The turtles from ‘Gilmore’s slab’: evolution and diversity of the extinct turtle Echmatemys (Testudines: Testudinoidea: Geoemydidae) from the central United States

This contribution presents a complete revision of the extinct geoemydid Echmatemys from North America, based on new information and a re-evaluation of previously published specimens. Echmatemys is the most diverse and widely distributed geoemydid turtle in the fossil record of North America, known from numerous shells. The different morphotypes are assigned to the currently accepted valid species and their evolutionary history is evaluated in a cladistic context. Their function is further evaluated with an anatomical network analysis. This re-appraisal is based mainly on ‘Gilmore’s slab’, a slab containing several turtle shells collected from the Bridgerian of Levett Creek (Wyoming, USA). The fossils from this slab are described in detail for the first time and are completely re-identified. As this slab represents a snapshot of the turtle community in past Wyoming during the Bridgerian it offers a unique opportunity to discuss the evolution and diversity of Echmatemys. The morphological variability in the shells of Echmatemys challenges its recovery as a monophyletic group under parsimony analysis. Also, most Echmatemys spp. are recovered in a derived position within Geoemydidae, a result that poses new problems in the interpretation of the origin and the palaeobiogeographical evolution of this clade in North America.