Sauropsida: Class Aves 



541 



of Archaeopteryx were present in upper and lower jaws. Best known of 

 these toothed birds are Ichthyornis and Hesperornis (Fig. 418), 

 found in Cretaceous strata of Kansas, which was then at the bot- 

 tom of a sea. Ichthyornis, except for the teeth, must have resembled 

 a modern tern in size and form, and probably in mode of life. Its wings 

 were strongly developed. Hesperornis was an enormous bird (3 or 4 

 feet long) having no functional wings. The pectoral limb was repre- 

 sented only by the vestigial girdle and humerus. The feet projected 

 laterally outward and probably served as powerful swimming paddles. 

 The sternum had no carina. No remains of toothed birds have been 

 found in strata later than the Cretaceous. 



So far as can be judged from the scanty evidence, the dominant 

 birds since early Cenozoic times have been toothless and otherwise 

 very much like modern flying birds. By the middle of the age, however, 

 appeared birds with wings much reduced or entirely lacking and legs 

 powerfully developed — the "running birds" such as the ostriches. 



The geologic history of birds, fragmentary as it is, gives us a clue 

 to the general course of development of the Class. Three periods may 

 be recognized, characterized as follows: (1) Animals of distinctly 

 reptilian features including long tail and teeth, but feathered and 

 having a pectoral wing adequate for at least a short gliding flight. (2) 

 Birds closely approaching modern birds in their numerous anatomic 

 adaptations for effective sustained flight; strong sternum with well- 

 developed carina; tail short, but teeth of reptilian type still persist- 

 ing; contemporaneous with these flying birds are some swimmers and 



Fig. 



418. (Left) Ichthyornis. (Right) Hesperornis. (Courtesy, Coues: 

 North American Birds," Boston, L. C. Page & Co.) 



Key to 



