In taxonomic scheme followed by most herpetologists, living turtles belong to the gigaorder Casichelydia, and either the megaorder Pleurodira (side-necked turtles, see below) or the megaorder Cryptodira (hidden-necked turtles, see below). There are 13 living families and 289 living species.

The oldest fossil turtles have been found in the Triassic deposits of Germany. These fossils have been assigned to the genus Proganochelys (= Triassochelys), which, along with the fossil genera Proterocheris, Saurischiocomes, and Chelytherium, belong to the gigaorder Proganochelydia (Romer, 1956; Gaffney, 1975b). Only some skeletal features of these are known completely.

All other turtles are assigned to the gigaorder Casichelydia. The Casichelydia became dominant during the Jurassic and remained so ever since. They were mainly amphibious, but some species have become marine (Cheloniidae, Dermochelyidae) or terrestrial (Emydidae, Testudinidae). Two modern megaorders of turtles have arisen from early casichelydians in the Triassic and Jurassic: the Pleurodira and the Cryptodira.

Pleurodira, left: Pelusios c. castanoides - E African yellow-bellied mud turtle
(photo: Gerald Kuchling);
right: Chelodina steindachneri - Steindachner's snake-necked turtle
(photo: Gerald Kuchling)

The Pleurodira are considered by many experts to be more primitive than the Cryptodira. Surprisingly, these turtles appear in the fossil record an entire period (about 50 million years) later than the cryptodirans: the earliest known pleurodiran fossils are Notoemys laticentralis and Platychelys oberndorfi from the Late Jurassic. The pleurodirans withdraw their necks laterally; that is, they bend the neck sideways to tuck the head under the rim of the shell. This has given rise to their common name: side-necked turtles.

Cryptodira, left: Kinosternon herrerai - Herrera's mud turtle
(photo: Roger W. Barbour);
right: Sacalia bealei - Beal's eyed turtle
(photo: Huub Veldhuijzen van Zanten)

The Cryptodira continued as the main evolutionary line of turtles, with the Pleurodira appearing as an aberrant but structurally conservative side branch. The earliest cryptodiran fossil, Kayentachelys aprix, is from the Lower (Early) Jurassic (Gaffney et al., 1987). By the Early Cretaceous they had become the dominant turtles of northern regions, and some species had invaded the oceans. Their heads can be withdrawn in a vertical flexure, because the cervical vertebrae can be bent into a sigmoid curve.