GENERAL CHARACTERISTICS AND GEOLOGICAL HISTORY 35! 



Fig. 171. Mitmiocene basin (Zhizhchenko, 1940). 



Sannatian basin 



During the Upper Miocene a Sarmatian basin, cut off from the open seas, 

 was formed in the place of the Middle Miocene basin (Fig. 172). 



A number of the most typical members of the Mediterranean fauna dis- 

 appeared in the Sarmatian basin as, for example, the sea urchins, the bivalves 

 Area, Pectunculus, Leda and its most typical representatives Cardium, 

 Pecten, Venus, Corbula, Conus, Natica, Turitella and others. Pleurotoma, 

 Murex, Lucina, Loripes, Corbula and others continued to exist there for some 

 time. The hardiest forms survived : the gastropods Cerithium, Trochus, Buc- 

 cinum, Nassa and the bivalves Cardium (small size), Modiola, Tapes, 

 Mactra, Syndesmya, Donax, Ervilia. As has been pointed out by V. Boga- 

 chev (1933) a peculiar vertebrate fauna was also associated with the Sar- 

 matian basin: among fish: grey mullet, gadidae, Clupea, dolphins and 

 other Cetotheria, and seals (very similar to the present Caspian seal). Later 

 the Sarmatian basin lost much of its salinity, becoming possibly much less 

 saline than the present Black Sea. Conditions favourable for the development 

 of a hydrogen sulphide zone were created by the existence of a considerable 

 difference in the salinity of the surface and deep layers of the sea. Almost the 

 whole Sarmatian basin fauna rapidly died off under the effect of considerable 

 general loss of salinity and the contamination of the deep layers by hydrogen 



Fig. 172. Sarmatian basin (Kolesnikov). 



