Planetochelys savoiei Weems 1988

Robert & Weems, E., 2014, Paleogene chelonians from Maryland and Virginia, PaleoBios 31 (1), pp. 1-32 : 10

Planetochelys savoiei Weems 1988

Specimen —USNM 412107, posterior half of carapace, described by Weems (1988).

Locality, horizon, and age —Found along the western bank of Aquia Creek at the base of “zone 2” of the lower Piscataway Member of the Aquia Formation ( Clark and Martin 1901); late Paleocene (early Thanetian).

Remarks — Weems (1988) assigned this taxon to the family Sinemydidae Yeh 1963 but new material and more extensive descriptions of Sinemys Wiman 1930 ( Brinkman and Peng 1993, Sukhanov 2000) have documented a suite of diagnostic features that do not support close relationship between Sinemys and Planetochelys . Hutchison (2012) has restudied the type specimen of Planetochelys savoiei , reinterpreted its suprapygal region, and described a new closely related species from the western United States, P.dithyros Hutchison 2012 . Unlike the type specimen of P. savoiei , the new species includes the anterior portion of the carapace and plastron.The occasional presence in P. dithyros of extragular scales indicates that it is too primitive to be included within the chelydrid-testudinoid lineage and most likely lies near the adocid grade of trionychians ( Hutchison 2012). Planetochelys has a number of specializations that show it to have been a terrestrial turtle that had evolved a carapace and plastron much like that of a “box turtle,” even though it is not closely related to the emydid and kinosternid turtles that independently have evolved a plastral hinge to allow the shell to partly or wholly close. Because of its unique combination of primitive and derived features, Planetochelys is not particularly close to any other turtles and has been placed in its own family ( Hutchison 2012).