614 The Animal Kingdom 



and sphenodon all were differentiated at a very early time from this 

 stem form. Also from this cotylosaur group were developed the first 

 primitive mammal-like reptiles which later gave rise to the mammals 

 and the thecodonts which were destined to give rise to the dinosaur line. 

 The thecodonts or ruling reptiles were small, predaceous forms 

 with somewhat elongated hind legs, thus suggesting the beginnings of 

 bipedal development. From these animals arose the tremendous and 

 varied dinosaurs. These dinosaurs ranged in size from small species 

 to some weighing fully 50 tons. Some were large and carnivorous — 

 forms such as the 20-foot long Tyrannosaiirus and the 34-foot long 

 Allosanrns were examples of this group. From forms ancestral to 

 these carnivorous dinosaurs there developed the tremendous herbivorous 

 forms such as Brontosaiirus and Diplodocus. So large were these (they 

 weighed up to 50 tons) that it seems unlikely that they could have 

 walked on land ; more probable is their having been semiaquatic forms 

 which relied on the support of the water for their mighty bodies. These 

 represented one line of development, the so-called reptilelike dinosaurs. 



A second line of development was that of the birdlike dinosaurs 

 which were primitively biped. Most famous of these are the duckbills 

 (hadrosaurs). Many of these forms reverted to the four-footed method 

 of locomotion and some developed remarkable dorsal spines. These 

 included Stegosaurus, Ankylosaurus, and the famous Triceratops with 

 its three horns and great bony protection around the neck. 



The winged reptiles were the third group to develop from the 

 thecodonts. These are the pterosaurs. Though most were small, no 

 larger than sparrows in fact, others were very large. Pteranodon was 

 the largest of these, and had a wing spread of 20 feet and a skull 2 

 feet long. It had no teeth and the tail was very short. 



The crocodiles are the only living survivors of this once mighty 

 group. The birds, however, represent direct descendants of the 

 thecodont stalk. 



THE CLASS AVES 



While most of the descendants of the thecodonts are but fossil 

 remains, one branch gave rise to the birds which at present are at the 

 height of their development and abundance. These "glorified reptiles" 

 have become so completely adapted to flight that they seem far re- 

 moved from their reptilian progenitors. Secondarily some forms have 

 even lost their power of flight, thus demonstrating that like other sue- 



