publication. The potential for radio-metric dating and magnetostratigraphic analysis of the Ferris Formation has not been evaluated adequately. The western Hanna Basin has become, from biostratigraphic points of view, among the most closely constrained of latest Cretaceous and early Paleocene nonmarine depocenters. Such calibration is particularly relevant to better understanding the end of the Mesozoic and approximately the first million years of Cenozoic history in western North America.
Our research was supported by grants from the National Science Foundation (EAR-9205567 and EAR-9506462), National Geographic Society, and Office of Research at The University of Wyoming (awarded to Lillegraven), as well as through personal generosity of Malcolm C. McKenna and Donald W. Boyd. Support for summer field work also was provided to Eberle by the Geological Society of America, College of Arts and Sciences at The University of Wyoming, Department of Geology and Geophysics at UW, Society of the Sigma Xi, Paleontological Society, and the American Federation of Mineralogical Societies. The research also benefited from the generosity of a John H. Hanley Memorial Scholarship awarded to Eberle.
We thank members of the Miller family (represented by The Miller Estate Company) for providing full access to the research area. Bob Faulkner, the company's land manager, was particularly helpful in practical ways. Members of the U.S. Bureau of Land Management were helpful in many ways, and the collections were conducted under auspices of Paleontological Resource Permit numbers 184-WY-PA91, 184-WY-PA94, and 184-WY-PA94 (extended). Laurie J. Bryant helped in many ways, both logistically and scientifically.
Results of preliminary palynological analyses are included in this study thanks to Douglas J. Nichols, who visited the field area, collected and analyzed several samples for palynomorphs, and furnished identifications. For technical help and access to specialized facilities related to sorting paleontological samples, we thank Donald W. Boyd, Kevin R. Chamberlain, Carol D. Frost, Peter George, and Keith A. Krugh. We thank the following, who provided loans of casts and original specimens: Eric Dewar, Richard C. Fox, John P. Hunter, Donald L. Lofgren, Malcolm C. McKenna, Miao Desui, Peter Robinson, John E. Storer, Leigh Van Valen, and Thomas E. Williamson. James Finley designed the code and perfected operating formats of “Finley1."
The following individuals shared their knowledge and provided constructive criticisms to our research: Lisa M. Amati, J. David Archibald, Donald W. Boyd, Brent H. Breithaupt, Gregory A. Buckley, Keith E. Clarey, William A. Clemens, Jr., Eric Dewar, Lowell W. Dingus, Jason F. Hicks, John P. Hunter, Kirk R. Johnson, Michael B. Leite, Donald L. Lofgren, Malcolm C. McKenna, Virginia Maiorana, Michael D. Middleton, Douglas J. Nichols, Peter N. Shive, Arthur W. Snoke, Nancy L. Stanton, John E. Storer, Carl C. Swisher, III, Leigh Van Valen, and Thomas E. Williamson. Although many individuals helped in the field, prime contributors include Jean-Pierre Cavigelli, Seaghan Uibreaslain, and Anton F.-J. Wroblewski. The manuscript benefited greatly from the careful reviews by Kirk R. Johnson and an anonymous reader.
Finally, we thank our respective spouses, David A. Taylor and Linda E. Lillegraven, for the many ways in which they contributed to completion of this research.
- Received December 9, 1996.
- Revision received June 16, 1997.
- Accepted July 18, 1997.