Pleurodira

The Pleurodira are one of the two living suborders of turtles (the other being the Cryptodira) more known commonly as the side-necked turtles. The name "Pleurodira" translates to "side neck", whereas the Cryptodira are known as hidden-necked turtles; these names reflect the distinct neck retraction mechanisms between the two suborders, which are the product of diverging evolutionary histories. Within the Pleurodira, three living families are represented: Chelidae, the Austro-South American side-necked turtles, the Pelomedusidae, the African mud terrapins, and the Podocnemididae, the American side-neck river turtles. Pleurodiran turtles are currently restricted to the freshwater ecosystems of former Gondwanan landmasses, including in Australia, South America, Arabia, Madagascar and Africa. Prehistorically, they were a cosmopolitan clade during the Cretaceous and most of the Cenozoic, and even occurred in marine environments around the world. The Pleurodira are identified by the method with which they withdraw their heads into their shells. In these turtles, the neck is bent in the horizontal plane, drawing the head into a space in front of one of the front legs. A larger overhang of the carapace helps to protect the neck, which remains partially exposed after retraction. This differs from the method employed by a cryptodiran, which tucks its head and neck between its forelegs, within the shell. The different methods of bending the neck require completely different anatomies of the cervical vertebrae.