REPTILIA 217 



Palaeozoic, but actually ran out their entire course of specialization 

 and became entirely extinct before the Mesozoic age began. Thus the 

 cotylosaurs, proganosaurs, anomodonts, pelycosaurs, and phytosaurs 

 died out either in Permian or at least not later than early Triassic 

 times. The remaining orders that arose in the Palaeozoic were able 

 to weather the climatic crisis at the end of this age and were the an- 

 cestors of the great Mesozoic orders of reptiles. 



THE GOLDEN AGE OF REPTILES 



The reptiles, as we have seen, made a modest start in the Carbonifer- 

 ous, underwent a considerable degree of adaptive specialization during 

 the Permian, and in some lines became senescent and died out. On 

 the whole, however, the Palaeozoic reptiles were of generalized or 

 primitive types and gained no great ascendency. It was not until 

 the Mesozoic that the reptiles really came into their own. It was 

 during this "Age of Reptiles," an immense period involving several 

 millions of years, that they gained their world supremacy and came 

 to exercise undisputed sway over the land habitats, and disputed 

 with the fishes the right to rule the waters. The dominance of the 

 reptiles of this period was due largely to five great groups: ichthyo- 

 saurs, plesiosaurs, carnivorous and herbivorous dinosaurs, and ptero- 

 saurs. Each of these assemblages deserves individual attention. 



ICHTHYOSAURIA 



No more extreme case of adaptation of a member of an essen- 

 tially terrestrial class for an aquatic habitat could be given. The 

 first reptiles are believed to have acquired their main characters in 

 adaptation to land life, so we have no alternative than to believe 

 that the ichthyosaurs have been derived from land forms that found 

 the sea a rich hunting ground and developed the habiliments of a fish 

 to facilitate their aquatic activities. A change involving, first, adapta- 

 tions for land life and, second, a return of aquatic adaptations is re- 

 ferred to as an example of reversed aquatic adaptation, and is by no 

 means uncommon among the higher vertebrates. The external form 

 of an ichthyosaur (Fig. 124, C) is strikingly like that of a sword-fish. 

 The pectoral and pelvic limbs are flipper-like fins, the tail has a re- 

 markable caudal fin externally precisely like that of a fish, a very 

 fish-like dorsal fin plays the same role as that in a fish. Unlike other 

 reptiles and like the fishes, these creatures have no real neck, but the 

 head seems to be joined broadly with the trunk. All of these adapta- 



