A HISTORY OF VERTEBRATES: BIRDS AND MAMMALS 



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however, there is Httle fusion of bones and each finger bears a claw. If 

 the skeleton alone were known, the creature would probably have been 

 regarded as a peculiar archosaur, but it is evident that this was a primi- 

 tive bird, and not a reptile, for there are clear impressions of feathers 

 (Fig. 35.1). The feathers would suggest that Archaeopteiyx was active 

 and warm-blooded. The ratio of its wing surface to its body size, together 

 with the poorly developed sternum, indicates that it was not a strong 

 flier. These most primitive birds are placed in the subclass Archaeor- 



nithes. 



The next group of fossil birds lived in the Cretaceous. These birds 

 had lost the long reptilian tail, had evolved a well developed sternum, 

 and were modern in many other ways. A true pygostyle had not yet 

 evolved, and teeth were present in at least certain species. There are 

 clear indications of teeth in fossils of Hesperornis (Fig. 22.7 B), a large 

 diving species with powerful hind legs and vestigial wings. The nature 



B - _ _ 



Figure 24.8. Representative paleognathous birds. A, Ostriches; B, a kiwi with its 

 relatively huge egg. (Courtesy of the American Museum of Natural History.) 



