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Chelonia:
The Chelonia form a well-defined group of tetrapods which is mainly characterized by the development of a shell and the derivation of a stegocephalian skull. A saltatory formation of a chelonian shell in the geological record, which led to the migration of the limb girdles to positions inside the rib cage, has caused a big interest to the questions connected with the formation of the shell and origin of the group. The existence of a temporal and morphological hiatus between most turtles, on the one hand, which lost teeth but already had a shell, and their presumable ancestors among Paleozoic and Mesozoic tetrapods, on the other hand, having teeth but didn’t developed a shell of chelonian type made the chelonian origin one of the most fascinating problems in herpetology and paleoherpetology.
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