The earliest cryptodiran fossil, Kayentachelys aprix, is from the Lower 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 Their heads can be withdrawn in a vertical flexure, because the cervical vertebrae can be bent into a sigmoid curve.
The Cryptodira is considered to be the more advanced group of turtles. According to Gaffney (1984) the cryptodiran line is composed of two subdivisions called hyperorders. The extinct hyperorder Pleurosternoidea is comprised of the family Pleurosternidae (= Glyptopsidae, Jurassic). The second hyperorder, Daiocryptodira, includes the parvorder Baenoidea (family Baenidae, Cretaceous-Eocene), and the parvorder Eucryptodira, which includes living cryptodirans (Relationships within Testudines).
Pleurosternids and baenoids retain several primitive characters. They retain nasal bones and thus have the prefrontals separated and not in contact at the midline of the skull. The foramen for the internal posterior cortical canal lies midway along the length of the basisphenoid-pterygoid suture. They also have a well-developed stapedial artery and reduced orbital and palatine arteries. The neck vertebrae lacked mechanisms to retract the head, and no formed central articulations were present in the early species. Mesoplastral bones and paired intergular scutes were also present.
The parvorder Eucryptodira is composed of several superfamilies: the Trionychoidea (Cretaceous-Recent), including the living families Kinosternidae, Dermatemydidae, Carettochelyidae, and Trionychidae; the marine Chelonioidea (Jurassic-Recent), including the extinct families Plesiochelyidae (Jurassic), Protostegidae (Cretaceous), and Toxochelyidae (Cretaceous-Eocene) and the living families Cheloniidae and Dermochelyidae; the Testudinoidea (Paleocene-Recent), including the living families Chelydridae, Platysternidae, Emydidae, and Testudinidae. Gaffney (1984) places the Chelydridae in the separate infraorder Chelydroidea.
See Pleurodiran/cryptodiran turtles for a comparison of pleurodiran and cryptodiran turtles (modified from Romer, 1956, and Gaffney, 1975b).