Dr Robert Sturm
Over the last few decades, local amateur collectors, as well as professional palaeontologists, have collected a large number of fossils from quarries and sandpits on the northern margin of the Central European Alps. With the help of these marine and terrestrial fossils, it has been possible to reconstruct a picture of the animal life in the early Tertiary (about 50Ma). Interestingly, many animals that lived in these ancient tropical habitats can still be found in the oceans and on the coasts today.
Main geological characteristics of the area
As you can see from the map in Fig. 1, the geology of the northern Alpine margin can be subdivided into three main, east-west striking units:
- The Flysch Zone (green) is situated directly north of the limestone Alps, the exposed part of which reaches a width of up to 20km. The grey marls and sandstones belonging to this unit were deposited in a deep, oceanic basin during the Cretaceous (144 to 65Ma) and measure more than 3,500m in thickness. In the south, the Flysch Zone was successively superimposed on by the Northern Limestone Alps.
- The Helvetic zone (violet) borders on the Flysch Zone in the north, forming a three to four kilometre-wide strip. The most recent strata of this geological unit are exposed in a small quarry at St Pankraz (red arrow) and are Eocene in age (50Ma). In the south, the Helvetic zone was superimposed on by the Flysch Zone.
- The Molasse Zone (yellow) extends far to the north (with a width of up to 50km) and usually consists of gravels and sands measuring several thousand metres in thickness. These sediments represent the former detritus that was produced during uplift of the alpine mountain belt in the early Miocene (25Ma). During this time, the rocks of the Molasse Zone were also superimposed on by the Helvetic zone.
The sediments and rocks of the Flysch and Helvetic zones, which appear at the northern margin of the Central European Alps and which can be attributed to different oceanic basins and geological eras, were certainly not deposited where they can be found today. This can easily be explained with the help of the geological north-south profile in Fig. 1.
The rocks of the Northern Limestone Alps were deposited during the early Triassic (250Ma) at the latitude where today’s Sahara can be found, while the marls and sandstones of the geological units mentioned above were most probably formed south of where the present alpine mountain belt is situated. As a result of the movement of the African plate northwards over a time period of almost 100myrs (Late Cretaceous to late Tertiary), several kilometres of thick rock layers were folded and lifted up, as well as thrust over the Eurasian plate. In this way, plate tectonics were responsible for rocks that were originally formed and deposited far to the south ending up in Central European regions.
The fossils of the Flysch and Helvetic zones
One of the main sites within the Flysch and Helvetic zones where you can collect marine and terrestrial fossils is at St Pankraz, about 15km north of Salzburg. There is a specific fossil layer, measuring up to one metre in thickness, which contains large amounts of glauconite, masses of fossil bivalves and gastropods, as well as calcareous shells of protists, corals and other organisms. Fossil teeth and shark vertebrae are also common. Some of the teeth belong to the species, Carcharocles auriculatus, a shark whose size is comparable to the great white shark, Carcharodon carcharias. The teeth of several other shark species (for example, the grey reef shark, mako shark and so on), the jaw plates and spines of sting rays, and the shield plates of teleosts are also common.
As well as various fossils originating from a subtropical oceanic basin, further sensational finds have been made by collectors over the last few decades. For instance, the teeth of a primitive whale (Archaeoceti: Basilosauridae), the oldest in Europe, have been found here. In addition, the remains of land animals, such as the complete armour of giant tortoises (Osteopygis salisburgensis, Puppigerus camperi and Cheirogaster steinbacheri), as well as crocodiles and tapirs, together with petrified fruits, nuts and even pieces of amber, indicating the presence of a nearby continent or island arc, some 50Ma.
In Figs. 3 and 4, there is a selection of fossils that were collected from the Flysch zone and Helvetic zone of St Pankraz. Most fossils from the sandstone and loam layers are shells of gastropods, bivalves and cephalopods. About 100 different species of gastropods have been found, most of which may be also found in today’s tropical coastal regions. The fossilised shells of the neogastropod Conus sp. are very interesting. This species was a predator that used its radula tooth as a ‘poisoned arrow’. The genus, Pleurotomaria, is also interesting as a result of being a ‘living fossil’ that first occurred in the late Palaeozoic, about 300Ma.
Today, this gastropod is chiefly found in the deep sea, east of Japan. The diversity of fossil bivalves is more or less the same as that of the gastropods, but the most common are the genera, Cardium and Spondylus. Cardium is marked by an increased ability to free-swim, not typical of bivalves, and reached its peak during the Miocene. The bivalve, Pholadomya, is conspicuous as a result of its elongated, date-like shape and its ability to bore into wood and calcareous rock with its foot acting as a kind of sucker disk. Important examples of cephalopods that have been found include the genera Eutrephoceras, Nautilus and Aturia, of which only Nautilus can still be found today.
Another group of invertebrates, whose fossils can be found in the quarries and sandpits at St Pankraz, are echinoderms, of which sea urchins and sea lilies are the most common. The main genera of the sea urchins are Micraster, Conoclypus, and Echinolampas. Of these, Micraster is the most conspicuous as a result of its petrified shell having the shape of an idealised heart. As for sea lilies, only small parts of their stalks have been preserved. The most common genera of these echinoderms include Isocrinus and Trochocyathus. The remains of marine arthropods include parts of the armour (for example, Ranina marestiana) and the claws (for example, Harpactocarcinus and Titanocarcinus).
The marine life of the Eocene
Based upon fossils found at St Pankraz, I have crudely tried to reconstruct Eocene marine life – especially marine vertebrates – of this ancient tropical oceanic basin. The result of this reconstruction is shown in Fig. 5.
In this fossil-based reconstruction, the vertebrate population of the tropical oceanic basin was dominated by predators, with various kinds of sharks playing an essential role. Marine life was also characterised by primitive whales, which reached impressive body sizes.
The shark and whale fauna was most probably accompanied by swarms of teleosts (bony fishes common today) as well as different species of ray fishes. On the seafloor, invertebrates such as gastropods, bivalves and sea urchins were the dominant fauna, most of which were either acting as endobenthic organisms (that is, buried in the sediment) or living between dense associations of aquatic plants.