462 VERTEBRATE LIFE AND ORGANIZATION 



lated to bones of the upper jaw and palate in such a way that they are 

 loldetl against the rool ol the mouth when the mouth is closed, and 

 automatically brought forward when the mouth is opened (Fig. 

 23.13 B). The poison ol these snakes is hemolytic, and causes a break- 

 down ol the red blood cells in the animal bitten. Coral snakes belong 

 to a group related to the Old World cobras. Their poison is neurotoxic, 

 and their fangs are a pair of hollow, short, stationary teeth attached to 

 the front of the upper jaw. The poison of snakes is used to immobilize 

 and kill their prey, which they swallow whole. In addition, the poison 

 of some snakes contains digestive enzymes that are carried by the vic- 

 tim's blood stream throughout its body before its death. 



Dinosaurs and Their Allies. Lizards and snakes are the successful 

 reptiles today, but during the Mesozoic era the land was dominated by 

 another offshoot of primitive rhynchocephalian-like reptiles. These 

 "ruling reptiles" were the archosaurs— an assemblage of several orders 

 that shared many features, including a tendency to evolve a two-legged 

 gait. Reduced pectoral appendages, enlarged pelvic appendages, and a 

 heavy tail that could act as a counterbalance for the trvmk were cor- 

 related with this mode of life. 



Saurischian dinosaurs (order Saurischia) evolved from ancestors 

 that were only three or four feet long, but later saurischians became 

 giants of the land and swamps. Tyrannosaiirus (Fig. 23.14 A) was the 

 largest terrestrial carnivore that the world has ever seen. It stood about 

 20 feet high, and had large jaws armed with dagger-like teeth six inches 

 long— a truly formidable creature! Other saurischian dinosaurs were 

 herbivorous swamp-dwellers that reverted to a quadruped gait, but the 

 bipedal gait of their ancestors was reflected in their long hind legs. 

 The buoyancy of the water permitted some to grow to enormous size. 

 Brontosaurus (Fig. 23.14 B) and certain of its allies attained lengths of 

 80 feet and weights of 50 tons. Only certain modern whales have ex- 

 ceeded them in size. 



Many dinosaurs in another group (order Ornithischia) became ter- 

 restrial, rather than swamp herbivores. These also reverted to a quad- 

 ruped gait and increased in size, though none was as large as the 

 saurischians. These animals undoubtedly formed much of the diet of 

 carnivores such as Tyrannosaurus, and many evolved protective devices 

 such as spiked tails, bony plates on the body and horned skulls. Stego- 

 saurus and Triceratops (Fig. 23.14 C and D) are examples of this group. 



The reasons for the evolution of large size are not entirely clear. 

 Within limits, large size has a protective value, but it may also have 

 been a way of achieving a more nearly constant body temperature. 

 Reptiles, being poikilothermic, derive a great deal of their body heat 

 during warm weather from the external environment. As mass in- 

 creases, the relative amount of body surface available for the absorption 

 of heat decreases, and body temperature would fluctuate less. An adap- 

 tation of this type may have been particularly important for animals that 

 lived in a warm climate and were too big to shelter by burrowing or 

 hiding beneath debris, for it would help prevent body temperature from 



