170 INTRODUCTION TO EVOLUTION 



birds avoided the necessity of laying their eggs in the water as their ances- 

 tors had always done. This was one of the greatest achievements in the 

 entire history of vertebrate evolution. 



What other advances over their amphibian ancestors did the reptiles 

 achieve? Like amphibians, reptiles are "cold blooded," meaning that they 

 have but little ability to regulate their body temperature. Reptiles achieve 

 more of such regulation than do amphibians, but to a considerable extent 

 body temperature fluctuates with fluctuations of the surrounding tempera- 

 ture. 



Most modern amphibians lack scales on their skin, while reptiles have 

 coverings of horny scales. This difference in body covering aids reptiles in 

 living in drier habitats than are possible for amphibians, since the scaly 

 covering decreases water loss by evaporation from the surface of the body. 

 It is noteworthy that when scales are present in amphibians, as they were 

 in labyrinthodonts and are in reduced form in the modern limbless caecil- 

 ians, they are of the bony type characteristic of fishes. Apparently such 

 scales were inherited from the crossopterygian ancestors of amphibians but 

 have been lost by most modern representatives of the latter. Horny scales 

 of the type characterizing the surface of reptilian skin formed a "new" 

 evolutionary development. 



Reptiles have larger brains than have amphibians, the enlargement of 

 the cerebral hemispheres in particular forming a portent of better things 

 to come. 



Reptiles differ from modern amphibians by having one occipital con- 

 dyle, the bony knob by which the skull is articulated to the first ver- 

 tebra of the backbone. Modern amphibians have two occipital condyles, 

 but labyrinthodonts had only one. Apparently, therefore, a single con- 

 dyle was the primitive condition, retained by reptiles but not by later 

 amphibians. 



Reptiles are distinguished from amphibians by other differences in skele- 

 tal details, including number of joints characteristic of fingers and toes. 

 Nevertheless it is difficult to pick out one diagnostic characteristic by 

 which the anatomy of all reptiles differs from the anatomy of all amphib- 

 ians. 



The cotylosaur reptiles are of greatest interest to us because they were 

 the ancestors of higher reptiles and, indeed, the distant ancestors of birds 

 and mammals. During the Permian and the early part of the Triassic (Ta- 

 ble 7.1, p. 137) the cotylosaurs and their immediate descendants formed 

 a diversified group of reptiles. A few Permian reptiles were surprisingly 

 specialized. One of the commonest, Dimetrodon, is sometimes called a 



