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Late Cretaceous and early Paleocene turtles of the Denver Basin, Colorado

Museum of Paleontology, 1101 Valley Life Sciences Building, University of California, Berkeley, CA 94720-4780, U.S.A.

^* Author to whom correspondence should be addressed: pholroyd{at}uclink4.berkeley.edu.

The record of turtles in the Denver Basin spans four formations (Fox Hills, Laramie, Arapahoe, and Denver) that range from Late Cretaceous (Lancian) to early Paleocene (Puercan) in age. We recognize at least fifteen distinct, although fragmentary, species-level 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 the best-preserved skull of Compsemys, 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. We also present evidence for synonymy of Paleotrionyx and Conchochelys with Axestemys, and the first cranial material assignable to Axestemys. The early Paleocene (Puercan) part of the Denver Formation yielded the most diverse assemblage, followed by the Cretaceous (Lancian) part of the Denver Formation and Laramie Formation. The Cretaceous samples are not demonstrably different from more northerly comparable faunas, 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.

Key Words: Chelonii (= Chelonia) • Paleocene • Cretaceous • biogeography • Denver Basin • Arapahoe Formation • Fox Hills Sandstone • Denver Formation • Laramie Formation

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