New cranial material of Gilmoremys lancensis (Testudines, Trionychidae) from the Hell Creek Formation of southeastern Montana, U.S.A.

ABSTRACT

Plastomenidae is a speciose clade of soft-shelled turtles (Trionychidae) known from Campanian to Eocene deposits throughout western North America. We here describe two large skulls from the Upper Cretaceous (Maastrichtian) Hell Creek Formation of Carter County, Montana, that document the adult morphology of the plastomenid Gilmoremys lancensis. Whereas juveniles of this species, as previously documented by five subadult skulls, have narrow skulls, a narrow processus trochlearis oticum, a deep and narrow median palatal groove, low accessory ridges, and a secondary palate fully formed by the maxilla, skeletally mature individuals have notably broad skulls, a broad processus trochlearis oticum, a shallow but broad median palatal groove, high accessory ridges, and a substantial contribution of the vomer to the secondary palate. An expanded phylogenetic analysis reveals that the Campanian Aspideretoides foveatus and the Paleocene Aspideretoides superstes, nov. comb., are situated within Plastomenidae as sister to all previously identified plastomenid turtles, despite their general resemblance to trionychine soft-shelled turtles. The name Aspideretoides should therefore not be used as a taxonomic wastebasket for fossil trionychids with unclear phylogenetic relationships.

SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP

Citation for this article: Joyce, W. G., T. R. Lyson, and S. Williams. 2016. New cranial material of Gilmoremys lancensis (Testudines, Trionychidae) from the Hell Creek Formation of southeastern Montana, U.S.A. Journal of Vertebrate Paleontology, e1225748. DOI: 10.1080/02724634.2016.1225748.

ACKNOWLEDGMENTS

All Burpee Museum of Natural History paleontological field work in Carter County, Montana, was conducted on Public Lands managed by the Bureau of Land Management (BLM), under permit no. 100239. We wish to thank G. Liggett and D. Melton at the Miles City BLM office for all their efforts. We thank Bob and Mark Youngren of the Estwing Company, Jay and Barbara Brost, C. Fowler, S. Landi, R. Moore and N. Englehardt-Moore, and Dr. Chris and Krissy Vittore for funding Burpee field work. We would like to thank K. Tremaine and J. Mathews for field work supervision and fossil preparation and S. Clawson for fossil preparation. For additional logistical assistance while in the field, we thank Tom and Margaret Blair, N. Carroll and the Carter County Museum, Camp Needmore and the town of Ekalaka, Montana, as well as all of the Burpee Highway to Hell Creek field volunteers and support staff. We finally wish to thank P. Meylan, J. Sterli, and N. Vitek for providing us with constructive comments that helped improve the quality of the manuscript.