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INTRODUCTION TO EVOLUTION 



to that of the legs of some of the bipedal dinosaurs. Pterosaurs, on the 

 other hand, had very weak legs. The wing surface composed of feathers is 

 much more efficient, maneuverable, and readily repaired than was the 

 flying mechanism of pterosaurs. 



Discussions of evolution in former years frequently included mention of 

 "missing Hnks." The term was used in various senses but always included 

 the idea of a form standing midway between two groups of animals now 



clearly separate from each other. In most 

 cases such exactly intermediate forms have 

 not been preserved to us, but the Jurassic 

 birds, Archaeopteryx and Archaeornis, form 

 a fortunate exception to this lack. They are 

 clearly birds, since the imprints of the 

 feathers are preserved in the Archaeopteryx 

 fossils (Fig. 9.11). Yet they are so reptilian 

 that if the imprints of the feathers had not 

 been preserved they would probably have 

 been classified as small, bipedal dinosaurs. 

 Relative to body size the wings were small, 

 and three of the digits of each hand per- 

 sisted, armed with claws. Thus the forelimbs 

 were probably used for climbing as well as 

 for flight. Indeed, the small wingspread sug- 

 gests that gliding from a height was a more 

 probable activity than was sustained flight. 

 The tail was long, with a row of feathers 

 along either side of the slender chain of 

 vertebrae (Fig. 9.11), and clearly reminis- 

 cent of thecodont ancestry (Fig. 9.3). This 

 tail is strikingly unlike that of modern birds. 

 In them the bony portion of the tail is very 

 short; length of tail is due entirely to length 

 of feathers. Finally we may note that the jaws of these Jurassic birds, as 

 well as the jaws of Cretaceous birds, were equipped with teeth. In this 

 respect again they resembled reptiles more than they did modern birds, 

 since the latter are always characterized by toothless, horny beaks. It is 

 interesting that the Jurassic birds did not possess the hollow bones 

 characteristic of modern birds, on the one hand, and of pterosaurs, on 

 the other. Apparently birds and pterosaurs, both descended from theco- 

 donts, developed hollow bones independently and at widely differing 



FIG. 9.11. Archaeopieryx, a 

 Jurassic toothed bird, c/, clavi- 

 cle; h, humerus; r, radius; sc, 

 scapula; u, ulna. (After Stein- 

 mann-Doderlein; from Guyer, 

 Animal Biology, Harper & Broth- 

 ers, 1948, p. 615.) 



