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Puercan mammalian systematics and biostratigraphy in the Denver Formation, Denver Basin, Colorado

University of Colorado Museum, 265 UCB, Boulder, CO 80309, U.S.A.

As a result of the Denver Basin Project, several more fossils of Puercan mammals are reported here from five areas in the Denver Formation: South Table Mountain, Greater Denver, the West Bijou Site, Big Gulch, and Corral Bluffs. Systematic description and discussion are provided for one multituberculate and 11 ungulate taxa, including a new species of Baioconodon. Some taxa represent extensions of previously recognized temporal and geographic ranges. Notably, the ungulate Protungulatum donnae from strata of early Puercan (Pu1) age in the Denver Formation represents the southernmost occurrence of the species, while Oxyclaenus simplex, in probable early Puercan strata of the Denver Formation, appears to represent both a temporal and geographic range extension from middle Puercan (Pu2) strata of the San Juan Basin, New Mexico. Additionally, I report the first occurrence in the Denver Basin of the arctocyonid Loxolophus faulkneri. Refined biostratigraphic interpretations, resulting from new discoveries and incorporating paleomagnetic, palynological, and radioisotopic analyses presented elsewhere, suggest that Puercan interval zones Pu1 and Pu2 are both represented by mammalian faunas in the Denver Formation. Localities at South Table Mountain, as well as the Denver Oxyclaenodon Site (DMNH loc. 299) and Nicole's Mammal Jaw locality (DMNH loc. 2557), are Pu1 correlatives. Discoveries reported here support previous interpretations that the Alexander and South Table Mountain localities are probably similar in age (i.e., Pu1) and are included here in the Littleton fauna. Based upon comparison to other Puercan faunas, the Littleton fauna may be temporally intermediate between typical Pu1 assemblages known north of the Denver Basin and earliest Pu2 assemblages from the Hanna Basin, Wyoming. Alternatively, faunal differences between the Littleton fauna and other Pu1 faunas may reflect ecological and biogeographic differences. The unusually high diversity of ungulates in the Littleton fauna suggests some of the diversification that elsewhere characterizes the onset of Pu2 may already have begun by Pu1 in the Denver Basin. The mammalian assemblage at Corral Bluffs is interpreted here as a probable Pu2 correlative, based upon occurrence of Loxolophus faulkneri, Conacodon entoconus and C. delphae, and absence of Pu3 index taxa. Pu2/Pu3 correlatives (i.e., faunal assemblages that are either Pu2 or Pu3) also are reported here from the Big Gulch area, although more fossils are needed to refine their ages. The present study and others in this issue demonstrate that the Denver Basin is among the few places wherein correlation between Puercan mammalian biostratigraphy and paleomagnetic, palynological, and radioisotopic analyses is an attainable goal.

Key Words: biostratigraphy • Paleocene • Puercan • Denver Formation • Denver Basin • Colorado • Eutheria • Multituberculata • Ungulata