Subfamilia Emydinae

Rafinesque, 1815
Pond turtles

The Clemmys complex of McDowell (1964). It has four genera, 10 species, including the genera Clemmys, Emydoidea, Emys, and Terrapene. The plastron of Clemmys is ridged, but that of the other three genera is hinged and movable. Clemmys is thought to be ancestral to the hinged forms of which Emys is most primitive, Emydoidea intermediate. and Terrapene most derived (Bramble, 1974). These turtles have the triturating surfaces of the jaws narrow and ridgeless with the upper triturating surface lacking parts of the palatine or the pterygoid. The orbito-nasal foramen is small (the posterior palatine foramen is much larger). The interorbital region is coarsely sculptured, and the postorbital bar is relatively wide. The jugal bone is separated from the palatine. On the plastron, the humero-pectoral seam crosses the entoplastron (except in some Emydoidea). The cervical vertebrae are not elongated in Clemmys, Emys, and Terrapene, but are in Emydoidea, which also has the cervical extensor muscle hypertrophied. The epipubes are ossified. Musk glands are present in all four genera.

Burke et al. (1996) studied the relationships among turtles of the Clemmys complex using a wide range of behavioral, morphological, life history, and ribosomal DNA (rDNA) characters, and found that, contrary to analysis of rDNA data alone, the genera Emydoidea, Emys and Terrapene form a monophyletic clade, and that, contrary to most previous studies, the genus Clemmys is paraphyletic. Within the genus Terrapene, T. nelsoni and T. ornata are more closely related to each other than to either T. carolina or T. coahuila, which is more widely separated from the other three species. Emydoidea and Emys form a close clade, but among the species of Clemmys, C. insculpta and C. muhlenbergii are closely related, while C. guttata, although closer to the other two eastern species, and the Pacific Coast C. marmorata are separate lineages. In fact, guttata and marmorata are more closely related to the Emydoidea and Emys clade than to C. muhlenbergii and C. insculpta, with marmorata showing the most affinity as a sister species.
Formerly, Emydoidea was included in a separate group with Deirochelys (Loveridge and Williams, 1957; McDowell, 1964). The two genera share similar neck, jaw, and rib structure, but these are probably convergent features related to similar feeding mechanism (Bramble, 1974). In addition, Jackson's (1978b) studies on fossil Deirochelys and electrophoretic studies by Seidel and Adkins (1989) indicate that it is more closely related to the genera Chrysemys, Graptemys, Malaclemys, Pseudemys, and Trachemys.