CHAPTER XXVIII 



REPTILES, BIRDS, AND MAMMALS 



Thus far our account of the post-Cambrian evolution of animals has 

 traversed some 260 million years of geological time. During most of the 

 immensely long Paleozoic era the vertebrates lived in the waters of the 

 earth, and only toward its end did some of them finally emerge onto the 

 land. Once this step had been taken, the further evolution of the land 

 dwellers was rapid, and the succeeding Mesozoic era saw the origin and 

 establishment of all the truly modern groups of terrestrial organisms. This 

 era is often called the Age of Reptiles, for a horde of reptilian types domi- 

 nated the scene. It was also the time during which the birds, mammals, 

 modern insects, and flowering plants arose and became established. The 

 Mesozoic era seems brief and its evolutionary developments rapid by 

 contrast with the interminable Paleozoic. How long it actually lasted can 

 be better appreciated if we bear in mind that dinosaurs walked the earth 

 during 120 million years, while the whole existence of mankind scarcely 

 spans 1 million. 



The evolutionary lines which lead from the early vertebrates to man 

 form the main thread of our story. These lines do not, however, lead from 

 evolutionary climax to evolutionary climax; they run along the edges of 

 the picture, so to speak, until very recent times. Thus of the 20 or more 

 reptilian orders that arose in the Mesozoic era, it was not the largest and 

 most successful that lie in our own ancestry but an early and relatively 

 obscure side branch from the primitive reptile stock. Nevertheless, the 

 history of the reptiles as a group, and of their descendants the birds, was 

 a most dramatic and instructive evolutionary episode, which warrants our 

 attention. 



The origin of reptiles. To see the start of the reptilian dynasty we 

 must return to the Carboniferous coal forests, where lived, along with 

 the dominant labyrinthodonts, other similar-appearing animals which 

 were neither amphibians nor reptiles; they combined characteristics of 

 both. One such form was Seymouria, a clumsy sprawling creature 

 about 2 feet long. From this or some similar animal all the later reptiles 

 evolved. 



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