PHYLOGENY 



tional genera. Thus Eryops, which is now usually classified as an amphib- 

 ian, and Seijmouria, which is now usually classified as a reptile, have, with 

 much justification, been placed in both classes by different authorities, or 

 even by the same authority at different times. The reptiles first appear in 

 the fossil record in the Pennsylvanian. By the Permian, they had already 

 begun a great diversification which led to the formation of six orders in 

 the Permian and ten more in the Triassic. Throughout the Mesozoic, they 

 were the dominant vertebrates, and hence the ordinary designation of this 

 era as the "Age of Reptiles." 



The original reptiles were substantially just amphibians adapted for 

 permanent land life. Like modern reptiles, the skin was probably thick- 

 ened and cornified to prevent drying of the animal. It had four short limbs 

 set more or less at right angles to the body, so that it could lift its weight 

 only clumsily. It had a large number of undifferentiated conical teeth. 

 And, perhaps most important of all, the developing embryo was enclosed 

 in embryonic membranes including the amnion and chorion, and it re- 

 spired by means of a third membrane, the allantois. Thus the reptiles 

 were freed of the necessity of returning to water for reproductive pur- 

 poses. The eggs, once laid, were unattended. 



Adaptive radiation within the class Reptilia has been extremely varied 

 (Figure 58). Three orders returned to the water and again developed 

 specializations appropriate to that habitat. Most extreme of these was the 

 order Ichthyosauria, in which the external form became completely fish- 

 like, but the skeletons prove that these animals were reptiles. The plesio- 

 saurs were less extremely modified. They had turtle-like bodies, and large 

 flippers. The neck was often very long and serpentine in appearance. The 

 third aquatic order is the Chelonia, including the turtles, which are gen- 

 erally adapted to an amphibious mode of life. But, as is well known, some 

 turtles have become exclusively terrestrial and others have become ex- 

 clusively marine. Representatives of most of the terrestrial orders have 

 also invaded the water. The best known modern reptiles are the snakes 

 and lizards, and these occur in both terrestrial and aquatic forms, and are 

 adapted to predation upon almost every type of animal. The greatest 

 range of adaptive radiation occurred within the several orders of dino- 

 saurs, the ruling reptiles of the Mesozoic era. Their principal types are 

 illustrated in Figure 59. 



Why the Ruling Reptiles became extinct at the end of the Cretaceous 

 is not known, but a number of plausible theories have been suggested. 

 The disappearance of the great reptiles coincides roughly with the rise of 

 the birds and mammals, and it has been suggested that the reptiles were 

 simply unable to compete with these progressive newcomers. But this is 

 improbable, because these groups had been present since the Jurassic, 

 perhaps since the Triassic in the case of the mammals, and they had not 

 been able to achieve a place of importance in competition with the domi- 

 nant reptiles. It is more probable that the rise of the birds and mammals 

 in the Cenozoic era occurred because their reptilian competitors disap- 

 peared. Another suggestion which has been given more credence is that 

 the world climate became more severe, and that the great reptiles were 



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