MAMMAL-LIKE REPTILES I91 



of inches to a yard in length, and the toothless jaws are sheathed 

 in horn and beaked like those of turtles. This is a nearly 

 typical social group: large and small, herbivorous, omnivorous, 

 and carnivorous, toothed, toothless and horny-beaked, swift- 

 moving, slow-moving, unarmored, partly armored; it lacks 

 only the completely armored, slow-moving type to be a perfect 

 complex. 



In the Upper Permian the fauna includes pareiasaurs and 

 gorganopsians, which are similar to a large group of reptiles of 

 the same geologic age discovered in Russia by Amalitzky. 



In Lower and Middle Triassic time the last and most highly 

 specialized of the beaked anomodonts appear together with di- 

 minished survivors (ProcolopJion) of the very ancient solid-headed 

 order (Pareiasauria of South Africa, Cotylosauria of Texas). 

 Here also are found the true cynodonts, which are the most 

 mammal-like of all known reptiles. In the Upper Triassic of 

 South Africa occur carnivorous dinosaurs, also crocodile-like phy- 

 tosaurs (Fig. 75), allied to those of Europe and North America. 



Origin of the Mammals and Adaptive Radiation of the 

 Eighteen Orders of Reptiles 



The most notable element in this complex reptilian society 

 of South Africa are those remarkable pro-mammalian types of 

 reptiles (cynodont, theriodont), from which our own most 

 remote ancestors, the stem forms of the Mammalia, the next 

 higher class of vertebrates above the Reptilia, were destined to 

 arise. This is another instance where palaeontology has dis- 

 lodged a descent theory based upon anatomy, for at one time 

 from anatomical evidence alone Huxley was disposed to derive 

 the mammals directly from the amphibians. 



The question at once arises, why were these particular reptiles 

 so highly favored as to become the potential ancestors of the 



