ABSTRACT
Chelonians are in many ways unique among the tetrapods. Above all, the presence of the shell, with girdles that lie within the rib cage and limbs emerging laterally from the anterior and posterior apertures of the shell (Webb et al., 1978), affects various aspects of the biology and morphology of all turtles. Various chelonian species inhabit a variety of marine, freshwater, and terrestrial habitats. Morphologically, a turtle’s leg design corresponds to its habitat: marine chelonians have paddles, whereas terrestrial and freshwater turtles have more typical tetrapod limbs. At the current time, the plesiomorphic limb design remains unknown. Extant marine turtles appear to be derived from an amphibious ancestor but it is possible that other extinct groups of marine turtles existed that had different derivations (Renous, 1995; Pritchard, 1979). In cryptodires, it appears that the oldest turtles were amphibious (Romer, 1968; Gaffney et al., 1987). In this case, both marine and terrestrial conditions would be apomorphic. There are important differences in the long bones of marine and non-marine turtles, mainly in proximal bones (humerus and femur); these bones tend to have curved diaphyses in non-marine forms, whereas they are straighter in marine forms (Romer 1968). Alternatively, there is a lengthening of both metacarpus and metatarsus in the autopods of marine turtles (Guibé, 1970).
