A NEW EARLIEST PALEOCENE (PUERCAN) FAUNA FROM COLORADO’S DENVER BASIN

Few areas preserve the Cretaceous-Paleogene (K-Pg) boundary and earliest Paleocene (Puercan) mammalian faunas of western North America better than the Denver Basin of Colorado. Research conducted more than a decade ago described a diverse Puercan assemblage – the Littleton Local Fauna – from late early Puercan sediments. Renewed collecting from one Puercan locality on the eastern side of the Denver Basin has produced a diverse assemblage of Puercan vertebrates. In addition to abundant remains of aquatic and semi-aquatic vertebrates including fishes (Lepisosteidae), amphibians (Anura, Caudata), and crocodylians (c.f. Alligatoroidea), the site contains isolated teeth belonging to approximately ten species of fossil mammal. Based on its faunal composition, low observed diversity, absence of larger, more derived middle/late Puercan taxa, and close stratigraphic proximity to the well-defined K-Pg boundary, the assemblage likely represents a fauna older than the Littleton Local Fauna and is temporally correlative to Pu1 faunas in NE Montana and Wyoming’s Hanna Basin. We report the occurrence of at least three species of multituberculates including two species of Mesodma, the marsupial Thylacodon pusillus, and several ‘condylarth’ species, including Protungulatum donnae, Oxyprimus galadrielae, and Ragnarok nordicum. The presence of Mesodma, O. galadrielae, and P. donnae is consistent with a Pu1 age. The presence of P. donnae alone defines the onset of the Puercan North American Land Mammal Age (NALMA).

Additionally, we report the occurrence of the ‘condylarth’ Ampliconus browni from another Puercan locality - the South Table Mountain locality near Golden, CO. Its presence helps to corroborate prior research by others that the South Table Mountain locality is correlative temporally to the late early Puercan Alexander locality.

This study documents the southernmost early Pu1 fauna yet known, extending the geographic range from Wyoming, and suggests that early Puercan mammalian diversity in the Denver Basin may be greater than previously thought. Future research is needed to further our understanding of early Puercan mammals and refine early Paleocene biostratigraphy in the Denver Formation. Doing so will further illuminate mammalian radiation right after the K-Pg boundary in the Rocky Mountain Region.