Dr Robert Sturm (Austria)
Unlike the British Isles, which contain large swathes of Palaeozoic rocks, Central Europe only features sporadic rock types belonging to this early geological era. Among the most salient geological terrains entirely or partially from the Palaeozoic are the Bohemian Massif, the Central Plateau in France, the Ardennes in Belgium and the Black Forrest in Germany. Within the alpine mountain belt, the frequency of Palaeozoic rock formations is even less, with such rock deposits being limited to the Greywacke Zone in the Central Alps, the Palaeozoic lithologies exposed around the city of Graz, the Gurktal nappe and the Carnic region/Karawanken in the Southern Alps.
In this article, I will discuss some important Palaeozoic index fossils from the Carnic region that have been found by Austrian palaeontologists over the last few decades.
Geology of the Carnian region in the Southern Alps – a brief overview
When visiting the Carnic region in the early nineteenth century, the famous natural scientist, Leopold von Buch, expressed his fascination of the virgin landscape he came across with the statement that it was “a fully unknown area [that] has to be discovered and comprehensively described”. Since then, the Carnic region has acquired a high reputation among geologists and palaeontologists in Europe and, indeed, all over the world, because it represents an outstanding ‘picture-book’ containing 500myrs of earth history. In this area, there is a considerable amount of evidence of specific sections of this long period of geological time and especially for those prehistoric organisms that populated the early oceans.
Since a detailed description of the geology in the Carnic region is far beyond the scope of this article, only the major cornerstones of the geological development of the area will be introduced here. There are traces of the geological past going back to the Ordovician (about 500Ma) – a period that was characterised by a transgression of the ocean into the Carnic region and a continuance of this thalassocratic phase (that is, a period of widespread distribution of seas over the surfaces of our modern-day continents) until the early Carboniferous. Within this time period, layers of sediment (consisting of clay, sand and lime) were deposited with a total thickness of 3,000m.
Single layers of lime located within this mighty sedimentary column were formed from the skeletons and shells of various marine organisms, as the remnants of gastropods, bivalves, cephalopods and microorganisms (such as foraminifera) were accumulated in considerable amounts. Notable sedimentary components are the trilobites, which populated the seafloor and were incorporated into the sediment after their death.
The Devonian period (400 to 360Ma) was characterised by a warm climate, which supported the radiation of corals and the successive growth of coral reefs. The reefs themselves offered optimal habitats for all kinds of marine organisms. As well as those animals mentioned to above, different species of sea lilies and stromatopores lived in between the corals.
During the Carboniferous period, sea levels rose significantly, leading to the extinction of the corals and the end of the growth of the reefs. The water basin formed by this transgression was again filled with fine and intermediately grained sediments, which emanated from huge mud avalanches and also from nearby areas of land. After this process came to a halt, the sedimentary layers were folded, tilted, thrust into each other and, finally, lifted out of the ocean, marking the birth of the Carnic Alps.
In the middle of the Carboniferous era (about 300Ma), the early Carnic Alps were subject to extensive and continuous weathering and erosion, and then, after a few million years, they were partially flooded again by the ocean. This second transgression took place from the southwest and ranged over several tens of kilometres, resulting in the deposition of new layers of sediment and the burial of folded sequences under horizontal layers of rock. As a result of oscillations in the sea level, the composition of the newly accumulated sediments varied between lime deposits, including marine organisms, and sequences of sands and clays containing plant fragments.
The youngest rocks of the Carnic region date back to the middle of the Triassic. During the early Cretaceous (90Ma), the entire area was again subjected to orogenesis (that is, mountain building), which affected the regional lithology, such that rock units were compressed, folded and thrust into one another.
The last natural process to have significant effects on the geomorphology of the Carnic area was the so-called ‘Wuerm ice age’ (about 20,000 years ago), which filled the valleys with glaciers that reached thicknesses of up to 2,000m. After the ice melted 14,000 years ago, what was left was a landscape free from any vegetation and characterised by large deposits of loam and gravel, which was rapidly populated by numerous plant species.
Fossil from the Carnian region
As already mentioned above, petrified shells and the remains of marine organisms played an essential role as major rock-forming constituents during the development of the Carnic Alps. Many of these fossils are probably marine animals and plants that lived during a relatively short period of geological time, so that they can be used to determine the relative age of their host rocks.
These so-called index fossils include both macro and micro-organisms. Important members of the first group are trilobites, cephalopods, graptolites, brachiopods, gastropods and bivalves, while the second group consists of foraminifera, ostracods, conodonts, coccoliths, spores and pollen (among others). Nowadays, after the continuous improvement of preparation and imaging techniques, microfossils have surpassed macrofossils in their value for dating rock and reconstructing earth history.
Within the Carnic region, the oldest index fossils include trilobites, graptolites, various cephalopods, corals, bivalves and brachiopods. Trilobites, which generally represent valuable index fossils from the early Cambrian to the Middle Devonian, are mostly found in the Silurian and Devonian limestone rocks and they can sometimes be found in high numbers. The most important taxa of these arthropods include Phacops, Harpes, Philippsia and Pseudophilippsia.
The defining feature of trilobites is their segmentation into three distinct parts – the cephalon (head), the thorax (body) and the pygidium (tail). The animals reached a total length between 3cm and 8cm, although in extreme cases they could reach a length of 75cm. As they grew, they are thought to have moulted about 30 times. Trilobites were benthic organisms, with the seafloor representing their main habitat, but some smaller species were also able to swim.
Brachiopods are generally used as index fossils for the Silurian to the Jurassic and are mainly found in the dark and rather soft schists of the Upper Carboniferous (300Ma), in which the genera with the greatest palaeontological significance include Isogramma, Enteletes, Derbyia, Neospirifer, Choristites, Brachythyrina and Linoproductus.
Brachiopods belong to the phylum, Tentaculata, and are often confused with bivalves (due to their shell consisting of two valves), although they are not related at all. The most eye-catching features of brachiopods are their tentacles, which are used to collect nutriments. Unlike most bivalves, the valves of the shell are completely different in shape and size and the dorsal valve has a hole for a stalk, which attaches the animal to the ground.
Within the rocks of the Carnic Alps, cephalopods occur as two distinct forms – belemnites, with linear elongated (internal) shells, and ammonites with tightly coiled (external) shells. Most fossils of these organisms are found in the so-called Wolaya limestone, which formed during the Upper Ordovician. Genera of particular interest include Orthoceras, Cyrtoceras, Gomphoceras, Goniatites and Clymenia.
All fossil cephalopods (other than belemnites) possessed a shell divided into numerous chambers, only the last and largest of which was inhabited by the animal. The chambers were filled with nitrogen and acted as a hydrostatic buoyancy organ regulating depth in the water column. Today’s descendants of the fossil belemnites and ammonites, mostly represented by the genus Nautilus, have up to 90 tentacles, while their feet are converted into motile funnels, which allow movement by propelling a jet of water out behind the animal.
Among the most important bivalves that are found in the rocks of the Carnic Alps, one has to mention the species, Cardiola docens. This organism colonised the seafloor during the Upper Silurian (420Ma) and is found accumulated in a 0.5 to 4m-thick rock layer (the Cardiola Formation).
Graptolites are chiefly found in Silurian quartz schists and are represented by the genus, Monograptus, which were colony-forming marine organisms, whose biology and ecology were not fully understood until recently. By analogy with their extant relatives in the phylum, Pterobranchia (for example, Raptopleura), they lived in a multi-chambered tube system composed of collagen. As a result of rock-forming processes, this substance was converted into coal-like compounds, as well as a bright mineral called guembelite. It is highly likely that graptolites had a planktonic lifestyle and consisted of single housing tubes mounted on a buoyant body.
Corals reached their highest value as index fossils during the Devonian and the Lower Carboniferous, during which most organisms belonging to the class Anthozoa formed huge reefs that are preserved in the Carnic Alps as mighty limestone strata. The most significant genera include Heliolites, Favosites, Hexagonaria and Cyatophyllum.
As well as the index fossils described in the preceding paragraphs, there are two more important fossil groups occurring in the Carnic region – the crinoids (or sea lilies) and the bryozoa. Crinoids are found in almost all limestone layers of the Carnic Alps. However, they are particularly concentrated in sediments of the middle Devonian. Most important genera include Actinocrinus and Poteriocrinus. From a biological point of view, crinoids are characterised by a rather simple anatomy, consisting of an apical calyx with numerous tentacles that supplied the animal with nutriments, a middle stalk, and basal roots stabilising the animal on the seafloor. Ecologically, the animals formed huge colonies, which preferred large amounts of sunlight, warm water and increased oxygen levels.
Bryozoa are related to the brachiopods and are mostly limited to the Auernig Formation of the Upper Carboniferous (290Ma). The genus, Fenestella, is found in an 8m-thick limestone layer, consisting of net-shaped, residential colonies (zooarium). It preferentially lived in shallow waters with reduced wave activity.
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