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.
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