Genus Terrapene

Merrem, 1820
Box turtles

These four New World emydids are characterized by a high domed carapace and a plastron with a single hinge between the pectoral and abdominal scutes which divides the plastron into two movable lobes. A medial keel is present on some species but not others, and the unserrated posterior carapacial rim may or may not be flared. Growth annuli may sculpture the carapacial surface in younger individuals, but become worn smooth with age. Neurals are hexagonal and shortest anteriorly; the 8th is absent so the corresponding costals meet at the carapacial midline. There is no bridge or corresponding buttresses; the plastron and carapace are joined at a non-bony connection. The entoplastron is intersected by the humero-pectoral seam. The skull is short with an incomplete temporal arch. The pterygoid contacts the basioccipital. The jugal is tapered to a point ventrally and does not touch the pterygoid, and the frontal bone barely enters the orbit. Orbito-nasal foramina are small. Triturating surfaces are narrow and lack ridges. The toes are partially webbed, and the dorsal surface of the head is covered with smooth skin.

Minx (1996) phylogenetically analyzed 32 morphological characters to determine the most parsimonious relationships among all 11 taxa of in the genus Terrapene. Two species groups were recognized: the ornata group (T. ornata, T. nelsoni) and the carolina group (T. carolina, T. coahuila). Terrapene ornata and T. nelsoni are sister taxa in the ornata group, with T. nelsoni slightly more basal. Terrapene coahuila is the basal clade of the carolina group. Among the various subspecies of T. carolina, T. c. major is the most basal clade with the others comprising two monophyletic groups. The first is slightly more basal than the second, and contains T. c. carolina and T. c. bauri. The second contains the two Mexican subspecies T. c. yucatana and T. c. mexicana, in addition to T. c. triunguis, which seems to be the most derived of the subspecies of T. carolina.

Species identification
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