Creature Feature 26

Hello, fans! It's been a long time since I've posted, but we now have a new model ready to present. Meet Emarginochelys cretacea, the earliest known Chelydrid turtle!

 Emarginochelys cretacea model, unpainted.

As stated above, Emarginochelys cretacea is the earliest known member of the family Chelydridae. The only extant members of this family are Chelydra and Macrochelys - the snapping turtles. The genus was spelled as Emarginachelys in the original description (Whetstone, 1978), but has since been changed to Emarginochelys (Bryant, 1989; Holroyd & Hutchison, 2002). Luckily, the holotype for this species is actually housed here at KU, so I was able to examine the specimen myself to make observations on anatomy.

Several features unite Emarginochelys with Chelydridae. These include a cross-shaped plastron attached to the carpace by ligaments, a reduced entoplastron, and several other skeletal characters (Whetstone, 1978). Furthermore, its robust limbs and heavy body indicate that it likely would have walked along the bottom of channels or swampy areas.

However, there are several notable differences that separate this basal species from its more derived kin. Several differences involve the structure of the shell. In Emarginochelys, the neurals (the scutes running down the midline on the carapace) and peripherals (the scutes along the border of the carapace) are much thicker than in derived forms. Furthermore, there is no emargination at the anterior end nor scalloping at the posterior end.

Perhaps the most interesting differences from its modern relatives are the features seen in the skull. Emarginochelys completely lacks the premaxillary "hook" seen in derived forms, nor does it have the extremely developed parietals - both of these features are important in feeding for modern chelydrids. In modern chelydrids, the sharp hooked shape of the beak is critical in grasping prey, and the large parietals (at their most extreme in Macrochelys) provide a large area for jaw muscle attachments (which is why snapping turtles can do so much damage). Emargionchelys, therefore, would have likely had a significantly weaker bite than its derived kin, and would not have been able to grasp prey in the same way. It's quite possible that its diet was not the same as derived snappers, and it may have been a generalist omnivore, with derived features representing specialization towards carnivory. Interestingly enough, another species of turtle from Hell Creek, Compsemys victa, had a skull indicating a highly carnivorous diet, yet its modern relatives are herbivores.

Unfortunately, not much work has been done with this species as far as I've seen. I was lucky enough to see the holotype myself, which allowed us to reconstruct the neck since there were no figures showing the cervical vertebrae in the original publication (there are a few vertebrae missing but we took that into account with our model). Unfortunately, no caudal vertebrae were preserved, so the tail length is based on proportions for modern relatives.

Hope this has taught you a bit more about this interesting turtle!

Acknowledgements:
Whetstone, Kenneth N. 1978. A New Genus of Cryptodiran Turtles (Testudinoidea, Chelydridae)
From the Upper Cretaceous Hell Creek Formation of Montana. University of Kansas Science Bulletin 51(17): 539-563.
Bryant, L. J. 1989. Non-dinosaurian lower vertebrates across the Cretaceous-Tertiary boundary in northeastern Montana. University of California Publications in Geological Sciences 134:1-107
Holryod, P. A.; Hutchison, J. H. 2002. Patterns of geographic variation in latest Cretaceous vertebrates: evidence from the turtle component. Geological Society of America Special Paper 361:177-190