Turtles, properly called Testudines, are highly distinctive reptiles well represented in the Cretaceous fossil record, but known in the Crato Formation from just a few specimens (Fielding et al., 2005; Oliveira and Kellner, 2005). Turtles make their earliest fossil appearance in the Norian of the Upper Triassic (Gaffney, 1990) and survive in the extant fauna as approximately 290 species. Uniquely, the turtle skeleton is encased within a bony shell, the dorsal component of which is termed the carapace, and the ventral component the plastron. Bones, variously termed neurals, costals and peripherals, form most of the shell (with a single nuchal bone at the anterior end of the midline, and a suprapygal and pygal at the posterior end), and in life these are covered by keratinous scutes. Members of a few groups, namely leatherback and softshell turtles, and the bizarre African pancake tortoise Malacochersus tornieri, have reduced both the bones that form the shell, and the scutes that cover it. Isolated shell bones are among the commonest turtle fossils, and the Crato Formation turtle fossils are unusual in that all specimens found so far are complete examples.
Although turtles might be regarded as constrained in diversity by way of the shell, they still include terrestrial, amphibious and fully aquatic species that, even today, occur worldwide with the exception of the polar regions. How turtles are related to other reptiles remains controversial.