2o8 Evolution and Distribution of Fishes 



CHAPTER VIL 



The Physical and Biological Environment of Fishes. 

 (d) During the Cretaceous Period 



The fundamental changes in distribution of land and 

 sea, that were effected toward the close of the Liassic 

 period and that became even more pronounced as subse- 

 quent Jurassic deposits were laid down, proceeded with 

 equally striking results during Cretaceous time. The dia- 

 gram on p. 240 shows this for the later Cretaceous. 

 The period is an important one in our present inquiry. 

 For all accumulated evidence combines to show that a great 

 and varied extension of elasmobranchs into the sea now 

 took place. It also emphasizes the viev/ that the Dipnoans, 

 most of the Holosteans, the Chondrosteans, and lingering 

 Crossopterygians, as well as many of the derivative Teleo- 

 steans remained as freshwater dwellers. 



Here also it may be observed that the only genera of 

 the fishes which occur in Jurassic strata and have come down 

 through the Cretaceous to our own day in living repre- 

 sentatives are Rhina [Squatina) , Rhinobatus, Notidanus, 

 Cestracion, Pristhiriis^ and Ceratodiis^ all of which except 

 the last are finely preserved in the Solenhofen slates, and 

 all had apparently become thoroughly marine by the be- 

 ginning of the Cretaceous, except Ceratodiis^ which per- 

 sisted throughout in freshwater. 



The Cretaceous vegetation was in many places luxuriant 

 and even sufficed to furnish workable coal. Such coal beds 

 also are practically always associated with strata that indi- 

 cate inland lacustrine conditions. Thus the Wealden coal 

 of North West Germany, and to a less extent of England; 

 the Deister coal beds of North West Germany, the Pflan- 

 zen-quader of Bohemia; the enormous beds of the Laramie 

 formation, that are estimated to include "more than 

 100,000 square miles of coal-bearing land" (5:111: 159), 

 are a few examples. 



The marine strata, as now known, have bulked more 

 largely in geological literature, than the freshwater or 



