The semi-aquatic and semi-terrestrial New World pond turtles and their European relative Emys orbicularis compose the diverse family of turtles, the Emydidae. Today it is represented only in the Americas, Europe and northern Africa; however, the fossil record indicates that the family was formerly more widespread in Europe than at present. The oldest fossil emydids are Gyremys sectabilis from the Upper Cretaceous Judith River formation of Montana and Clemmys backmani from the Paleocene Ravenscrag formation of Big Muddy Valley, Saskatchewan. Several other genera are known from Eocene deposits in Europe (Emys) and North America (Echmatemys). Fossils dating from the Oligocene and the Pleistocene have shown increased diversity in this family, and the living forms are represented by 10 genera and 40 species. The literature on a few of the species is sparse, but most are among the best studied species of turtles.
Species included in Emydidae share several osteological characters. Their skulls are relatively small and similar to those of the batagurids (Bataguridae) and tortoises (Testudinidae). The temporal region is widely emarginated posteriorly preventing squamosal-parietal contact. The frontal bone enters the orbit, and the postorbital is wider than that found in the Testudinidae. The maxilla and quadratojugal are separated, and the quadrate is exposed posteriorly. The premaxillae usually do not meet to form a hooklike process. The triturating surface of the upper jaw varies from broad to narrow, and may be ridged or not. There is a tendency toward the development of a secondary palate. A vestigial splenial bone is present. The lower jaw has the angular bone touching Meckel's cartilage. The basioccipital is narrow, separated from the paracapsular sac and pterygoid, lacks a lateral tuberosity, and does not form the floor of the cavity scala tympani. A double articulation occurs between the centra of the 5th and 6th cervical vertebrae, and the suture between the supracaudals and last vertebral lies over the pygal bone, not the suprapygal. The carapace is usually low arched, but may be considerably domed in some species, particularly Pseudemys and Terrapene. A vertebral keel may be present or absent; lateral keels are normally absent. Dorsal rib heads are well-developed. Carapaceandplastron are usually united by a broad bridge (absent in Terrapene). The plastron is well-developed, and in several genera may bear a moveable hinge between or near the seam separating the pectoral and abdominal scutes. Mesoplastral bones are absent, as also are intergular and inframarginal scutes. Limbs are developed for swimming. Usually more than two phalanges occur in the second and third digits, and the femoral trochanteric fossa is reduced. In the pelvic girdle, the pubis normally contacts the ischium of the same side, but cartilage along the ventral midline of the girdle prevents the pubis and ischium from touching those from the opposite side.
With the exception of the genus Emys, which ranges from Europe and northern Africa into the Middle East, emydids are found from southern Canada to central South America and the West Indies. One species, the slider Trachemys scripta elegans, has been introduced through released pets and is living wild and reproducing on all hospitable continents and many islands.
In the Emydidae (together with the Bataguridae formerly considered subfamilies of one large family; see Ernst and Barbour, 1989), two complexes were recognized by McDowell (1964) and Bramble (1974): the Clemmys complex (including Clemmys, Terrapene, Emydoidea, and Emys) and the Deirochelys (Chrysemys) complex (including Graptemys, Malaclemys, Chrysemys, Deirochelys, Trachemys, and Pseudemys). On the basis of morphological traits, Gaffney and Meylan (1988) subdivided the Emydidae into two subfamilies—Emydinae (former Clemmys complex) and Deirochelyinae (former Deirochelys /Chrysemys complex). Recent studies by Seidel and Adkins (1989) and Bickham et al. (1996) have upheld this arrangement.