ENVIRONMENTAL CONDITIONS 405 



reasonably interpreted as due to local shiftings of conditions and imper- 

 fection of the fossil record. Taken, however, with their later absence 

 from fresh waters, their disappearance from the fresh-water record of the 

 Upper Devonian suggests a highly significant deployment of fishes during 

 the Devonian, owing to some environmental pressure by which the elasmo- 

 branchs passed out to and became dominant in the sea, at the same time 

 that the ganoids became dominant in the land waters. It was not until 

 far later, especially in the Mesozoic, that the ganoids in their turn began 

 to invade the sea. That movement is therefore not a part of the present 

 problem. 



The ganoids are first found as fossils in the Middle Old Bed Sandstone, 

 where they constituted, in number of genera and species, about one-half 

 of the fish fauna. From the time of their appearance they dominated in 

 their variety within the fresh waters over the competing subclass of elas- 

 mobranchs. The dipnoans appeared at the same time, at first few in 

 species, later increasing to equal importance with the ganoids. In the 

 Upper Old Eed Sandstone these became the two dominant groups. The 

 arthrodires, regarded now as an offshoot from dipnoans, appeared at the 

 same time, remained to a limited degree in the fresh waters, but found in 

 their armored head and fore body, their powerful jaws and tail, an organ- 

 ization fitting them to compete with the sharks in the open seas. The 

 competition was, however, temporary, for at the end of the Devonian they 

 became extinct, in contrast to which in the following period the sharks 

 attained a great expansion. 



If the question be raised as to what broad causes operated to bring- 

 about the differentiation in habitat between elasmobrancbs and crossop- 

 terygian ganoids, the answer must be found in the nature of environment, 

 since each, originally occupying the same habitat, became a dominant and 

 successful group, but in separated spheres of life. The environment must 

 have matched some fundamental difference in organization. Such a dif- 

 ference is found in the possession of an air-bladder, capable of use as a 

 supplemental breathing organ in dipnoans and ganoids, contrasted to its 

 complete absence in even rudimentary or embryonic form among the 

 elasmobranchs. The seasonal diyness, with the shrinkage and fouling 

 of the fresh waters, or even their complete local disappearance, was a 

 feature of the Devonian which appears to have increased in intensity to a 

 maximum in the epoch of the Upper Old Eed Sandstone. 



In the Appalachian geosyncline, as previously noted, this intensification 

 of climatic character is seen in the contrast of the color of the marginal 

 marine muds of the Middle to those of the Upper Devonian. The sand- 

 stones and shales of the Middle Devonian (Hamilton) are dark gray in 



