The subclass Anapsida (Sect. 2.4) is represented by a number of early reptilian taxa including the mesosaurs (e.g. Mesosaurus, Fig. 7), the procolophonids and the Testudines. The family Procolophonidae ranged throughout the world from the Upper Permian to the Upper Triassic. Its early members were small and agile insectivores, with numerous peg-like teeth. Procolophon (Fig. 77) from the Lower Triassic of South Africa and Antarctica was probably herbivorous or insectivorous. Later members, from the Middle Triassic onwards, were usually larger, herbivorous, and with broad cheek teeth suitable for grinding rough plant material. The spikes around its head were probably defensive. Apart from the Testudines, aquatic members of which were discussed in Chapter 4, few anapsids survived into the Mesozoic Era.
The Triassic Proganochelys and Triassochelys (Fig. 18) were remarkably similar to modern terrestrial terrapins except that they were unable to retract their heads and legs into the shell as completely as modern Testudines can. They
probably spent their lives grazing on low vegetation, and buried their eggs in the ground. Although the horn of their shells did not ever become fossilised, its presence is indicated by marks on the bones. So their defence against predators must have been exactly like that of extant forms.
The most successful group of chelonians - the suborder Cryptodira -evolved, along with the Pleurodira, during the Jurassic period. The Pleurodira, it may be remembered, retract their heads by flexing the neck sideways. Most of them were, as they still are, aquatic or amphibious. Many of the cryptodires retract their head into the shell by lowering the neck and pulling it directly backward. By the end of the Jurassic, the order had greatly diversified and had replaced the pleurodires in the seas, rivers, and lakes throughout the world. Cryptodires are represented in the world today by land tortoises, aquatic and amphibious terrapins, and marine turtles. Tortoises are vegetarian, while amphibious terrapins and marine turtles are more often omnivores or predatory carnivores. Food intake can be influenced by its availability. Moreover, many taxa of Testudines tend to be carnivorous when young, but switch to becoming largely vegetarian as they get older. Most probably, the same would have been true of Mesozoic forms (Sect. 4.3.2).
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