A HISTORY OF VERTEBRATES: AMPHIBIANS AND REPTILES 



453 



panded, become highly vascular and form an organ for gas exchange; 

 and the period of development is accelerated. 



Something similar to what has taken place among these frogs today 

 may have occurred among the amphibians which were ancestral to 

 reptiles. Ancient amphibians were certainly no more consciously trying 

 to improve upon their terrestrial adaptation than crossopterygians were 

 trying to get onto the land. U the aquatic larvae of the ancestral am- 

 phibians were subjected to a very high predation, any variation that 

 tended toward the suppression of the defenseless larval stage and toward 

 the direct development of their embryos in a less vulnerable environ- 

 ment would have a selective advantage. It is possible that the amphibians 

 that gave rise to the reptiles developed means of terrestrial reproduction 

 before the adults completely left the water, 



205. Characteristics of Reptiles 



The adjustment of most amphibians to terrestrial conditions is 

 deficient in three respects: (1) poor means of conserving body water, (2) 

 inability of most to reproduce on the land and (3) inability to maintain 

 their body temperature and metabolic processes at a fairly constant level. 

 Reptiles, as a group, evolved adequate solutions to the first two of these 

 problems, and certain extinct reptiles probably achieved some measine 

 of control over their body temperature. Reptiles also improved upon 

 the means of locomotion and gas exchange, and other terrestrial at- 

 tributes of their amphibian ancestors. 



A lizard, such as the collared lizard (Crotapliytits coUaris Fig. 23.6) 

 of the southwestern United States, is a typical reptile, llie surface of 

 the skin is covered with dry, horny scales that prevent water loss by this 

 route. These scales develop through the deposition of considerable 

 keratin (a very insoluble and hence waterproofing protein) in the super- 

 ficial layers of the epidermis. 



Figure 23.6. The collared lizard, Crotaphytus coUaris. (Courtesy of the New York 

 Zoological Society.) 



