- UW Department of Geology and Geophysics
The Hanna Formation, exposed in the northeastern Hanna Basin, Wyoming, represents deposition from late early Paleocene into earliest Eocene time in alluvial, floodplain, and lacustrine environments. A 600-m-thick section that yields abundant vertebrate fossils begins 975 m above the local base of the formation. This section has been dated as latest Torrejonian through middle Tiffanian using mammalian index fossils. The terrestrial mammals are accompanied by numerous elasmobranch teeth, representing species thought extinct since the end of Cretaceous time. They are species known from two locally widespread marine Cretaceous units, the Wall Creek Member of the Frontier Formation and the Steele Shale. These units are broadly exposed on flanks of the Sweetwater arch, which define the northern margin of the Hanna Basin. The elasmobranch teeth from the Hanna Formation range in size from a few millimeters to over three centimeters and display transport-induced abrasion not seen on their in situ Cretaceous counterparts. Enameloid of cutting edges, crown points, and cusplets is rounded or sometimes broken, and bony bases commonly are etched or dissolved away. These teeth were reworked and transported from the Wall Creek Member and Steele Shale during uplift and erosion of the Sweetwater arch in middle Paleocene time. Lithic clasts from Paleozoic and Mesozoic strata derived from that arch also occur in association with the elasmobranch teeth in the Hanna Formation. The Cretaceous clasts and teeth indicate a local, northerly source for part of the Hanna Formation in the northeastern Hanna Basin.
- Frontier Formation
- Hanna Basin
- Hanna Formation
- Steele Shale
- Sweetwater arch
The Hanna Basin is a small, Laramide-style basin located in Carbon County, south-central Wyoming, on the southeast flank of the Sweetwater arch (Granite Mountains, sensu Love, 1970). Various structural subdivisions of the Sweetwater arch bound the northern margin of the Hanna Basin. From east to west, these are the Freezeout Hills, Shirley Mountains, and Seminoe Mountains. The greater Hanna Basin extends as far west as the Rawlins uplift and south to the northern edge of the Medicine Bow Mountains and Sierra Madre (Fig. 1).
The focus of present research is the northeastern corner of the Hanna Basin, in an area of badlands known as The Breaks (Como West 7.5 min topographic quadrangle). Although the badlands are composed primarily of Paleocene Hanna Formation, their margins expose diverse Upper Cretaceous formations. Approximately 975 m (3,220 ft) above the local base of the Hanna Formation is the base of a 609-m-thick fossiliferous sequence, hereinafter referred to as the vertebrate fossil-bearing zone (VFBZ). Most fossils discussed in this paper were recovered from the VFBZ.
SEDIMENT AND DEPOSITION OF THE HANNA FORMATION
This paper focuses on the magnificent exposures of Hanna Formation located in The Breaks. The formation there exceeds 3536 m (11,600 ft) in thickness, making it the thickest formal rock unit of the Hanna Basin. It represents essentially continuous deposition from late in the early Paleocene into earliest Eocene time, an interval less than 10 million years (Lillegraven and Snoke, 1996). Basin-margin erosion has greatly reduced distribution of the Hanna Formation from its original lateral extent. The Hanna Formation is found in the eastern Hanna Basin (Fig. 1) to the base of the northern and southern surrounding uplifts. Its immense thickness, representing a short interval of time, suggests extremely rapid rates of deposition.
The Hanna Formation in The Breaks is a wholly nonmarine unit, dominated by carbonaceous mudstone (Fig. 2). Although some sandstone and conglomerate beds occur throughout the formation, the coarse-grained strata are volumetrically unimportant. Coarse-grained strata in the Hanna Formation in the northeastern corner of the Hanna Basin typically have been interpreted as alluvial in origin, interdigitating with mud deposited on floodplains (e.g., Knight, 1951; Ryan, 1977; Hansen, 1983). Sandstone in the lower two-thirds of the Hanna Formation in The Breaks is comprised mainly of quartz.
The VFBZ in The Breaks is laterally continuous for several miles, and the strata above and below the VFBZ are essentially devoid of vertebrate fossils (Lillegraven and Snoke, 1996; Higgins, 2000). The depositional environment for the strata above and below the VFBZ seems not to have been strikingly different from that of the VFBZ. Fossils probably were present at one time, but have been removed through diagenesis (Lillegraven and Snoke, 1996; Higgins, 1999). The VFBZ fossils generally consist of disarticulated, weathered bone fragments, isolated teeth, and abundant scales of Paleocene gars. Fragments of turtle shells and other bones are among the most abundant remains. Skeletal remains and isolated teeth of crocodiles are also found, along with a diverse latest Torrejonian through middle Tiffanian mammalian fauna. A fairly diverse shark fauna, the focus of this paper, has been discovered throughout the VFBZ in the Hanna Formation.
The VFBZ is similar to most of the rest of the Hanna Formation in that it is dominated by carbonaceous mudstone. It does, however, lack significant coal. Occasional beds of sandstone and pebble conglomerate are interbedded with the fine-grained rocks. Material coarser than pebbles is not typically found in this area, although cobble conglomerate can be found above and below the VFBZ. While volumetrically insignificant in the VFBZ, the pebble conglomerates are important because they are the only rock types that typically yield abundant vertebrate fossils. The sandstones and pebble conglomerates were deposited in stream channels. Sandstone of the VFBZ is typically of two types. The first is a white to buff, fine-grained, massive, quartz-rich sandstone that usually is found in thick outcrops. This variety is fairly uninformative paleontologically, as only a few vertebrate fossils have been collected from it. The other major type found in the VFBZ is a yellow, variously bedded, quartz-rich, muddy sandstone. These layers typically grade laterally and vertically into pebble conglomerate.
The following taxonomic hierarchy comes from Cappetta (1987). Dental terminology, unless indicated otherwise, follows Compagno (1988). Locality and specimen numbers are for the Collection of Fossil Vertebrates, Departmental Scientific Collections, Department of Geology and Geophysics, The University of Wyoming.
Class CHONDRICHTHYES Huxley, 1880
Subclass ELASMOBRANCHII Bonaparte, 1838
Order LAMNIFORMES Berg, 1958
ANACORACIDAE Casier, 1947
Squalicorax Whitley, 1939
Squalicorax falcatus Agassiz, 1843
Squalicorax falcatus Agassiz, 1843, p. 226.
Holotype.—Agassiz, 1843, p. 226, plate 26, figure 14, plate 26a, figures 1–15.
Type locality.—Maastricht, Netherlands; no other information available.
Referred specimens.—Frontier Formation: UW 26737, 26748, 26751, 26757, 26778, 26780, 26782, and 26889–26892; and Hanna Formation: UW 26628, 26631, 26633, 26634, 26638, 26643, 26645, 26649, 26652, 26654, 26657, 26659, 26660, 26665, 26671, 26674, 26675, 26679, 26683, 26684, 26691, 26695, 26698, 26702, 26709, 26711, 26712, 26718, 26721, 26725, 26728, 26732, 26733, 26775–26777, 26779, 26781, 26800, 26816, 26829, 26840, 26844, 26848, 26850, 26877–26886, 26893–26895, and 36154.
Localities.—Frontier Formation: UW localities V-96014, V-96015, V-96016, and V-96049; and Hanna Formation: UW localities V-90029, V-90032, V-90035, V-90036, V-90039, V-90040, V-90043, V-90044, V-90047, V-90048, V-90050, V-90052, V-90053, V-90055, V-90056, V-90057, V-90060, V-90061, V-90062, V-90063, V-90064, V-90066, V-90067, V-90068, V-90070, V-90071, V-90082, V-90086, V-90089, V-90107, V-90109, V-90112, V-90113, V-90119, V-90123, V-91007, V-93021, V-93024, V-96005, V-96006, V-96010, V-96020, V-96035, V-96036, V-96040, V-96048, V-97020, V-97024, V-97029, and V-97031.
Known distribution (of unreworked specimens).—Turonian through late Santonian of North America (Schwimmer et al., 1997); Coniacian through Campanian of Europe; and Cenomanian of Africa (Cappetta, 1987).
Description and discussion.—Squalicorax falcatus was a wide-ranging species that lived in a variety of marine environments across the Western Interior Cretaceous Seaway (Shimada, 1996). Squalicorax falcatus has been confused often with Squalicorax kaupi due to their similar size and morphology. Squalicorax kaupi succeeded S. falcatus during the early Santonian (Case et al., 1990). Teeth of S. falcatus can be distinguished from those of S. kaupi by the acute apex of the main cusp (Schwimmer et al., 1997), by the sharp notch that separates the crown from the distal trenchant edge, and the outer face of the crown that overhangs the tooth base (Cappetta, 1973). On smaller specimens, it is almost impossible to distinguish S. falcatus from S. kaupi, as these teeth tend to converge in morphology (Williamson et al., 1993).
Teeth of Squalicorax falcatus represent the most abundant elasmobranch fossils in both the VFBZ of the Hanna Formation and beds of the Wall Creek Member located on uplifts to the north. Fossils of S. falcatus have been recovered from virtually all localities of the Hanna Formation in The Breaks that are known to bear shark teeth. Fossils of this species have not been found in other local Cretaceous marine units adjacent to the northeastern margin of the Hanna Basin.
Squalicorax kaupi Agassiz, 1843
Squalicorax kaupi Agassiz, 1843, p. 225.
Holotype.—Agassiz, 1843, p. 225, plate 26a, figures 25–34, plate 26, figures 4–8.
Type locality.—Maastricht, Netherlands; no other information available.
Referred specimens.—Steele Shale: UW 26801, 26896–26898, and 36338; and Hanna Formation: UW 26704, and 26793–26796.
Localities.—Steele Shale: UW locality V-97026; and Hanna Formation: UW localities V-90044, V-90064, V-90079, V-90112, and V-93024.
Known distribution (of unreworked specimens).—Campanian of North America; and Campanian of Europe, Japan, and New Zealand (Cappetta, 1987).
Description and discussion.—A few teeth of Squalicorax kaupi have been found in local beds of the Steele Shale and the VFBZ of the Hanna Formation. This species is thought to have descended from S. falcatus, with only minor differences evident in their teeth (Bilelo, 1969). Squalicorax kaupi is distinguished from S. falcatus primarily by a more obtuse apical angle of the main cusp (Schwimmer et al., 1997).
MITSUKURINIDAE Jordan, 1898
Scapanorhynchus Woodward, 1889
Referred specimens.—Frontier Formation: UW 26738, 26753, 26756, 26812, 26813, and 26900; Steele Shale: UW 26818, 26888, and 36339; and Hanna Formation: UW 26630, 26637, 26641, 26644, 26647, 26650, 26651, 26655, 26666, 26669, 26673, 26676, 26680, 26681, 26692, 26700, 26717, 26726, 26727, 26731, 26735, 26809–26811, 26814, 26841, 26852, 26899, 26901, 26902, and 36155.
Localities.—Frontier Formation: UW localities V-96014, V-96016, and V-96049; Steele Shale: UW localities V-97030 and V-98003; and Hanna Formation: UW localities V-90029, V-90032, V-90039, V-90043, V-90044, V-90047, V-90055, V-90060, V-90061, V-90062, V-90063, V-90064, V-90067, V-90068, V-90082, V-90089, V-90107, V-90112, V-91007, V-93024, V-96010, V-96035, V-96036, V-96040, and V-97020.
Description and discussion.—Although teeth of Scapanorhyncus sp. are common in local beds of the Wall Creek Member of the Frontier Formation, they are comparatively rare in the Steele Shale. Fossils of this genus also are common in the Hanna Formation in the Breaks, although they are less abundant than in the Frontier Formation. Scapanorhyncus was a wide-ranging genus temporally and geographically, and it also was present in a wide range of marine environments (Williamson et al., 1993).
Cohort EUSELACHII Hay, 1902
Superfamily HYBODONTOIDEA Zangerl, 1981
PTYCHODONTIDAE Jaekel, 1898
Ptychodus Agassiz, 1838
Ptychodus whipplei Marcou, 1858
Ptychodus whipplei Marcou, 1858, p. 33.
Holotype.—Marcou, 1858, p. 33, plate 1, figure 4.
Type locality.—Gray sandy marls, 3 miles north of Galisteo, road from Galisteo to Pecos, New Mexico.
Referred specimens.—Frontier Formation: UW 26736, 26752, 26754, 26805, 26806, 26903; and Hanna Formation: UW 26640, 26663, 26685, 26688, 26694, 26706, 26714, 26803, 26804, 26807, 26808, 26817, 26830, 26839, 26843, 26851, 26873, and 26874.
Localities.—Frontier Formation: UW localities V-96014, V-96016, and V-96049; and Hanna Formation: UW localities V-90029, V-90039, V-90043, V-90057, V-90064, V-90066, V-90068, V-90084, V-90123, V-93024, V-96006, V-96010, V-96020, V-96033, and V-97020.
Known distribution (of unreworked specimens). North America, extending from the Turonian into the early Santonian (Cappetta, 1987).
Description and discussion.—Teeth of Ptychodus whipplei are abundant in the Wall Creek Member and widespread throughout fossil-bearing localities in the Hanna Formation in The Breaks. Ptychodus whipplei is the most common species of benthic elas-mobranchs in the Frontier Formation and is the most commonly reworked benthic taxon in the Hanna Formation. Teeth of P. whipplei have not been found in other local Cretaceous units.
Referred specimens.—Hanna Formation: UW 26661, 26783, and 26887.
Localities.—Hanna Formation: UW localities V-90064, V-90112, and V-96010.
Description and discussion.—Teeth of Ptychodus sp. are extremely robust, with high crowns and broad tooth margins. The tops of the crowns are characterized by ridges radiating outward from the center that end at the base of the crown, where they are replaced by a granular pattern. On some specimens, the granular pattern forms concentric rings along the tooth base. The lingual edges of the teeth are strongly indented. In occlusal view, the teeth are rectangular-to diamond-shaped, with a concave lingual side. Ptychodus sp. can be differentiated easily from Ptychodus whipplei by a relatively lower crown with a strong cone shape in the former. All specimens of Ptychodus sp. are larger than teeth typical of Ptychodus whipplei, with one tooth measuring about 3 cm across the base. These three teeth appear to represent the same species.
Only three teeth of Ptychodus sp. have been found in the Hanna Formation, and they have not been discovered in the Frontier Formation or any other local Cretaceous units. As true for other species of Ptychodus, this species was well-suited for crushing shellfish. These specimens have been identified only to the generic level because they cannot be recognized as a member of any known species.
EVIDENCE FOR REWORKING OF SHARK TEETH
The Hanna Formation in vicinity of The Breaks is an extensively channeled terrestrial unit that shows no evidence of marine or estuarine influences. While there have been recent suggestions in published abstracts that the paleoshoreline was nearby, and that sandbodies in the Hanna Formation a few miles to the southwest represent incised valley-fills with tidally influenced fluvial and distributary channels (Wroblewski and Secord, 1998; Wroblewski and Steel, 1999), necessary supporting evidence has yet to be provided. Belt et al. (1997) found and described marine-influenced facies of the Fort Union Formation in southeastern Montana, central North Dakota, and north-central South Dakota, based on marine ichnogenera. Kroeger and Hartman (1997) investigated brackish water tongues of the Cannonball Formation for palynological analysis. These tongues represent the westernmost transgression of the Cannonball Sea; probably the greatest western extent of the Paleocene sea was eastern Montana. Lillegraven and Ostresh (1990) show the paleoshoreline hundreds of miles to the east at the end of the Cretaceous. If the sharks discovered in the Hanna Formation were indeed alive during Paleocene time, this would imply that they swam far upstream to their sites of preservation in the Hanna Basin. Alternatively, they lived more permanently in fresh waters of the Hanna Basin at the time of deposition. Although modern marine sharks are sometimes found upstream in large rivers (Budker, 1971), such occurrences appear too improbable to justify this interpretation for the great number of fossil teeth found in the Hanna Formation.
As a result of extinctions associated with the Cretaceous-Tertiary transition, the North American elasmobranch fauna was greatly diminished (Eaton et al., 1989) and replaced by a new one (Cvancara and Hoganson, 1993; Fig. 5). Fossil shark teeth from the Hanna Formation represent what appears to be an exclusively Cretaceous fauna. In the present study, shark fossils from The Breaks were compared with specimens collected from surrounding Cretaceous marine units. Squalicorax falcatus, Scapanorhynchus sp., and Ptychodus whipplei occur in abundance in the Wall Creek Member of the Frontier Formation. Scapanorhynchus sp. was found in local beds of the Steele Shale as was Squalicorax kaupi. All of these species were found in relative abundance in the channel deposits of Hanna Formation in The Breaks.
The fact that shark teeth from the Hanna Formation in The Breaks show obvious signs of heavy abrasion not seen in their Cretaceous counterparts indicates that they were eroded from marine Cretaceous strata, fluvially transported, and then redeposited. Shark teeth in the VFBZ of the Hanna Formation occur in pebble conglomerate, a rock type indicating a fairly high-energy environment of transport for the components. While these teeth seem to have been transported only a few miles or tens of miles from the surrounding uplifts, they were carried under conditions that led to rapid abrasion.
Typical evidence of transport-induced abrasion includes broken or generally incomplete teeth. Commonly, points on primary crowns have been removed and originally sharp cutting-edges are blunted. Fractured areas show rounding, and bony tooth bases (sensu Peyer, 1968) are typically lost. Enameloid of the crown exhibits polishing not present on teeth from in situ Cretaceous counterparts.
Interestingly, Paleocene mammal teeth and other Paleocene vertebrate fossils within the Hanna Formation also show transport-induced abrasion, and to an extent exceeding that of the reworked Cretaceous shark teeth. Teeth and bones of these Paleocene mammals, therefore, were subjected to the same high-energy environment of transport prior to deposition as were Cretaceous shark teeth. In some instances, therefore, the Paleocene mammal teeth were exposed either to more abrasive forces or to more distant transport than were the reworked Cretaceous shark teeth. Another possibility is that the mammal teeth were less mineralized and thus subject to faster rates of damage prior to completion of the steps of fossilization.
PROVENANCE OF SEDIMENTS CONTRIBUTING TO HANNA FORMATION
Sediments that were deposited as the Hanna Formation probably came from several different geographic areas. A traditional response to the question of provenance has been that the sediments came from the Sweetwater arch to the north and the Sierra Madre and Medicine Bow Mountains to the south (Knight, 1951; Ryan, 1977; Hansen, 1986). The Green River Basin during the Late Cretaceous and Paleocene has also been cited as an eastward-flowing conduit for fine-grained sediment that became trapped and preserved in the Hanna Formation (Lillegraven, 1994).
The Sweetwater arch clearly was an important source for much of the material in the Hanna Formation. As the Freezeout Hills (a southeastern component of the Sweetwater arch) were uplifted during the Laramide orogeny, the Mesozoic and Paleozoic sedimentary cover became deeply eroded. By the end of deformation, erosion had exposed Paleozoic strata to just above the Precambrian core. This period of latest Cretaceous and Paleocene uplift and erosion cut through about 7000 m (23,000 ft) of sedimentary rock on the most deeply eroded part of the Freezeout Mountain anticline (Lillegraven and Snoke, 1996). The Shirley Mountains, west of the Freezeout Hills and also a southern part of the Sweetwater arch, underwent intense erosion as well. In this case, the crystalline Precambrian core of the mountains became broadly exposed during Paleocene time. Strongly deformed remnants of Paleozoic and Mesozoic strata crop out along all flanks of the Shirley Mountains.
Comparatively minor beds of cross-bedded fluvial sandstone exist throughout most of the Hanna Formation. Analysis of cross-bedding in the northern Hanna Basin indicates that flow from the north delivered much of the coarser-grained detrital sediment (Ryan, 1977). Also, clasts from various Mesozoic and Paleozoic formations adjacent to the basin have been recognized in the Hanna Formation since it was first described (Bowen, 1918). These clasts were eroded from formations exposed on flanks of the Sweetwater arch to the north. A remarkable section of upper Paleozoic, Mesozoic, and lower Cenozoic sedimentary rocks is located in the northern Hanna Basin and on flanks of adjacent uplifts. This succession is approximately 13,150 m (43,000 ft) thick and includes a wide diversity of rock types. From that succession, rounded pebbles and cobbles of Mowry Shale, Cloverly Formation, and Tensleep Sandstone have been identified in shark-bearing conglomerates of the Hanna Formation.
The Mowry Shale contributed the most readily recognizable, locally derived pebbles found in the Hanna Formation in The Breaks. The Mowry Shale represents late Albian and/or early Cenomanian marine deposition (Cobban and Kennedy, 1989). It is almost completely comprised of black to gray, siliceous shales that weather to a distinctive steelgray. Rounded clasts of Mowry Shale are found in nearly every pebble conglomerate in the VFBZ, and they make up a large volume of the clasts in all of the conglomeratic beds in the lower half of the Hanna Formation. In a few instances, these pebbles comprise more than half the volume of the conglomerate. Many of these clasts contain fish scales and other bones.
Clasts of Cloverly Formation and Tensleep Sandstone commonly are found throughout conglomeratic layers of the lower Hanna Formation, although they are not as omnipresent as the Mowry pebbles. The Lower Cretaceous, terrestrial Cloverly Formation is represented in the Hanna Formation by pieces of quartzitic conglomerate with well-rounded clasts. Reworked clasts from the Pennsylvanian Tensleep Sandstone vary from pebble- to cobblesized fragments of medium-grained, gray to buff, sugary sandstone.
SUMMARY AND CONCLUSIONS
Fossil shark teeth recovered from the vertebrate fossil-bearing zone in the Paleocene Hanna Formation were reworked from Cretaceous units undergoing erosion in surrounding uplifts. The assemblage of fossil elasmobranchs from the Hanna Formation shares most taxa with the faunas known from the Wall Creek Member of the Frontier Formation and the Steele Shale, but differs from faunas of unquestionable Paleocene age recognized elsewhere. Elasmobranch teeth from the Hanna Formation show signs of abrasion due to transport that are not evident on teeth recovered in situ from Cretaceous rocks. Rounding of crown points and cutting-edges is common in the reworked teeth, as well as loss of accessory cusplets, tooth bases, and distal parts of crowns. The reworked teeth were deposited in paleochannels together with gravel and skeletal material from contemporary terrestrial, Paleocene vertebrates.
The reworked shark teeth provide additional evidence for erosional sources along the Sweetwater arch for sediments that constitute the Hanna Formation in The Breaks. More resistant clasts of Tensleep Sandstone, Mowry Shale, and Cloverly Formation in pebble- and cobble-conglomerate of the Hanna Formation in The Breaks also support local, northerly sources.
The National Science Foundation provided funding that made this research possible through grants EAR-9506462 and EAR-9909354, both awarded to The University of Wyoming through Drs. Jason A. Lillegraven and Arthur W. Snoke.
- Received July 22, 2000.
- Revision received January 16, 2001.
- Accepted January 17, 2001.