A NEW LATE CRETACEOUS MAMMAL FAUNULE FROM NORTHEASTERN COLORADO AND COMPARISONS WITH OTHERS FROM THE WESTERN INTERIOR OF NORTH AMERICA

Mammalian faunas from the Latest Cretaceous of North America are best known from the northern Western Interior. Few Cretaceous mammal fossils have been recovered from the Denver Basin; however, other components of the fauna hint at potential biogeographic differences with contemporaneous northern faunas. In the nearby Julesburg Basin (Weld County, NE Colorado), a mammalian fossil sample was recovered from the Laramie Formation in 1977. Although initially considered contemporaneous with faunas from the Lancian NALMA, recent extension of an age model developed for the Denver Basin suggests that it may be older (69-67 Ma) than most known Lancian faunas (67-65.5 Ma).

In 2006, we returned to the Julesburg Basin to initiate an intensive paleontological survey of Laramie Formation exposures on Pawnee National Grasslands. Ongoing exploration has led to the discovery of 10 microvertebrate localities, which have thus far yielded 25 mammalian specimens via surface collection and screenwashing. Most specimens are referable to known Lancian genera, but a lower jaw with three teeth (p4-m2) is referable to a new genus and species of neoplagiaulacid multituberculate. Using a model based on electric well logs from the area, we have stratigraphically arranged fossil localities relative to the underlying Fox Hills Sandstone. As we increase fossil sample sizes and integrate the Julesburg Basin into the age model developed for the Denver Basin, we aim to document the succession of Late Cretaceous mammalian faunas in the Julesburg Basin and draw biogeographic and temporal comparisons with faunas from other areas in the Western Interior of North America.