The 12 species of Graptemys are North American riverine turtles highly adapted to a molluscivorous diet. Their oval carapaces bear a medial keel of varying development from a low, raised mound to well-developed, spiny or blunt protuberances; the posterior rim is strongly serrated. Neurals are usually hexagonal and anteriorly short sided, but the second is sometimes octagonal. The carapace usually bears a pattern of narrow, light, meandering lines or ocelli on the pleurals, vertebrals, and, sometimes, marginals which resembles contour lines on a map; thus, their common name of "map" turtles. The plastron lacks a hinge; axillary and inguinal buttresses are not well-developed. The entoplastron lies anterior to the humero-pectoral seam. The skull is usually narrow to moderately broad in males, but broader in females. It has a complete temporal arch, and the orbito-nasal foramen is enlarged into a fenestra. The flattened triturating surfaces of the jaws are broad and ridgeless. The anterior border of the inferior process of the parietal is thickened, and the pterygoid touches the exoccipital. The toes are webbed.
McDowell (1964) revived Boulenger's inclusion (1889) of Graptemys in the genus Malaclemys. Graptemys was not recognized even as a subgenus, because the differences between M. terrapin and G. pseudogeographica kohnii are no greater than those between G. p. kohnii and G. geographica. This arrangement was not widely accepted (see, for example, Ernst and Barbour, 1972; and Dobie and Jackson, 1979). Wood (1977) felt that McDowell had not made a convincing case for the all-inclusive Malaclemys and presented other evidence suggesting the origin of many, if not all, Graptemys from Malaclemys. However, Dobie (1981) showed Malaclemys to differ substantially from Graptemys in osteology. This point was settled most decisively by Lamb and Osentoski (1997) who generated a data set composed of DNA sequences for the mitochondrial control region and cytochrome b gene. They generated 32 different parsimonious trees from the sequence data that, all of which showed Graptemys and Malaclemys as distinct, monophyletic clades, refuting Wood's (1977) hypothesis.
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