RISE OF MODERN FISHES 



175 



finless stage is through convergent adaptation either approached 

 or actually passed. 



The bony fishes (teleosts), which first emerge as a distinct 

 group in Jurassic time, radiate adaptively into all the great 

 body-form types which 

 had been previously at- 

 tained by the older 

 groups, more or less 

 closely imitating each 

 in turn, so that it is not 

 easy to distinguish su- 

 perficially between the 

 armored catfishes {Lori- 

 caria) of the existing 

 South American waters 

 and their prototypes 

 (Cephalaspis) of the 

 early Palaeozoic. The 

 most extreme specializa- 

 tion in the great group 

 of bony fishes is to be 

 found in the radiations 

 of abyssal fishes into 

 slow- and swift-moving 

 forms which inhabit the 

 great depths of the 

 ocean and are adapted 

 to tons of water-pres- 

 sure, to temperatures 

 just above the freezing 

 point, and to total absence of sunlight which is compensated 

 for by the evolution of a great variety of phosphorescent light- 



FiG. 56. NorthAmerica in Upper Devonian Time. 



The maximum evolution of the Arthrodiran fishes 

 {Dinic/illiys, etc.) and of the ganoids of the Upper 

 Devonian of Scotland, the establishment of all the 

 great modern orders of fishes excepting the bony- 

 fishes (Teleosts), and the appearance of the first 

 land vertebrates, the amphibians (Tliiuopus), 

 took place during this period of depression of the 

 western centre of the North American continent. 

 Modified after Schuchert. 



