Date of Award


Document Type

Restricted Thesis

Terms of Use

© 2006 Tyler R. Lyson. All rights reserved. Access to this work is restricted to users within the Swarthmore College network and may only be used for non-commercial, educational, and research purposes. Sharing with users outside of the Swarthmore College network is expressly prohibited. For all other uses, including reproduction and distribution, please contact the copyright holder.

Degree Name

Bachelor of Arts


Biology Department

First Advisor

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.