EVOLUTION

Turtle evolution goes back to the Triasic Period during the Mesozoic Era, 200 million years ago, even before dinosaurs completely ruled the Earth. The task of establishing the origin of turtles has been complicated by the scarcity of fossils from the most primitive forms known to have existed. Nonetheless there are those who classify the cotilosaurus as a possible ancestor. These animals, from the Lower Permian, had an anapside skull like modern turtles, which is considered an important characteristic in evolution.

Fossils of the genera Meiolania from the Jurassic Period have been found. This was a gigantic terrestrial turtle that had horns on the head and a tail ending in spiky armor (both structures are covered by horny sheaths). Numerous species of the group became extinct in the Superior Cretacic period during which gigantic sea turtles, contemporaneous of ictiosaurus and mosaurus (Ictiform lizards with several meters long) existed in North America. Two examples of this sea turtles are Archelon and Protostega  (suborder Cryptodira), animals more than three meters long and four meters from one end of a flipper to the other.  Both had a carapace and plastron covered with horny scutes like modern turtles.

Cheloniidae, the family that includes most of the modern sea turtle species, had its first representatives at least 100 million years ago at the end of the Mezosoic era, in the Superior Cretacic. It had a great number of turtle genera that proliferated in all the tropical oceans, but most of them became extinct before the end of the Cenozoic era. In the present, the family consists of seven genera that comprise six species.

Currently, only two families of marine turtles exist: Cheloniidae with 6 species and Dermochelydae
with just one species.

The exact moment in which Dermochelydae originated is not clear.  Nonetheless, individuals of four different genera appear in the fossil records since the Eocene, in the Cenozoic Period.

All the species of present sea turtles belong to the suborder Cryptodira, which includes turtles that can retract their neck in a vertical plane and whose plastron bones usually do not include a mesoplastron. Most of the species of sea turtles have bony scutes that cover the shell dorsally as well as ventrally.