REPTILES, BIRDS, AND MAMMALS 



447 



far back on the skull between the eyes. Some of them grew to great size. 

 By Jurassic times the phytosaurs had been replaced by their distant 

 relatives the crocodiles, in which, among other differences, there is a bony 

 palate in the roof of the mouth that forms an air passage from the throat 

 to nostrils placed at the tip of the snout. The crocodiles and related 

 alligators have been among the least progressive of the ruling reptiles 

 but are the only members of the archosaur stock which survived beyond 

 the Mesozoic era. The marine crocodiles of Jurassic time, already men- 

 tioned, are the only archosaurs that ever took to the sea. 



Fig. 28.11. The American crocodile (Crocodilus acutus, narrow snout) and American alli- 

 gator {Alligator mis sis sip pie n sis, broad snout) are surviving representatives of the subclass 

 Archosauria. (Courtesy American Museum of Natural History.) 



The pterosaurs, or flying reptiles. Twice in the history of the 

 archosaurs the front legs, freed from walking, were modified into wings 

 — once by the ancestors of the birds, as will be described later; again, 

 and for the time being, more successfully, by the pterosaurs. These were 

 light-bodied flying reptiles, with membranous wings supported by the 

 enormously elongated fourth finger of the forelimb; the bones were hollow 

 and air-filled as in birds. Since they had no large breastbone for the 

 attachment of strong flying muscles, they are believed to have flown by 

 soaring, taking off and landing on high cliffs or trees. They varied from 

 the size of sparrows to the largest flying animal that has ever existed, 

 the giant Pteranodon of Cretaceous times, which had a wingspread of 27 

 feet. 



The saurischian dinosaurs. Of all extinct animals, the dinosaurs are 

 those most generally known. The great size and bizarre forms of some of 

 them appeal strongly to the imagination, and the fact that the last 



