• DOI:10.3374/014.050.0202
• Corpus ID: 85505337

Two New Plastomenine Softshell Turtles from the Paleocene of Montana and Wyoming

@inproceedings{Joyce2009TwoNP,
  title={Two New Plastomenine Softshell Turtles from the Paleocene of Montana and Wyoming},
  author={Walter G. Joyce and Ariel Revan and Tyler R. Lyson and Igor G. Danilov},
  year={2009}
}

• W. Joyce, A. Revan, +1 author I. Danilov
• Published 10 November 2009
• Biology

ABSTRACT The many fossil turtle remains collected by Princeton University throughout the 1950s and 1960s from Paleocene sediments in Montana and Wyoming include two new species of plastomenine softshell turtles (Trionychidae) that are referred to Hutchemys gen. nov., which corresponds to the previously used informal taxon “Plastomenine Type A.” Hutchemys is unique among trionychids in having a broadly rounded entoplastron that is underlain by a large rectangular callosity and fully immobilized through lateral processes of the hyoplastra. Hutchemys rememdium sp. nov. is documented by specimens from the Torrejonian of Montana and the Tiffanian of Wyoming, whereas Hutchemys arctochelys sp. nov. is only known from a collection of shells from a single Clarkforkian locality in Montana. The holotype of a third taxon, Aspideretes nassau Hay 1908, is diagnostic for Hutchemys, but cannot be diagnosed to the species level and is thus considered a nomen dubium. A phylogenetic analysis places the new species in a clade that is sister to Plastomenus aff. thomasii along the phylogenetic stem of Cydanorbinae, thus tentatively confirming monophyly of Plastomeninae and a close relationship to cyclanorbines. However, given that many supposedly diagnostic postcranial characters of Plastomeninae are often found among trionychines as well, in particular characters pertaining to the extensively ossified plastron, we suggest that a wide and rounded entoplastron is a more useful character in diagnosing this clade. Representatives of Hutchemys seem to have preferred swampy habitats and their shell converges on that of testudinoid or pleurodiran pond turtles in the degree of ossification and the presence of an anterior plastral hinge.