458 



THE CHANGING GENERATIONS 



that less energy is used in lifting the body and more in propelling it. The 

 bones of the digits have been reduced in number from the reptilian 2-3- 

 4-5-3 to 2-3-3-3-3, beginning with the thumb. 



Fig. 28.23. Permian fin-backed reptiles ("ship lizards," or pelycosaurs), as drawn by 

 Charles II. Knight. (Courtesy American Museum of Natural History.) 



Reptilian ancestors. The earliest animals showing traces of mam- 

 malian characters were the primitive synapsid reptiles called pelycosaurs, 

 which lived in late Carboniferous and early Permian times. They included 



the bizarre "fin backs," or "ship 

 lizards," some of them 7 feet long 

 and with a 3-foot high membranous 

 dorsal "sail," supported by enor- 

 mously elongated vertebral spines. 

 It was not, however, these spe- 

 cialized pelycosaurs but more primi- 

 tive ones with lizardlike bodies and 

 sprawling limbs that lie in the 

 mammalian line. Features that 

 indicate their relationship to mam- 

 mals are the position of the skull 

 opening and the fact that the skull 

 is nearly closed behind. 



The next step in the development 



Fig. 28.24. Lycaenops, a mammal-like 

 reptile or therapsid from the Permian of 

 South Africa, restored by E. H. Colbert. 

 This is one of the cynodont or dog-toothed 

 reptiles, the stock from which the mammals 

 probably arose. (Courtesy American Mu- 

 seum of Natural History.) 



of mammals is represented by the 

 mammal-like reptiles or therapsids common in late Permian and Triassic 

 strata; most of them have been found in South Africa. They were a varied 

 group, some quite like the pelycosaurs from which they came, others 



