Dr Robert Strum (Austria)
About 23mya, the Paratethys Ocean covered a large area, including what is now the Vienna basin and Alpine Foreland. This mostly shallow ocean was a habitat for a large number of plant and animal species, which included numerous marine gastropods, some of which are discussed in this article. To give the reader a better impression of their volume and shape, the fossil shells have been photographed by using the stereoscopic technique I described in Issue 18 of Deposits (see 3D photography of fossils: ammonites from the Northern Limestone Alps of Austria).
Today, beautiful shells with various shapes and patterns, of recent marine gastropods, can be collected in large numbers along Mediterranean and tropical coasts. While this seems normal to us, finding marine molluscs in Austrian sandpits, far away from any ocean, is a little more surprising. Of course, the simple answer is that these shells are fossils from the ocean referred to above, which once covered what is now Central Europe. The fossils commonly originate from the Tertiary period, which lasted from 65mya to about 1.8mya. More precisely, the fossils are from the Eggenburgian of the Miocene (23mya to 5.3mya), named after Eggenburg, a lovely village north-west of Vienna. (The Eggenburgian is a Lower Miocene stage of the Central Paratethys regional chronostratigraphic classification.)
Geological development of Central Europe during the Neogene
The palaeogeographic development of Central Europe during the Neogene (23mya to 1.8mya) was characterised by an extensive transgression (that is, an expansion of the oceans due to an elevation of sea levels), causing large areas of Central Europe to be covered with water. The east-west trending branch of the resulting ocean, covering (among other areas) Austria and large parts of Germany, is referred to as the Paratethys by today’s palaeontologists, emphasising its role as a water basin adjacent to the huge Tethys Ocean. The period of transgression persisted into the Tortonian stage (11mya), at which time the Paratethys began its withdrawal from the area and became more and more brackish. The end of this so-called regression phase was reached about 6.5mya. The former oceanic landscape completely disappeared from Central Europe and, what was once open ocean, became small, brackish lakes and evaporation pans (that is, evaporation basins with highly increased amounts of sea salt). In Eastern Europe, the water withdrew to the region of today’s Black Sea, while, in Mediterranean regions, small predecessors of the Mediterranean Sea were formed. From the middle of the Pliocene (3.5mya) onwards, the Mediterranean and Black Sea basins were successively filled with sea water and the subsequent geological development finally resulted in the formation of the European coastline that is visible today (for further information, see Steininger and Senes, 1971).