LIFE 



a conclusion supported by the occurrence of their remains in marine 

 sediments free from other land fossils. Some of them had singular 

 elongate rod-like tails, with a rudder-like expansion at the end. 

 Pterodactyls (Fig. 443) 

 had short tails, and 

 were mostly small and 

 slender. Fully differ- 

 entiated as first found, 

 they underwent no 

 radical change of struc- 

 ture during their career, 

 and the steps of their 

 remarkable evolution 

 are for the most part 

 unknown. Flying rep- 

 tiles are extremely rare 

 among the Jurassic fos- 

 sils of North America. 



Turtles, which had 

 lived elsewhere since 

 the Middle Trias, made 

 their first appearance 

 in North America in 

 the Morrison beds, and 

 the crocodilians became 

 differentiated into sev- 

 eral branches. Primi- 

 tive lizards were doubtless abundant, but because of their terrestrial 

 habits and small size, they have little representation among the 

 fossils, and none have been found in our continent. 



A less bizarre, but really greater evolution, was the differentia- 

 tion of true birds. The remote ancestors of the pterosaurs and the 

 birds may have been closely allied, but there is no evidence that 

 birds descended from pterosaurs. The two are examples of analo- 

 gous and parallel evolution, not of relationship. 



The oldest known bird, Arch&opteryx macrura (Fig. 444), shows 

 clear traces of a reptilian ancestry. From this ancestry it retained a 

 long, vertebrated tail, reptile-like claws, teeth set in sockets, biconcave 

 vertebrae, and separate pelvic bones. On the other hand, its head 

 and brain were bird-like, its anterior limbs adapted to flying in bird- 



. 443. Skeleton of pterodactyl, Ptcrodac- 

 lylns sfxrltibilis, from the lithographic stone at 

 Eicfastadt, Bavaria; about }4, natural size. (After 

 H. v. Meyer.) 



