CHAPTER 31 



THE TETRAPODA: MAMMALS 



The cotylosaurs or stem reptiles gave rise to many diverging lines 

 of animals. One of these ultimately produced the most successful of 

 the modern vertebrates, the mammals. The development from the stem 

 reptiles to the modern mammals was a long process, lasting from the 

 Upper Carboniferous to the present time (Fig. 203). While it is pos- 

 sible to trace the main line of development, much detail is obscured by 

 the sheer diversity of species. Many forms flourished and died through 

 the millions of years before finally the modern mammals emerged from 

 this seeming chaos. 



The first group of reptiles in this line were the pelycosaurs which 

 arose in the Upper Carboniferous and reached their height of develop- 

 ment during the early Permian. These were very primitive reptiles, 

 but they had skull characters which indicate that they were the first 

 step toward the mammals. A primitive pelycosaur such as Varanops 

 shows but little evidence of its potentialities ; it appeared to be a pe- 

 culiar lizard. More highly specialized forms such as Dimetrodon, 

 with its strange dorsal sail, had skull and teeth characters that were 

 somewhat mammal-like. 



During the late Permian and early Triassic another group of un- 

 usual reptiles arose. These were the therapsids or mammal-like rep- 

 tiles. Genera such as Cynognathus and Lycaenops are characteristic of 

 this period of development. Cynognathus, the dog-jawed reptile had 

 teeth that were in sockets and also were differentiated into various 

 types. Further the legs supported the body in a very definitely mammal- 

 like manner, and as nearly as can be judged, Cynognathus was homoio- 

 thermic. This conclusion is based on the presence of structures which 

 allowed for uninterrupted breathing and the possible presence of a 

 diaphragm. Although these forms disappeared from the fossil record 

 of the Jurassic, they were exceedingly abundant during the early Tri- 

 assic. It appears that they were unable to compete with the highly 



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