A HISTORY OF VERTEBRATES: AMPHIBIANS AND REPTILES 



459 



Lizards and snakes, though superficially different from each other, 

 are similar enough in basic structure to be placed in the single order 

 Squamata. Li/ards (suborder Lacertilia) are the older and more primi- 

 tive. They doubtless evolved from some rhynchocephalian-like ancestor 

 early in the Mesozoic era. For the most part lizards are diurnal, ter- 

 restrial quadrupeds, but, like other successful groups, they have under- 

 gone an extensive adaptive radiation (Fig. 23.11). 



Several groups have become arboreal and evolved interesting 

 adaptations for climbing. The true chameleon of Africa (not to be 

 confused with the circus chameleon of our Southeast) has a prehensile 

 tail, and an odd foot structure in which the toes of each foot are fused 

 together into two groups that oppose each other like the jaws of a pair 

 of pliers. Geckos, in contrast, cling to trees by means of expanded 

 digital pads. Numerous fine ridges on the under surface of the pads 

 increase the friction. 



Many lizards, including the horned toads of our Southwest {Phryn- 



Figure 23. 11. Adaptive radiation among lizards. A, The Old World chameleon has 

 grasping feet and a prehensile tail with which to climb about the trees. B, The gecko 

 climbs by means of digital pads. C, The horned-toad, Phrynosoma, is a ground-dwelling 

 species that often burrows. D, The glass snake, Ophisaurus, also burrows. E, The Gila 

 monster, Heloderma, and a related Mexican species are the only poisonous lizards in 

 the world. (Courtesy of the New York Zoological Society.) 



