ABSTRACT
Testudinata comprises all crown-group turtles, the Testudines, as well as the stem group, with the oldest representatives from the Triassic period. The turtle body plan is unique among vertebrates in that part of the endoskeleton, i.e., the ribs and parts of the vertebral column, are incorporated in a more or less rigid shell. Of the skeleton, the limb bones and the bones of the dorsal and ventral parts of the shell (carapace and plastron) have been reasonably well sampled for histology. Many studies have focused on skeletochronology and its ability to assess the age of individuals and life history patterns, especially of extant species, whereas other studies have looked for phylogenetic and functional signals in the bones of extant and extinct turtles. The shell bones were found to present a mixture of slow-growing tissues (parallel-fibered and lamellar bone) and parts of metaplastically ossified skin. Long bones usually show slowly deposited tissues, but a direct relationship between bone compactness and habitat, as in other amniotes, is not evident. Shell and long bones of exceedingly aquatic taxa typically remodel compact and cancellous components into a homogenous and well-vascularized spongy tissue.
