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The Late Cretaceous (Maastrichtian) Hell Creek and Lance formations in the states of North Dakota, South Dakota, Montana, and Wyoming, USA, have yielded rich remains of trionychid turtles, but the alpha taxonomy of the group is still poorly resolved, as most fossils are fragmentary. We here describe a new species of trionychid, Axestemys infernalis sp. nov., based on a rich assortment of skulls, shells, and postcranial remains collected from nine fossiliferous sites spread throughout these formations. The new species is attributable to the Axestemys lineage based on the combined presence of large size, a preneural, and a single lateral hyoplastral process. Axestemys infernalis sp. nov. differs from all Paleogene representatives of Axestemys by lacking elongate free ribs and by possessing costal sculpturing that reaches the margin of the shell. It further differs from the only other Mesozoic representative of the lineage, Axestemys splendidus, a species herein restricted to the Campanian, by possessing a rounded carapace. The available Axestemys infernalis sp. nov. material shows much variation, in particular in regards to the maximum size of individuals and the relative size of callosities, but not enough to warrant recognizing multiple species. Instead, this variation is likely caused by random drift or phenotypic variation that accrued over the course of the approximately 1.36 Ma represented in the Hell Creek Formation. Walter G. Joyce. Departement für Geowissenschaften, Universität Freiburg, 1700 Freiburg, Switzerland. walter.joyce@unifr.ch Donald Brinkman. Royal Tyrell Museum of Palaeontology, Box 7500, Drumheller, Alberta, Canada and Department of Biological Sciences, University of Alberta, T6G 2E9, Edmonton, Alberta, Canada. Don.Brinkman@gov.ab.ca Tyler R. Lyson, Department of Earth Sciences, Denver Museum of Nature Science, Denver, Colorado 80205, USA. Tyler.Lyson@dmns.org JOYCE ET AL.: A NEW SPECIES OF TRIONYCHID 2

palaeontologia electronica

A new species of trionychid turtle, Axestemys infernalis sp. nov., from the Late Cretaceous (Maastrichtian) Hell Creek and Lance formations of the Northern Great Plains, USA