A HISTORY OF VERTEBRATES: AMPHIBIANS AND REPTILES 



465 



Figure 23.16. The alligators and ciooHiilcs are the only surviving members of the 

 archosaurian reptiles, a group to which the dinosaurs belonged. A, The American alli- 

 gator; B, the American crocodile. The southern part of the Florida Everglades is the 

 only place in the United States where crocodiles can be found in the wild. (Courtesy of 

 Ross Allen's Reptile Institute.) 



bipedal archosaurs. The extinction of the dinosaurs may have resulted 

 from climatic changes. .\n inability of the specialized herbivores to adapt 

 to the drying up of the large swamps and to changes in vegetation would 

 have led to their death. Their disappearance, in turn, would deprive the 

 huge carnivores of most of their food supply, so their days would be 

 numbered too. 



Only one group of archosaurs survived this wholesale extinction— 

 the alligators and crocodiles (order Crocodilia). Crocodiles have reverted 

 to a quadruped gait (though their hind legs are much longer than the 

 front) and an amphibious mode of life. Only two species occur in the 

 United States-the American alligator, which can be distinguished by its 

 rounded snout, and the American crocodile, which has a much more 

 pointed snout (Fig. 23.16). 



Mammal-like Reptiles. Another line of evolution, which was des- 

 tined to lead to mammals, diverged from the cotylosaurs millions of years 

 before the advent of lizard-like reptiles or archosaurs. Early mammal-like 

 reptiles (order Pelycosauria) were very similar to cotylosaurs. These were 

 medium-sized, somewhat clumsy,, terrestrial quadrupeds with limbs 

 sprawled out at right angles to the body. Their jaws contained numerous 

 conical teeth and were composed of many dermal bones covering the 

 mandibular arch. The jaw joint lay between the ossified posterior ends 

 of the mandibular arch, i.e., between the quadrate bone of the upper 

 jaw and the articular bone of the lower jaw. This is where the jaw joint 

 is located in the frog (Fig. 21.4) and other low^er vertebrates. The stapes 

 (a derivative of the hyoid arch of fishes) transmitted vibrations to the 

 inner ear. It, in turn, may have received air-borne vibrations from the 

 external environment by means of a tympanic membrane as it does in 

 frogs (Fig. 21.17), or ground-borne vibrations may have been picked up 



