LATE CRETACEOUS AND EARLY PALEOCENE TURTLE BIOGEOGRAPHY: NEW DATA FROM THE DENVER BASIN, COLORADO

Turtles are among the most abundant vertebrate fossils in the latest Cretaceous to early Tertiary and one of the few groups relatively unaffected by K-T extinction events. For these reasons, they are extremely useful for assessing hypothesized patterns of vertebrate biogeography and faunal provinciality through this interval. The Denver Basin, central Colorado, occupies a notable gap between better-known northern and southern faunas, and this fauna has not been studied for more than 100 years. The record of turtles in the Denver Basin spans five formations (Fox Hills, Laramie, Arapahoe, Dawson, and Denver) that range from Late Cretaceous (Lancian) to early Paleocene (Puercan) in age. Although fragmentary, we recognize at least fifteen distinct taxa, including Pleurosternidae (Compsemys), Baenidae (Neurankylus, Plesiobaena, Stygiochelys, and Palatobaena), Kinosternia (Hoplochelys), Adocidae (Adocus), Nanhsiungchelyidae (Basilemys), Trionychidae (Axestemys, Aspideretoides, Helopanoplia, and another plastomenine), Macrobaenidae and Chelydridae. Notable among these are a new chelydrid genus, the most northerly confirmed record of Hoplochelys, and the most southerly records of Helopanoplia, Stygiochelys, and Macrobaenidae in the Rocky Mountain region. The early Paleocene (Puercan) part of the Denver Formation yielded the most diverse assemblage, followed by the latest Cretaceous part of the Denver Formation and Laramie Formation. The Cretaceous samples are not demonstrably different from other latest Cretaceous faunas in the northern Great Plains, but the more diverse Paleocene part of the Denver Formation exhibits a unique combination of taxa compared to contemporaneous faunas to the north and south. These differences reflect both possible latitudinal gradients in distribution as well as the lack of coastal habitats in the Denver Basin.