REPTILIA 219 



PLESIOSAURIA 



These marine reptiles furnish less extreme examples of aquatic 

 adaptations than do the ichthyosaurs. Some of the plesiosaurs 

 reached a giant size, being upwards of fifty feet in length. The early 

 members of this group were of a rather generaUzed type and might 

 properly have been thought of as marine hzards. Later came a type 

 such as Elasmosaurus (Fig. 124, A), a slow-moving, long-necked, 

 short-bodied, small-headed type, with long, narrow paddles. The cli- 

 max of plesiosaurian specialization was reached by such forms as Tri- 

 nacromerion (Fig. 124, B), which is characterized by short body, rather 

 short neck, and fiercely predaceous jaws; a creature with all the ear- 

 marks of an aquatic speed demon, and doubtless as much of a terror to 

 the fishes as were the dinosaurs to the smaller denizens of the dry land. 



CARNIVOROUS DINOSAURS 



The dinosaurs are the last word in terrestrial specialization among 

 the reptiles. Doubtless the ancestors of this great group were rather 

 generalized lizard-like forms that lived in the late Permian, but as yet 

 the palaeontologists have not been able to place their hands upon an 

 unequivocal ancestral dinosaur. As has already been said, the group 

 had a dramatic rise to dominance and an equally dramatic extinction 

 at the close of the Cretaceous. While they lasted, their course was 

 an impressive one and far out-shadowed that of all contemporaneous 

 land creatures. On this account the middle and late Mesozoic period 

 has with some justification been called the "age of dinosaurs." 



The carnivorous dinosaurs were for the most part Saurischia (with 

 lizard-like pelvis), as opposed to the Ornithisda (with bird-like pel- 

 vis), a group to which most of the herbivorous dinosaurs belong. The 

 principal evolutionary changes that took place within the group of 

 carnivorous dinosaurs are associated with an absolute increase in 

 body size, relative decrease in the size of the fore limbs and increase 

 in that of the hind limbs, accompanied by a progressive tendency 

 toward bipedal locomotion and speed of running. The culminating 

 types were swift, cursorial creatures, with long tails for balancing, 

 short grasping fore limbs, long neck, and head armed with heavy 

 recurved teeth. One can readily imagine them as able to use their 

 powerful hind legs as effectively as does the ostrich. As a climax type 

 we may cite the great Tyrannosaurus rex (Fig. 124, F) of which Mat- 



