The archosauromorph branch of the diapsids (see p. 113) includes several groups, the trilophosaurids, rhyn-chosaurs, prolacertiforms and archosaurs. The first two are known only from the Triassic, but the prolacertiforms had arisen in the mid-Permian, and the archosaurs by the Early Triassic. The most important of the archosauromorph groups is the Archosauria, and their evolution in the Triassic was critical for the later history of vertebrate life on land: here were laid the foundations of the radiation of the dinosaurs, pterosaurs and crocodiles, and ultimately of the birds. These later stages of archosaur evolution are described in Chapters 8 and 9.
During the Triassic the archosaurs radiated into several groups. There were some short-lived forms in the Early Triassic,before a major split in the archosaur clade took place. One line led to the crocodilians, and the other to the pterosaurs, dinosaurs and birds (see Box 6.1). The Triassic archosaurs that do not belong to any of these terminal clades were previously called 'thecodontians', a paraphyletic group.
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