5o8 Evolution and Distribution of Fishes 



eurypterids, crustaceans, freshwater molluscs, and abundant 

 fishes. While the mailed fishes of earlier periods had large- 

 ly died out, a prolific dipnoan, elasmobranch, and ganoid 

 series stocked the freshwater areas. Sudden destruction 

 of great masses of the fishes by diastrophic, volcanic, and 

 other physical agencies, gave rise to free fish-oils that were 

 deposited with argillaceous, calcareous, or siliceous part- 

 icles to form the extensive beds in which, by transformation 

 of the oil, petroleum is now obtained, alike in the Old and 

 New Worlds. 



The beds of corresponding age, namely the Horton- 

 Albert and the Lower Mississippian in Canada and the 

 Eastern or Central States, show similar geologic and palae- 

 ontologic details to the last. But gradual migration sea- 

 ward of elasmobranch genera was now proceeding, and so 

 originated the marine elasmobranch life of the next higher 

 or Carboniferous Limestone or Upper Mississippian period. 

 This is further discussed in Chapter 9. 



The Coal Measures or Pennsylvania beds of the Old 

 and New Worlds are very largely of freshwater origin, and 

 so enclose evidences of an often teeming flora and fauna. 

 The remains of the former where appropriately deposited 

 originated coal. The latter include great aggregations of 

 crustaceans, arachnids, insects, freshwater molluscs, dipnoan 

 elasmobranch and ganoid fishes, as well as numerous species 

 of amphibians. 



Where fishes abounded In the lakes and swamps that 

 were forming coal, and were killed in masses, the product 

 became a cannel coal. But where as in Kansas, Oklahoma, 

 and Texas fish destruction was effected on a great scale, 

 there petroliferous shales were formed. 



The migrant elasmobranchs which had invaded the sea 

 seem largely If not wholly to have been obliterated by close 

 of the period. In transition to the Permian profound 

 physlco-geologic changes were proceeding that largely 

 altered the rocks, the flora, and the fauna. So a mixture of 

 palaeozoic and mesozolc types occurs. Volcanic activity 

 was frequent and widespread also, throughout the Old 

 and New Worlds. This may In part account for the obliter- 

 ation of marine fishes, and for their great diminution even In 



