2i8 Evolution and Distribution of Fishes 



It seems not unlikely, therefore, that the comparatively 

 sudden reduction in variety, in genera, and in individuals 

 of the hitherto dominant group of the Cephalopoda, was 

 largely brought about by the passage into the sea of car- 

 nivorous selachians, cestracionts, and teleost fishes, which 

 had themselves been driven from lakes, rivers and deep 

 marshes by evolving carnivorous reptiles, and in time be- 

 came the agents for destruction of the cephalopods. Still 

 later, in the battles that often took place — as fossil evi- 

 dence shows — between individuals of the evolving fresh- 

 water reptiles, whole groups migrated to the seashore and 

 into the sea. There they devoured alike the cephalo- 

 pods and the fishes. Thus Cope says of the giant snake- 

 like Elasmosaiirus that it "probably often swam many feet 

 below the surface, raising the head to the distant air for a 

 breath, then withdrawing it and exploring the depths 40 

 feet below without altering the position of its body. It 

 must have wandered far from land, and that many kinds 

 of fishes formed its food is shown by the teeth and scales 

 found in the position of its stomach." 



A new environal factor deserves further to be empha- 

 sized, as it probably contributed more or less to migration 

 seaward both of fishes and some reptiles. We refer to the 

 evolution of winged reptiles, and still later of birds. From 

 present evidence, both of these must have been evolving in 

 late Triassic and early Jurassic days. For already abund- 

 ant remains of the winged reptiles are met with in Liassic 

 freshwater rocks, and of birds from the Solenhofen Slates 

 onward. The former reached their climax of development 

 in the early or mid Cretaceous and had disappeared by the 

 end of that epoch. Provided with great stretch of wing 

 in relation to body-size, with powerful jaws holding equally 

 powerful teeth, they would be able to attack fish from a 

 new centre of action. Like the Ichthyosaurians also they 

 gradually carried their attack to sea-shores during Mid- 

 Cretaceous time. Their constant presence then as a hover- 

 ing foe, at first over inland waters, and later over seashores, 

 must have contributed to the seaward passage of fishes. 



The wading, and even more importantly the large div- 

 ing, birds of this epoch such as Hesperornis, must have been 



