Three-dimensional photographs of fossils (Part 1): Gastropods from the Paratethys Ocean
Dr Robert Strum (Austria)
About 23Ma, 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.
This is the first of three articles on three dimensional photograph of fossils. 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 describe in Three dimensional photography of fossils (Part 3): 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 65Ma to about 1.8Ma. More precisely, the fossils are from the Eggenburgian of the Miocene (23Ma to 5.3Ma), 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 (23Ma to 1.8Ma) 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 (11Ma), 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 6.5Ma.
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 (3.5Ma) 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).
Sampling sites and microphotography of the fossils
Collection of the fossil gastropod shells described in this article was, in part, conducted during an excursion near the village of Eggenburg, which is located about 30km north-west of Vienna. The most interesting fossil finds were made in a sandpit near Kühnring, where, as well as the gastropod shells, small foraminifera shells (foraminifera are unicellular organisms with calcareous shells and high species diversity) and bivalves were found in the layers of sediment. Many of the fossils found in the sandpit were characterised by excellent states of preservation.
In the field, those shells selected for 3D microphotography were carefully cleared of any sand particles and, as far as possible, sorted according to their species. (For details of the procedures for stereophotography, see my article referred to above.) Unlike ammonites, with their perfectly planar shape, gastropod shells can only be oriented for photography with the use of supporting material (by convention, the central axes of the shells have to be aligned parallel to a vertical line).
To achieve this, either small lumps of plasticine, chewing gum or liquid wax can be used. To obtain an optimal 3D effect, two images are used for one stereogram. The distance between where the object is photographed for each photo should not to exceed the mean distance between human eyes (65mm). This will result in a slight difference between the pictures. By doing this, distortion of the stereograms can be successfully avoided and a good 3D picture produced.
Which gastropod fossils can be found in high numbers?
One of the most prominent gastropods that can be found belongs to the genus Pleurotoma, the shells of which can reach a height of up to 8cm and displaying impressive surface structures. As well as the tall shell, the oval aperture, with its recess for the siphon (that is, a kind of breathing tube used by most meso and cenogastropods), is rather remarkable. Well-described species of this genus include Pleurotoma asperulata (h: 5cm, w: 2.2cm), Pleurotoma schreibersi (h: 4cm, w: 2.1cm) and Pleurotoma interrupta (h: 4.5cm, w: 1.8cm).
One of the most interesting species is Conus haueri (h: 2.7cm, w: 1.1cm), belonging to the highly poisonous cone snails, whose extant relatives primarily occur in tropical lagoons and other areas of shallow water. The most apparent characteristic of this genus is the so-called ‘toxogloss radula’, which can be shot at prey, somewhat like a poison arrow. The shape of the shell of Aporrhais (Chenopus) pes pelecani (h: 2.2cm, w: 1.4cm) probably attracts the most attention among fossil-collecting amateurs.
As befits the name of this gastropod, the shape of the aperture looks a bit like the foot of a pelican! Single whorls are clearly separated from each other and exhibit deep notches running horizontally and vertically. This latter characteristic can also be observed in a slightly diminished form in the species Buccinum rosthorni (h: 2.1cm, w: 1.2cm) and Buccinum costulatum (h: 2cm, w: 1.1cm), which together make up the smaller representatives of the fossil gastropods found in the sandpits and sandstone quarries of the area. In general, the genus Buccinum is characterised by rather stocky shells, with the last whorl exceeding the volume of its predecessors many times over.
The most frequently found gastropod in the sandpit at Kühnring belongs to the species Ancillaria glandiformis (h: 2.8cm, w: 1.5cm), which is remarkable for its smooth, unstructured shell surface, making differentiation of single whorls nearly impossible. The aperture of this species is perfectly oval, with a sharp edge on the upper side and a round hole at the bottom, enabling the passage of the siphon.
One of the largest fossil gastropod shells belongs to the species Turritella turris, which reaches a height of up to 8cm and a width of up to 1.5cm. Unlike Ancillaria glandiformis, this species is characterised by 10½ to 11½ clearly discernible whorls each of which is subdivided by several ridges running around the shell. As well as this, numerous shells of this species show clear evidence of drilling by other organisms (in the form of perfectly circular bore holes) and of other traces of interaction between competing animals.
The most extraordinary shell geometry is found in the so-called worm snail, Vermetus arenarius (h: 2.4cm, w: 2.3cm), which was a sessile gastropod species and is, among others, responsible for the formation of gastropod reefs. The food of this species was not based upon predatory strategies, as in the case of most other gastropods. Rather, by using a mucous net, it filtered microscopic organisms out of the water. The last gastropod described here belongs to the species Murex schöni (h: 2.1cm, w: 1.3cm). The genus Murex consists of numerous species, which are remarkable for their bizarre extensions.
What can we learn from this study?
Miocenic sediments exposed in the eastern part of Austria often have an enormous abundance of vertebrate and invertebrate fossils, of which gastropods are a prominent part. Spectacular finds, which are regularly made, attract enthusiastic palaeontologists and hobby collectors from all over Europe to this lovely region. As in the case of ammonites, 3D photographs of fossil gastropods are highly effective, since additional information on shell shape and structure is well illustrated using this technique.
Steininger, F. and Senes, J.: Chronostratigraphie und Neostratotypen. Miozän der zentralen Paratethys – M1 Eggenburgien. Vydavatel’stvo Slovenskej akadémie vied, Bratislava (1971).