A new baenid turtle from the Upper Cretaceous (Maastrichtian) Hell Creek Formation of North Dakota and a preliminary taxonomic review of Cretaceous Baenidae

ABSTRACT

A fragmentary skull from the Hell Creek Formation (Maastrichtian) of southwestern North Dakota represents a new taxon of baenid turtle named herein Gamerabaena sonsalla. The length of the frontals, jugal contribution to the labial ridge, and convex contact between the vomer and the pterygoids indicate its affinities with the clade Palatobaena, but the new taxon clearly lacks the great posterior expansion of the triturating surface, complete absence of a lingual ridge, subrectangular skull, and wide angle between the maxillae that diagnose Palatobaena spp. A maximum parsimony analysis provides strong support for G. sonsalla as sister taxon to Palatobaena spp. Gamerabaena sonsalla has several morphological features that are intermediate between Plesiobaena antiqua and the morphologically disparate Palatobaena spp., including orbits that are oriented slightly dorsally and moderately expanded posterior triturating surfaces. Our phylogenetic analysis, combined with stratigraphic arguments, indicates that our skull-based taxon G. sonsalla could belong to the shell-based taxon “Baena” hayi. Similarly, the skull taxa Hayemys latifrons and Eubaena cephalica may be synonymous with the shell taxa Thescelus insiliens and “Baena” hatcheri, respectively.

ACKNOWLEDGMENTS

We wish to thank the landowners, D. and M. Sonsalla, for donating the specimen to the North Dakota Heritage Center. Jim Parham, S. Modesto, and an anonymous reviewer made suggestions that significantly improved the manuscript. William Pinder assisted in the preparation of the specimens and J. Lyson and E. Schachner drew the figures. Funds for the preparation and illustrations of the specimen were provided by NSF grant IBN-0316025 to S. Gilbert. Additional funding for this project was provided by a National Science Foundation Graduate Research Fellowship to T.R.L. Much of the work for this paper was completed while T.R.L. was working with S. Gilbert at Swarthmore College.