35 



Reptiles — First Land Vertebrates 



The first land animals were reptiles. They were the ancestors of modern 

 turtles, lizards, snakes, and crocodilians, a small remnant compared with those 

 that once overspread the earth during the "Age of Reptiles," at least 150 

 million years ago. In the early part of that era, certain reptiles developed 

 structures and habits the like of which would eventually be those of birds and 

 mammals. 



The name of the Class Reptilia refers to the creeping habits of many of the 

 group (Fig. 35.1). Reptiles originated from primitive amphibians that then 

 and ever since have been bound to water by their unprotected eggs that de- 

 velop only in watery surroundings. Unlike amphibians, the reptiles made 

 permanent homes on land and laid their eggs there. In the course of time, the 

 eggs became truly land eggs with fluid held within them by their shells. Inside 

 the shell the young reptile was surrounded by membranes that had various 

 uses. One of these was the amnion, a sac of fluid, the private pond in which 

 every reptile, bird, and mammal now goes through its early stage of life (Fig. 

 35.2). 



In addition to the all important eggs, there were three other main keys to 

 reptilian success on land — their skins, respiratory organs, and means of loco- 

 motion. Necessity of being near a body of water and dependence upon warm 

 climate are like chains limiting the distribution of land animals. Reptiles broke 

 the first, but not the second chain. With their low rate of basic bodily activity 

 and "cold blood" they have continued heavily dependent upon a warm climate. 

 Only warm-blooded birds and mammals can live on the polar ice. 



Three Key Adjustments to Land Life. Skin. The skins of reptiles and am- 

 phibians are essentially similar except for one great difference. A snake's skin 

 resists drying; a frog's skin does not. The difference is in the outermost horny 

 layer of the epidermis (stratum corneum) that in frogs is soft and permeable 

 to water and in lizards is tough and waterproof (Fig. 35.3). 



713 



