All about Sea Turtles.
Sea turtles, along with other turtles and tortoises, are part of the order Testudines. All species except the leatherback are in the familyCheloniidae. The leatherback is the only extant member of the family Dermochelyidae.
In general, sea turtles have a more fusiform body plan than their terrestrial or freshwater counterparts. The reduced volume of a fusiform body means sea turtles can not retract their head, legs, and arms into their shells, like other turtles can.
The species are primarily distinguished by their anatomy: for instance, the prefrontal scales on the head, the number of and shape ofscutes on the carapace, and the type of inframarginal scutes on the plastron.
The leatherback is the only sea turtle that does not have a hard shell. Instead, it bears a mosaic of bony plates beneath its leathery skin. It is the largest sea turtle, measuring 6 to 9 feet (1.8 to 2.7 m) in length at maturity, and 3 to 5 feet (0.91 to 1.52 m) in width, weighing up to 1,500 pounds (680 kg). Other species are smaller, being mostly 2 to 4 feet (0.61 to 1.22 m) and proportionally narrower.
Sea turtles constitute a single radiation that became distinct from all other turtles at least 110 million years ago.[