128 EVOLUTION OF THE EARTH 



Origin of mammals. These forms are clearly derived from 

 a reptilian stock known as the cynodonts or theriodonts (Fig. 

 24), in which the dentition has become differentiated as in the 

 mammals into incisor, canine, and molar teeth. The reptiles 

 differ from the mammals in that the lower jaw is still a 

 complex of several bones, while in the mammal there is but 

 one on either side. Many cynodonts were long of limb and 

 must have traveled with the body well off the ground, which 

 seems to have been a prerequisite to the development of warm 

 blood. These cynodonts are widespread, but the evidence 

 points to certain of those of Africa as nearer the direct ances- 

 tors of the mammals; the record of the actual transition, 

 however, is as yet unrevealed. 



FIG. 24. Skull of cynodont reptile, Nythosaurus 

 !ar<vatus. Note the mammal-like tooth differ- 

 entiation, but complex reptilian jaw. After 

 Broom, from the Pirsson-Schuchert "Text- 

 book of Geology," published by John Wiley 

 & Sons, Inc. 



Origin of birds. Birds, on the other hand, came from 

 another, unrelated reptilian stock, that which also produced 

 the dinosaurs. True flight, such as that which the birds de- 

 veloped, has been thrice evolved among the vertebrates and 

 once among the invertebrates. In the two other vertebrate 

 groups, however, the flying mechanism involves not only the 

 fore limbs, as in the bird, but also the hinder pair. With the 

 birds, moreover, the hind limbs, as in dinosaurs, show a dis- 



