400. Evolution and Distribution of Fishes 



acclimated marine species to a freshwater environment is 

 a matter relatively of rare occurrence. 



What is probably the most condensed and highly modi- 

 fied group of theTeleosts the Plectognathi — requires short 

 notice here. For all are coastal or deep-sea types, whose 

 derivation and evolution from the Acanthuridae, as of it 

 again from brightly colored Chaetodontidae, has been re- 

 peatedly emphasized during the past half century. The 

 existence of genera of all of these great divisions, back in 

 Eocene strata, shows that marine fishes, allied to those now 

 living, must have been evolving through the greater part of 

 Eocene time, a conclusion that direct fossil evidence verifies. 



A brief review then of the detailed studies presented in 

 this chapter indicates that the Acanthopterygii first evolved 

 over the N. American continent during the Cretaceous 

 period, and amid extensive lakes, swamps and rivers, some 

 of the last of which may have flowed into the Cretaceous 

 sea, that existed then from Kansas southeastward, or west- 

 ward into the Pacific, before the western coastal ranges had 

 fully developed. The living groups Percesoces and Aphre- 

 doderidae known from that region, with extinct types of 

 the latter, are regarded as transitions from the soft-finned 

 families to the higher spine-finned ones. But these two 

 transition groups were probably derived from ancestral 

 soft-finned types allied to the clupeo-salmonids, which — 

 as judged by the abundance of Diplomystus — were wide- 

 spread over the above region. These again seem traceable 

 backward into Jurassic time as migrant derivatives from 

 freshwater Leptolepidae, like Leptolepis or Thrissops of 

 European origin. From derivatives of the Aphredoderidae 

 or from allies as yet unknown, the Centrarchidae and 

 Percidae evolved, largely over the N. American continent, 

 where they now abound in its rivers and lakes. But from 

 like ancestry the Berycidae early migrated into a marine 

 habitat, and soon spread over a wide sea territory. 



In further dispersion alike of the primitive freshwater 

 and derived marine ones, several important land-masses 

 that have now disappeared acted as connecting bridges, 

 while their shore-margins favored passage from a fresh- 

 water to a marine habitat for not a few of the evolving 



