xv. 16 THEROPODS 423 



of Europe and *Ornitholestes from that of North America (Fig. 241). 

 The front legs were short, with 4 or 3 digits, provided with claws ; the 

 pectoral girdle was reduced to scapula and small coracoid, with no 

 trace of clavicles. Some members of this line, the theropods, soon 

 developed into large carnivores, such as *Allosanrus (Fig. 246), over 

 30 ft long (Jurassic, North America). These animals apparently 

 swallowed their food whole and to help with this the quadrate was 

 movable and there was a joint between the frontals and parietals, as 

 in many lizards. In other respects the skull was very similar to that of 

 the pseudosuchians. 



At the end of the Cretaceous this theropod line produced the largest 

 carnivores that have appeared on the earth, such as *Tyrannosaurus 

 rex, nearly 50 ft long and 20 ft high, from North America. All the 

 previously mentioned tendencies were here accentuated, producing 

 creatures with bipedal habit, very powerful head and jaws, and much- 

 reduced fore-limbs. They presumably preyed upon the large herbi- 

 vorous dinosaurs of the Cretaceous and became extinct with their 

 prey, either from a common inability to meet the rigours of the climate 

 or in competition with the mammals and birds. Throughout most 

 of the Jurassic and Cretaceous the theropods were the dominant 

 carnivores of the world, taking the place occupied earlier by the 

 synapsid reptiles (p. 540) and later again by descendants of the synap- 

 sids, the carnivorous mammals. 



In the Cretaceous the organization of this saurischian line also 

 produced some exceedingly bird-like forms, *Struthiomimus and 

 *Ornitho)ithnus, walking on three toes and having three also in the 

 hand, one opposable and used for grasping. The skull became very 

 lightly built and the teeth disappeared, possibly in connexion with an 

 egg-eating habit (Fig. 246). 



All these carnivorous, bipedal saurichians may be grouped into a 

 suborder Theropoda. Another line of organization, starting from 

 bipedal, carnivorous Triassic theropods, adopted a herbivorous diet 

 and reverted to the quadrupedal habit. These animals, the suborder 

 Sauropoda, culminated in the immense Jurassic forms, * Apatosaurus 

 (= *Brontosaurus) and *Diplodocus, the largest of all terrestrial 

 vertebrates. Several stages of the transition from bipedal to quad- 

 rupedal habit can be traced. * Yaleosaimis from the Trias was a bipedal 

 creature 6 ft long but with rather long front and short hind legs. 

 *Plateosaurns, also of the Trias, was 20 ft long, but still bipedal. Soon 

 the front limbs became larger and more used for walking, though the 

 disparity always remained. The neck was immensely elongated and 



