XV 



EVOLUTION OF THE REPTILES 



1. The earliest reptile populations, Anapsida 



The organization of a reptile is well suited to maintain life on land. 

 Many features show a considerable advance in this respect over the 

 amphibia, for example, the dryness of the skin, the method of repro- 

 duction, and the devices for economizing in the use of water. The 

 immense radiation of the reptiles into every sort of land habitat during 

 the Mesozoic period shows the efficiency of these mechanisms, which 

 were probably present, at least in imperfect form, in the earliest Car- 

 boniferous and Permian offshoots from the ancestral Stegocephali 



(P- 356). 



We have sufficient knowledge to be able to trace the early stages of 

 reptilian evolution with considerable certainty. An animal known as 

 *Seymonria i found in the lower Permian of Texas (perhaps 250 million 

 years old), is of critical importance in our understanding of reptile 

 origins (Figs. 219, 220). It was a lizard-like creature, about 2 ft long, 

 probably living on insects and perhaps some larger animals. Its charac- 

 teristics are so exactly intermediate between those of amphibians and 

 reptiles that it is not possible to place it definitely with either group ; 

 many zoologists class it with the Amphibia. This intermediacy is 

 shown in almost every structure of the body and is often a subtle 

 matter of the shape or size of the parts. Although a list of anatomical 

 features is apt to give an unreal picture of any living organization it is 

 the only method available to us in the absence of any more ingenious 

 calculus, and we may therefore give first some of the characteristically 

 reptilian features of *Seymouria. (1) The neural arches of the verte- 

 brae were convex dorsally so that they have a 'swollen' appearance 

 which is also seen in early reptiles; (2) a canal for the lachrymal duct 

 was present; (3) the occipital condyle was single; (4) the pectoral 

 girdle possessed a long interclavicle; (5) the pelvic girdle was attached 

 to the vertebral column by two sacral vertebrae ; (6) the blade of the 

 ilium was expanded for the attachment of the large muscles used in 

 walking; (7) there were five digits in the hand instead of four as in 

 many labyrinthodonts and in living amphibia; (8) the phalangeal 

 formula was 2:3:4:5:3 or 4, approximating to the reptilian, rather 

 than the usual labyrinthodont condition, in which the phalanges are 

 less numerous. 



