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New material of Gilmoremys lancensis nov. comb. (Testudines: Trionychidae) from the Hell Creek Formation and the diagnosis of plastomenid turtles

Published online by Cambridge University Press:  14 July 2015

Walter G. Joyce  and
Tyler R. Lyson
Walter G. Joyce
Affiliation:
1Institut für Geowissenschaften, University of Tübingen, Sigwartstraße 10, 72076 Tübingen, Germany, 2Yale Peabody Museum of Natural History, 170 Whitney Avenue, New Haven, CT 06511, USA
Tyler R. Lyson
Affiliation:
3Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT 06511, USA, 4Marmarth Research Foundation, Marmarth, ND 58643, USA
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Abstract

Plastomenidae is a poorly diagnosed clade of extinct soft-shelled turtles (Trionychidae) known from the Campanian to Eocene of North America. Five skulls, a mandible, two carapaces, and numerous plastral remains from the Hell Creek Formation (Late Cretaceous, Maastrichtian) of North Dakota and Montana are referable to Gilmoremys lancensis nov. comb., a taxon previously known from a carapace and xiphiplastron only. Gilmoremys lancensis is diagnosed by a carapace that is covered by elongate sinusoidal grooves, distally expanded second costals, hyoplastral shoulders, an extensive secondary palate with accessory ridges, an extremely elongate mandible, a contribution of the parietal to the wall of the orbit, and a posterior ossified narial canal. A phylogenetic analysis of all well-known plastomenid turtles establishes Gilmoremys lancensis as the most basal known plastomenid and reveals that cranial characters are more reliable in diagnosing plastomenid turtles, in particular the contribution of the parietal to the orbit wall and the extensive secondary palate. All plastomenid turtles with a locked entoplastron are placed in Hutchemys. Assuming that all taxa are monophyletic, the phylogenetic analysis implies that the G. lancensis lineage is the only one to go extinct at the K/T boundary, whereas the four remaining plastomenid lineages survive. Extensive ghost ranges are nevertheless apparent. Taphonomic considerations indicate that G. lancensis was a riverine turtle, whereas more derived plastomenids preferred swampy habitats.

Type
Research Article
Information
Journal of Paleontology , Volume 85 , Issue 3 , May 2011 , pp. 442 - 459
Copyright
Copyright © The Paleontological Society 

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