Fauna of Europe of the Baltic Sea has led to the evolution of many slightly divergent species, such as the Baltic Sea herring, which is a smaller variant of the Atlantic herring. The most frequent benthic species are "Saduria entomon" and "Monoporeia affinis", which is originally a freshwater species. A great part of its bottom is anoxic and without animal life. The Baltic Sea and North Sea are also home to a variety of marine mammals (common seals, grey seals). Europe contains several important freshwater ecoregions, including the heavily developed rivers of Europe, the rivers of Russia, which flow into the Arctic, Baltic,
Geology of the Baltic Sea are however absent in the whole Baltic area except for the southern fringes. Scientists do not agree on how the present depression the Baltic Sea occupies formed. Scholars like Voipio (1981) and Šlaiupa (1995) consider the depression to have been formed in the Cenozoic, before the Quaternary, by tectonic processes. Some others like Marks (2004) stress erosion was key to the formation of the depression. In the Cenozoic, long before the Quaternary glaciations the Baltic was the site of a large river called Eridanos. This river drained west in direction towards the North Sea. The Neogene uplift of the South