There are fourteen extantfamilies of the orderTestudines, an order of reptile commonly known as turtles, tortoises and terrapins. The testudines are some of the most ancient reptiles alive, with only the tuataras considered more primitive. There are approximately 300 extant species and 97genera of testudines, split into two suborders: the Cryptodirans and the Pleurodirans. The distinction between these two suborders is based on the mode in which they cover their head and neck. The Pleurodirans, also called the side-necked turtles, have long necks, and fold them sideways to align them with the shell. The Pelomedusidae and Chelidae are the only extant families of pleurodires. The Cryptodirans pull their neck straight back to conceal their head within the shell. The Carettochelyidae, Cheloniidae, Chelydridae, Dermatemydidae, Dermochelyidae, Emydidae, Kinosternidae, Testudinidae and Trionychidae are all cryptodires, although the ability to retract the head has been lost in the sea turtles. A third order, the Paracryptodirans, are extinct. Reptiles are classified according to the pattern of in the temporal region of the skull. Testudines are placed in the subclass Anapsida because they lackfenestration. There are suggestions that this lack of fenestration is a secondary characteristic and that turtles belong in Diapsida. Both sidescite strong evidence, and the conflict has yet to be resolved. The shell of testudines distinguishes them from other vertebrates. The shell is not an exoskeleton, but a modified ribcage and part of the vertebral column. Because of the shell, the pectoral and pelvic girdles are located within the ribcage. The limb bones are also modified to accommodate to the shell. The earliest known turtles are from fossils in the Upper Triassic. These fossils are nearly indistinguishable from modern turtles anatomically. In these early fossils, the teeth have already been lost, and a keratin beak is suggested by the mandibles. Important differences between Proganochelys and modern turtles are the presence of the palatal teeth, the inability to retract the head within the shell, and the lack of a trochlear pulley in the jaw closing anatomy.
Families
The following lists the family—the Latin name of the family, date in which the species was formally described and classified, and the binomial authority on the species; the number of genera belonging to the family; the name or names commonly used; an example species and an example image.
