Mammal-Like Reptiles

Abstract

8.1 In the Shadow of the Dinosaurs When the thecodonts (``socket-toothed''), the group thought to include the ancestral stock of all other archosaurs, including birds, dinosaurs, pterosaurs and crocodiles, appeared on the evolutionary scene, another group of reptiles were also evolving. They were the mammal-like reptiles or therapsids. The group had a checkered history having survived the Permian boundary crisis and becoming virtually extinct by the end of the Triassic--possibly a consequence of competition from more efficient predators within the thecodonts. Their near complete demise would allow the dramatic evolution of the archosaurs, the pinnacle of which were the dinosaurs but it would also leave a small window of opportunity for what would later become the most highly evolved species on the planet--mammals. It is interesting to conjecture what would have happened if some of the early mammallike reptiles, bristling with so much potential, had not suffered such major extinctions and depletions early in their history and if so how their descendants, presumably fully fledged mammals, might have developed from some of these magnificent mammalian-reptile experiments (and not as it turned out much later from the small shrew-like survivors)--and indeed how our own human species might have evolved if at all? The mammal-like reptiles evolved from captorhinomorphs such as Eocaptorhinus (Fig. 8.1) during the Carboniferous and can be distinguished by the lower or synapsid temporal opening (see Chap. 6). They can be divided into the more primitive order Pelycosauria, of the Upper Carboniferous and Lower Permian (most notably from the Texas Red Beds) and from the more advanced Therapsida of the Upper Permian and Triassic (most notably from the South African Karoo and Russia).