During the Permian period, the dominant reptiles were pelycosaurs (Sect. 2.5) but, in the Triassic, these were replaced by therapsids or mammal-like reptiles (Sect. 2.6) from which the mammals evolved. During the same period, the early archosaurs of the Permian gave rise to thecodontians which were ancestral to the crocodilians (Sect. 4.6.3), the pterosaurs (Chap. 6) and the dinosaurs of the Mesozoic Era. The reptiles in nearly every one of these and other evolutionary sequences show an increase in size as time passed. Almost never, it seems, did big vertebrates evolve into smaller forms. On the contrary, they appear nearly always to have grown larger and larger until eventually they became extinct for one reason or another!
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