xv. 6 ICHTHYOSAURS 401 



Triassic forms were related to the plesiosaurs and were specialized for 

 mollusc-eating by the development of large grinding teeth on the 

 jaws and palate. 



5. Order *Ichthyopterygia 



These animals, found mainly in the Triassic and Jurassic seas, 

 were even more modified for aquatic life than the plesiosaurs (Fig. 

 228). They occupied a position comparable to that of the dolphins 

 and whales during the Tertiary period. The body possessed a stream- 

 lined fish shape and swimming was by lateral undulatory movements. 

 The vertebrae were amphicoelous disks and there were large dorsal 

 and caudal fins, with the vertebral column apparently continued into 

 the lower lobe of the latter. The paired fins were small and presumably 

 used as stabilizing and steering agents. The pelvic girdle did not 

 articulate with the backbone. In the limbs the number of digits was 

 often greater or less than the usual five, and there was often hyper- 

 phalangy. Evidently this type of skeleton gives better support for a 

 fish-like paddle than does the pentadactyl tetrapod type and it is 

 interesting to find it evolved again in vertebrate stocks that returned 

 to the water. This seems in a sense to be a case of reversal of evolution. 



The head was much modified for aquatic life, with a very long 

 snout armed with sharp teeth, and nostrils set far back. The eyes 

 were large and surrounded by a ring of sclerotic bony plates. The 

 temporal fossa, though in the parapsid position, had boundaries 

 different from that in the synaptosaurians. 



The Triassic ichthyosaurs were already greatly modified and we 

 have no trace of the origin of the group. Romer has suggested a pos- 

 sible derivation from cotylosaurs related to the ancestors of mammal- 

 like reptiles. The ichthyosaurs were more highly adapted to aquatic 

 life than any other reptiles known. They seem to have been viviparous, 

 since the skeletons of small specimens have been found within larger 

 ones. Like the plesiosaurs they developed a special mollusc-eating 

 type, *Omphalosanriis, in the Triassic. 



6. Subclass Lepidosauria 



Most of the animals popularly considered as characteristic of the 

 period of reptilian dominance have a two-arched or diapsid skull. This 

 condition, or some modification of it, is found in all the surviving 

 reptiles except the Chelonia, and in the birds. Formerly all the two- 

 arched reptiles were placed in a single subclass, the Diapsida, but it 

 is now customary to divide them into two subclasses, the Lepidosauria, 



