PRE-CAMBRIAN AND PALEOZOIC ERAS 167 



averaged smaller in size; cockroaches formed a decreased proportion of 

 them. New orders appeared: mayflies, dragonflies, beetles, among them. 



Labyrinthodont amphibians continued as inhabitants of moister portions 

 of the environment. Deposits containing their fossils indicate that as the 

 country became more arid they concentrated their habitations in and 

 around rivers and streams. Although a variety of forms appeared, the gen- 

 eral description given above continued to characterize the group. 



The most exciting occurrence in the Permian was the progressive devel- 

 opment of reptiles. Although, as we have seen, they arose in the Pennsyl- 

 vanian period, it was not until Permian times that reptiles formed a 

 prominent part of the vertebrate fauna. The first reptiles were the coty- 

 losaurs. In shape and bodily characteristics cotylosaurs closely resembled 

 labyrinthodont amphibians (Fig. 8.22). In fact the resemblance was so 



FIG. 8.22. A Permian cotylosaur reptile, Limnoscelis; length about 5 feet. (Romer, 

 1959, considered that this creature was amphibious and had webbed feet.) (After Case, 

 Publicafion No. 207, Carnegie Institution of Washington, 1 91 5.) 



great that concerning one creature living at the time, Seymouria (Fig. 

 8.23), there is still uncertainty as to whether it was a reptilelike amphibian 

 or an amphibianlike reptile. Concensus leans toward the latter interpreta- 

 tion today. Its skull was much like that of the labyrinthodonts but many 

 features of the remainder of its skeleton resembled those of primitive rep- 

 tiles. If we knew what kind of an egg it laid (see below) we could be 

 more certain as to whether it was an amphibian or a reptile. But our chief 

 interest lies in the fact that the existence of such a form demonstrates 

 the close relationship of labyrinthodonts to cotylosaurs. Cotylosaurs are 

 believed to have arisen from labyrinthodonts. 



Did the first reptiles live in the water or on land? Romer ( 1959) has con- 

 cluded that they spent most of their lives in the water, as their amphibian 

 ancestors had done, but that unlike the latter they laid eggs on land, as 

 aquatic turtles do today. He pointed out the advantages of laying eggs on 

 land, especially lessened danger of having the eggs and young eaten by 

 predatory fishes, insect larvae, and the like, and lessened danger of de- 



