262 



HISTORICAL GEOLOGY 



Reptiles. — Most of the great characteristic groups of Jurassic 

 Reptiles continued into the Cretaceous, while certain new forms, 

 such as Mosasaurs, Triceratops, and Snakes, were added. Mesozoic 

 Reptiles are discussed at the end of this chapter. 



Birds. — During the long time between the Jurassic, when the 

 first known Birds appeared, and the Upper Cretaceous, important 

 evolutionary changes took place in this class of animals, though 

 fossils of the interval are almost, if not wholly, absent. Creta- 

 ceous Birds were distinctly more advanced and modern in appear- 

 ance than were those of the 

 Jurassic. Thus the long, verte- 

 brated tail of the earlier forms 

 had become greatly shortened, 

 and the only important prim- 

 itive characteristic which they 

 retained was the possession of 

 teeth. Compared with modern 

 Birds, they had much smaller 

 brain cavities. 



At least 30 species of Cre- 

 taceous Birds are known, all 

 of these belonging to two great, 

 though very different, groups 

 (orders) e.g. Ichthyornis and 

 Hesperornis. All appear to 

 have been aquatic forms. The 

 Ichthyornis types were power- 

 ful fliers, as proved by the 

 strongly developed keel and 

 wing bones. The teeth were 

 set in distinct sockets. The 

 structure (biconcave) of their 

 vertebrae was quite distinctly reptilian. They averaged about the 

 size of a pigeon (see Fig. 163). 



Hesperornis comprised forms incapable of flight, but often of 

 great size — five to six feet in length. In marked contrast with the 

 Ichthyornis, these forms had powerfully developed legs which 

 served as swimming paddles in these almost wholly aquatic forms. 

 In every way they were adapted to rapid swimming. Their teeth 

 were set in grooves instead of sockets. 



Fig. 163 

 A Cretaceous toothed Bird, Ichthyornis 

 victor. Height, about 9 inches. (After 

 Marsh.) 



