Fossil Shells of Marine Gastropods and
Bivalves from THE PARATETHYS
by Robert Sturm, Austria
Some interested readers might ask for the originality of presenting shells of marine gastropods and bivalves, since you can find tons of them on the beach. However, the reason for making this contribution is that the shells were sampled in sand pits near Vienna, Austria, which we know is about 500 km away from the Adriatic Sea. They represent relics of the Tertiary era, when wide parts of Austria and adjacent countries were covered by an ocean called Paratethys. The peak of this transgression period was about 25 million years ago during the Oligocene. At this time, coral reefs and lagoons were colonized by lots of mollusc species, from which the shells of some typical exponents are presented here.
Shells of the presented molluscs have been sampled near Eggenburg, a village 30 km northwest of Vienna. The whole region is located near the Vienna Basin, which is embedded between the Alps and Carpats. Formation of the basin started in the Miocene (Badenien). During this time the region was exclusively colonized by a marine fauna, including molluscs, bryozoans, foraminifers, etc. About 11 million years ago, the basin was more and more marked by a brackish character. The end of this development, however, was reached about 6 million years ago with the formation of large freshwater bodies. Regression of the water bodies took place in the Quaternary, about 1 million years ago.
3…Aporrhais (Chenopus) pes pelecani
(please note: Determination of the shells was carried out according to the key of Hoernes, 1856, using the original names for the taxa. Some of these taxa have been changed due to new species descriptions and re-organisation of the gastropod system.)
In general, gastropods represent the mollusc group with the highest species diversity, but it is difficult to list some characteristics that are typical only for gastropods. The most specific properties of snails, however, include the torsion of their body and their spiralled shell. While most of the fossil gastropods inhabited marine biotopes, numerous recent species can be also found in terrestrial habitats. Gastropods act either as carnivores, herbivores, or as omnivores, taking up both nutriments of animal and plant origin.
The systematic classification of the gastropods depends upon five main factors:
The gastropods presented on Plates 1 and 2 belong to the subclass of the Caenogastropoda, being characterized by typical embryonic and larval shells. The larvae of these gastropods often nourish themselves with plankton. Turitella and Vermetus can be assigned to the order of the Littorinimorpha, whereby the latter species is characterized by its sessile life and its importance as a reef-forming organism. Aporrhais belongs to the order of the Strombimorpha, whereas all remaining species presented on the plates may be attributed to the so-called Neogastropoda. As exhibited in the sketch at the end of this contribution, Conus acts as a predator, being able to shoot its malicious tooth like an arrow.
Most bivalves have a shell consisting of two symmetric flaps. Further characteristics are the reduction of the head and the radula. Besides 20,000 fossil species, the same number of recent species has been described until now. As most bivalves are sessile, they obtain their nutriments by filtering plankton particles out of the water. Bivalve classification is mostly based on the morphology of the dentition and ligament, thereby distinguishing between eight groups. The two species presented on Plate 3 belong to the order of the Heterodonta, which, from a phylogenetic point of view, are the youngest bivalve group colonizing recent marine habitats.
Plate 4: (Click image to view larger)
The last plate (height: 6 centimetres) was especially made for all those microscopists, who want to get a more spatial impression of the mollusc shells. For an ideal perception of the stereographic effect, respective stereographic glasses (left: red, right: blue) are needed.
Conus taking up a small fish.
Published in the October 2005 edition of Micscape.
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