Cryptodira
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Cryptodira is the taxonomic suborder of Testudines that includes most living tortoises and turtles. Cryptodira differ from Pleurodira, or side-neck turtles, in that they lower their neck, and pull the head straight back into the shell, instead of folding the neck sideways along the body.
Classification and evolution
Cryptodires evolved primarily through the Jurassic period, and by the end of the Jurassic, had almost completely replaced Pleurodires in the lakes and rivers, while beginning to develop land-based species. Cryptodira has four main families:
Meiolaniidae
The Meiolaniidae family of giant turtles became extinct around 2 million years ago, and was noted for having developed large bony spikes on the head and tail for protection. Meiolania is a Pleistocene era example of this family.
Testudinidae
The most successful of the cryptodires belonged to the family Testudinidae, which encompasses all modern day land tortoises. The largest known tortoise species is a now extinct creature belonging to this family known as the Testudo. Examples have been found indicating a length of over 8 feet, and a weight of over 4 tons.
Protostegidae
Protostegidae, an ancient family of sea turtles, these large, highly specialized turtles had lightweight shells, and broad flippers. They lacked the ability to retract their heads.
Trionychidae
The family Trionychidae makes up most of our modern soft-shell turtles, and comprises over 30 species living in the freshwater lakes and rivers of North America, Asia and Africa.
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