xv. 15 DINOSAURS 421 



presumably assists in respiration. It is a development for this purpose 

 quite distinct from the mammalian diaphragm. It is not impossible 

 that the dinosaurs possessed further developments of this arrangement 

 of the heart and lungs, and that they owed some of their success to 

 this mechanism. 



The modern crocodiles represent only the survivors of a once much 

 more abundant group. Crocodilus is the most widespread genus, 

 occurring in Central America, Africa and Asia, Malay and East 

 Indies, and North Australia. Alligator, with each fourth lower tooth 

 penetrating into a hole in the maxilla, is found in North America and 

 in China. Caiman of Central and South America is related to Alligator. 

 The length of the snout varies considerably in different species, and 

 is extremely long and slender in the fish-eating Gavialis, Indian 

 gharial, and Tomistoma of the East Indies. Crocodiles lay hard-shelled 

 eggs in large clutches, depositing them in the sand or in nests com- 

 posed of vegetation. The crocodiles seem to have changed little since 

 they first appeared in the late Triassic, perhaps 190 million years ago. 

 *Protosuchns of that time had a pelvis like that of crocodiles but was 

 otherwise very like a pseudosuchian. There were numerous types of 

 crocodile in the Jurassic and Cretaceous, living both in fresh water 

 and in the sea. In these forms the palate was closed only as far back as 

 the palatine bones; the addition of flanges of the pterygoids took 

 place only in the Eocene crocodiles, which were numerous in many 

 parts of the world, including northern continental regions that today 

 are too cold for such animals. In spite of their specializations for 

 aquatic life, the crocodiles show us many features that were present 

 in the earliest archosaurs and they therefore give some idea of the 

 characteristics of the ancestors of the pterodactyls, dinosaurs, 

 and birds. 



15. The 'Terrible Lizards', Dinosaurs 



In the 10 million or so years at the end of the Triassic some of the 

 descendants of the pseudosuchians became very successful and numer- 

 ous and many of them were very large. The large size was not a 

 characteristic only of one line but of two quite distinct ones, each with 

 several sub-divisions. The term dinosaur is applied to all of them, but 

 the two main lines have little in common beyond the characters 

 common to all archosaurs. The desire to explain this extraordinary 

 exuberance of reptiles has attracted much attention to these 

 giants. 



