Date of Award
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Bachelor of Arts
Scott F. Gilbert
Two localities located in the Hell Creek Formation (latest Maestrichtian) of southwestern North Dakota have produced material from over 60 baenid turtles, including Eubaena cephalica, Palatobaena, two new taxa, and the first known occurrence of Plesiobaena putorius in the Cretaceous period. The two new baenid taxa are described based on skull material. Gamerabaena sonsallai, gen. et sp. n., and Stygiochelys gilberti, sp. n., are identified as Eubaenines based on the presence of the following characters: a broadly concave maxillary triturating surface, a deep temporal emargination with no parietal/squamosal contact, and a C-shaped quadratojugal. A high resolution baenid phylogenetic analysis supports the Baenidae phylogenetic framework of Gaffney and Meylan. The analysis regards S. estesi and S. gilberti as a monophyletic group and considers G. sol1sallai as the sister taxon of those Eubaenine genera. An analysis of the large amount of material collected indicates intraspecific variation within the shells of E. cephalica and the skulls of the closely related P. bairdi. Like many extant turtles, the shell of E. cephalica is sexually dimorphic. The male's plastron concavity is significantly greater than the female's plastron concavity. There is also considerable variation among the three complete Palatobaeno skulls found at one of the localities. The taxon varies in the overall height of the skull, degree of pterygoid contact, degree of temporal emargination, bones that make up the stapedial foramen, and degree of nasal excavation. The considerable variation, which makes up two distinct groups, indicates that the naming of an additional species may be warranted.
Lyson, Tyler R. , '06, "An assemblage of baenid turtles from the Hell Creek Formation: intraspecific variation, two new taxa, and a phylogenetic analysis of the Baenidae" (2006). Senior Theses, Projects, and Awards. 71.