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



immigrants (pseudorelicts) from the Arctic basin (chiefly in the Caspian). 

 Atlantic (Mediterranean) fauna comes in vigorously from the west, individual 

 forms penetrating as far as the Aral Sea. 



The difference between all these seas in respect of their fauna is most 

 marked. The Red Sea is populated by the tropical fauna of the Indian Ocean. 

 The Mediterranean fauna is a descendant of the south boreal fauna of the 

 Atlantic Ocean ; the Caspian Sea preserves in its fullest form the remarkable 

 relict ' Caspian ' fauna ; the least saline parts of the Black and Azov Seas are 

 inhabited by the 'Caspian' fauna, while the Mediterranean (Atlantic) fauna 

 populates the main basin. 



II. THE GEOLOGICAL PAST 



Evolution of the Seas of the Neogene System 



The geological history of our South Russian Seas has been traced in its main 

 features by the work of a number of investigators. Special credit in this field 

 is due to Andrussov and, lately, Archangelsky. 



N. Andrussov (1918) writes: 'the main characteristic of the history of the 

 Neogene ... of the Ponto-Caspian regions is their continuous and ever 

 increasing isolation from the ocean, leading to a change in the salt content of 

 the inland water basins which were being formed there, mainly in the direction 

 of lesser salinity, although at times an increase of salinity has also been 

 observed. . . . Owing to this isolation and the change in the salinity of the 

 waters covering different parts of the regions, the history of the fauna of these 

 waters affords a series of most interesting and instructive phenomena. The 

 marine fauna which originally inhabited them during the middle Miocene era 

 underwent a number of changes under the influence of changes in the composi- 

 tion of the water. On the one hand it is simply a gradual disappearance of the 

 stenohaline forms ; on the other it is a survival of forms less sensitive to 

 fluctuations in salinity (euryhaline forms), which is accompanied by con- 

 siderable morphological and anatomical changes, by great mutability of 

 species and the evolution of numerous new species and even genera . . .' 



Lower and Middle Miocene Periods 



During the Lower and Middle Miocene Periods a fully saline sea, with a 

 typically marine fauna of Mediterranean type and wide connections with the 

 ocean, stretched throughout the south of the European part of the u.s.s.r., 

 extending far to both the west and east (Fig. 171).* The process of the separa- 

 tion of this huge sea, part of the disappearing Tethys, from the ocean may 

 already have begun by the end of the Middle Miocene, individual parts of the 

 Sea losing some of their salinity. The rise of the mountains and the formation 

 of watersheds broke up the Middle Miocene Sea into more or less isolated 

 parts, which collected masses of river water and lost some of their salinity. 



* B. Zhizhchenko (1940) thinks that in the southern part of the u.s.s.r. there was a 

 much diluted basin (Aral Stage) by the end of the Oligocene and the beginning of the 

 Miocene Period, after which normal oceanic conditions were restored. 



