New latest Cretaceous mammals from northeastern Colorado with biochronologic and biogeographic implications

Abstract

ABSTRACT Few latest Cretaceous mammalian faunas are known from the central part of the Western Interior of North America. Here we report a collection of 28 mammalian fossils (23 isolated teeth, 5 dentulous jaw fragments) from five Upper Cretaceous localities in and around the Pawnee National Grassland (Weld County) in the Cheyenne Basin, northeastern Colorado. At least 10 taxa are represented, including two new multituberculates, a neoplagiaulacid Parikimys carpenteri and a ?cimolomyid Paressonodon nelsoni, which has greatest affinities to Essonodon browni. The new specimens combine with a previously reported sample for a total of 40 mammalian specimens from the area. Using a subsurface model based on geophysical log data, we stratigraphically arranged the fossil localities relative to the top of the Fox Hills Sandstone. Our stratigraphically lowest site is 95 m above the datum and likely near the top of the Laramie Formation. Presence of Meniscoessus collomensis at this site implies a late ‘Edmontonian’ or early Lancian age. The remaining sites are 220, 330, and 410 m above the datum and temporally equivalent to the Cretaceous part of the D1 sequence in the Denver Basin; the stratigraphically highest site is near the predicted Cretaceous–Tertiary boundary. The faunal samples from the latter sites include mostly typical Lancian taxa, except for two new taxa that suggest biogeographic differences with contemporaneous localities from farther north. This stratigraphic succession of fossil localities from northeastern Colorado may offer new perspectives on temporal and biogeographic changes of mammalian faunas leading up to the Cretaceous–Tertiary boundary.