GENERAL ZOOLOGY 



Fig. 18.24. A representative 

 crocodilian: the American 

 alligator, Allioator mississip- 

 piemis, a modern amphibious 

 reptile. (Photograph cour- 

 tesy New York Zoological 

 Society.) 



The Aves. Birds may be exactly defined as feathered animals,- no bird is 

 without feathers, and no other animal possesses them. In addition, birds are 

 warm-blooded vertebrates closely resembling reptiles, from which they have 

 obviously arisen. Among the earliest reptiles one line, from which dinosaurs 

 and pterosaurs originated, was ancestral also to crocodiles and to birds. 

 Toward the end of the Paleozoic or the beginning of the Mesozoic, it is 

 probable that small reptiles of this group, already specialized for locomotion 

 with the hind limbs, were further modified for an arboreal habitat and for 

 flight. The nature of the earliest adaptations for flight is uncertain. The 

 earliest known birds, now assigned to the single genus Archaeopteryx, whose 

 fossils were found in the Jurassic of Germany, had feathers and presumably 

 were moderately good fliers (Fig. 18.25). They retained, however, several 

 reptilian characteristics which have since been lost, or much reduced, in the 

 avian line. Among these primitive features were teeth, an elongate tail, and 

 claws upon some of the digits of the fore limb. No other fossils of birds are 

 known between the Jurassic and the Upper Cretaceous, when the toothed 

 aquatic forms Hesperornis and Ichthyornis flourished. These two genera represent 

 lines that became highly specialized for aquatic life before they became ex- 

 tinct. It is assumed that birds were abundant in the Upper Cretaceous, be- 

 cause the birds of the early Tertiary are diversified and essentially like those 

 of today. In these modern forms the most important divergence is between 

 the flightless birds, such as the existing emus, cassowaries, and ostriches, and 



574 



