Few areas preserve the Cretaceous–Paleogene (K–Pg) boundary and earliest Paleocene (Puercan) mammalian faunas better than Colorado's Denver Basin. Research conducted decades ago described a diverse Puercan fauna from the Denver Basin, but many would agree that the best-known fauna in the basin—the Littleton fauna—probably represents a late early-Puercan (i.e., late Pu1) assemblage. Renewed collecting of fossil mammals from Denver Museum of Nature & Science (DMNH) loc. 2560 in the Denver Formation on the eastern side of the Denver Basin—approximately nine meters stratigraphically above the K–Pg boundary—has produced a Puercan faunal assemblage containing isolated teeth of at least nine species of fossil mammal. Based upon its faunal composition, low diversity, absence of taxa characteristic of middle and late Puercan faunal assemblages, and close stratigraphic proximity to the K–Pg boundary, the fauna from DMNH loc. 2560 probably represents a Puercan fauna that is earlier than the Littleton fauna. It is most similar to early Puercan (Pu1) faunas in Wyoming and northeast Montana. We document the occurrence of three species of the multituberculate Mesodma (M. ambigua, M. formosa, and M. hensleighi), the cimolodont Cimexomys minor, the marsupial Thylacodon montanensis, and four species of archaic ungulate (or ‘condylarth’), including Protungulatum donnae, Oxyprimus sp., Baioconodon nordicus, and Maiorana sp. The presence of P. donnae defines the onset of the Puercan age, while Oxyprimus, Maiorana, and B. nordicus are restricted elsewhere to Pu1. C. minor is known from Lancian and early Puercan (Pu1) localities. The faunal assemblage from DMNH loc. 2560 indicates that the characteristic, low-diversity Pu1 fauna found in Montana and Wyoming extended as far south as Colorado.

Additionally, we report the occurrence of the ‘condylarth’ Ampliconus browni from the South Table Mountain locality (DHMH loc. 2814) near Golden, Colorado. Its presence corroborates research by others that this locality is probably temporally correlative to the late early-Puercan (late Pu1) Alexander locality south of Denver, Colorado (UCM loc. 77267 [UCM = University of Colorado Museum of Natural History]). Our research provides a more complete picture of early Puercan mammalian diversity in the Denver Basin and a means to temporally correlate early Puercan faunas from Montana to Colorado.

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