Early Kinosterninae (Reptilia: Testudines) and their phylogenetic significance

Two genera of kinosternids in the Eocene of North America exhibit specializations of the Kinosteminae such as the presence of incised musk ducts, loss of one suprapygal, and a generally derived neural pattern. The shell of Baltemys staurogastros, gen., et sp. no v., from the Wasatchian (early Eocene) of Wyoming shows striking phenetic resemblance to that of Staurotypus but these are judged as primitive features on the basis of out-group analysis. Xenochelys lostcabinensis, sp. nov., from the Wasatchian and X. bridgerensis, sp. nov., from the Bridgerian of Wyoming exhibit additional derived features of the extant kinosternines such as the reduction in musk duct length, development of an intergular scale, and expansion of the plastron. Baltemys and Xenochelys lack plastral kinesis. The morphology of the fossil forms bridges much of the morphological gap between the extant Kinosterninae and Staurotypinae. The wealth of derived morphological features that the two subfamilies share indicate that they are sister groups and best retained within the concept of the Kinosternidae. Baltemys and Xenochelys were contemporaneous during the Wasatchian and Bridgerian. Their cladistic relationships and stratigraphie occurrence indicate that the division of Staurotypinae and Kinosteminae (sensu lato) predates the early Eocene. The fragmentary record of the family suggests divergence times between 56 to 65 million years for the Staurotypinae and Kinosteminae.