54 BIOLOGY OF THE SEAS OF THE U.S.S.R. 



repopulation of the Arctic by Pacific forms took place as early as the post- 

 glacial period. 



The genesis of the Arctic basin fauna is closely connected with its geological 

 past, which so far is insufficiently known. 



Some geologists (Du Toit, 1939) assume the formation of the central part 

 of the Arctic basin in the Mesozoic and Tertiary periods. 



Others (D. Panov, 1945) think that a depression (900 to 1,000 m deep) was 

 formed in the Tertiary Era and that this only became deeper in the Ice Age. 



Finally, according to Wegener's theory (1922) the depression of the polar 

 basin as it exists now was formed in the Quaternary Era. 



In spite of the obscurity surrounding the geological past of the Arctic basin 

 it can be assumed that some components of the modern Arctic fauna were 

 evolved in preglacial times. Considering the orographic, climatic and hydro- 

 logical changes of the Quaternary Era with its most unfavourable conditions 

 for the life of sea fauna, we can only accept the genetic descent of the modern 

 fauna from the Tertiary one on the assumption that the latter survived the 

 Ice Age only in certain parts of the North Atlantic. The formation of the 

 main autochthonous fauna community of the Arctic basin should probably 

 be connected with the fall in temperature characteristic of the Ice Age. 



The high Arctic aspect of the present fauna is a result of it. So far it is not 

 known whether it was formed inside the polar basin or at the ' approaches ' to 

 the ice barriers of the Quaternary Arctic, but considerable geographical move- 

 ment of this fauna during the Ice Age must be accepted. 



Among the most typical and ancient endemic genera of crustaceans of the 

 Arctic region it is possible to establish a most curious division into species, 

 adapted to specific conditions of life. First of all we could single out groups of 

 species adapted to various degrees of salinity. Changes in the salinity of the 

 Arctic basin during the glacial and post-glacial periods played the main part 

 in the formation of these groups. If at the end of the Tertiary and especially 

 during the Quaternary Era there were long intervals when the Arctic basin 

 was completely or partially enclosed, then under the conditions of a temperate 

 or cold climate its waters must have lost much of their salinity. If this was 

 accompanied by the formation of brackish or fresh-water seas, they may have 

 acquired the character of whole interconnected systems. In the complex 

 system of transgressions and regressions these systems of semi-closed bodies of 

 water may have been connected at some time with the Atlantic, at other times 

 with the Pacific, becoming more saline once more and receiving some sea 

 fauna communities and later again losing some of their salinity. The bottom 

 topography of the epicontinental water bodies of the Arctic basin is such that 

 even a slight rise of the floor would have led to the formation of closed or semi- 

 closed bodies of water (Figs. 9 and 10). What effect would these changes of 

 salinity^have had on the marine fauna? The original fauna had either to die 

 during the decrease of salinity or to adapt itself to the new environment. 

 Nearly all the original fauna died out, but a definite number of species, mostly 

 crustaceans and fish, two groups most resistant to a decrease in salinity, 

 adapted themselves to life in less saline water. During the subsequent phase of 

 increasing salinity these forms were concentrated in the areas of the river 



