Dr Robert Sturm
As a result of their great diversity in shape and long-lasting occurrence in earth history (from the Devonian to the Cretaceous), ammonites are equally fascinating objects for the professional and amateur palaeontologist. By definition, ammonites exclusively comprise a group of extinct marine cephalopods that, according to the present store of knowledge, include about 1,500 genera and between 30,000 and 40,000 species. The shell size of adult animals ranged from a few centimetres to two metres in the case of Parapuzosia seppenradensis (Lehmann, 1981; Monks and Palmer, 2002).
The introduction of ammonites into zoological systematics was carried out by Carl Alfred von Zittel in 1884, who defined the sub-class ‘Ammonoidea’. This unconventional term dates back to the first century AD, when the elder Pliny interpreted these fossils as horns of the ancient Egyptian god Amun. Since the petrified shells represent the most important relics of ammonites, information on their biology and anatomy is characterised by a number of uncertainties.
For example, it is assumed that these cephalopods only possessed a small number of tentacles (eight to ten) and also an ink pouch, or bursa, for protection against natural enemies. Most species lived in a water depth of between 50m and 250m, where they mainly fed on crustaceans, foraminifers, and ostracods. Ammonites were also characterised by sexual dimorphism – the smaller individuals were males and the larger ones were females. Palaeontological determination of single species is chiefly based on the shape, size, sculpture and torsion of the shell, as well as the so-called suture lines accessible on the shell surface.
Accompanying this article are some 3D photographs of ammonites that I collected some years ago in the Northern Limestone Alps, not far from the city of Salzburg. To my mind, these images are a very appropriate way of describing cephalopod fossils, because additional information on the shape and depth of the shells is obtained by the application of a rather simple technique, which I will describe below.
How to generate the third dimension
Stereographic photography is a well-defined and widely accepted procedure in the fields of both panoramic and detailed photography. I will briefly describe the process to give some insight into the technique.
Before starting to photograph an object, it has to be fixed to a surface using either wax, some modelling clay or, if none of these are available, chewing gum. After doing this, the object is photographed from two different positions, separated by an angle of 5° to 7°. Photography of larger ammonites is conducted with a digital camera selecting the ‘macro’ setup, while smaller fossils are photographed either with a magnifying glass placed in front of the camera or under a stereomicroscope. Combination of the two photographs takes place with the help of special software for the production of anaglyphs (that is, pictures that give a stereoscopic 3D effect), which can be downloaded from the Internet for free. Another way of producing anaglyphic stereograms is provided by Adobe Photoshop, in which pictures can be trimmed and superimposed in a very elegant way (Russ and Russ, 2005; Sturm, 2005).
Geology of the Northern Limestone Alps – a brief outline
From a geographical point of view, the Northern Limestone Alps represent the northern part of the three regions of the eastern-alpine mountain belt. South of these mountains are the Central Alps, which are mainly composed of silicate rocks, while the southern margin of the Eastern Alps is formed by the Southern Limestone Alps.
Most of the calcareous sediments dominating the lithology of the Northern Limestone Alps were deposited on the so-called Adriatic plate (a part of the African continental plate) during the late Palaeozoic and Mesozoic. During the Tertiary, these sediments were lithified and overthrusted over the European continental plate, forming the complex nappe system of the northern-alpine mountain belt.
Ammonites can mainly be found in the Triassic and Jurassic limestone units and, in particular, the marbles (in the mason’s rather than the geologist’s sense as they are not actually metamorphic) exposed on the northern edge of the mountains contain a great number of cephalopod fossils (see below).
Ammonites from the Northern Limestone Alps
In the 3D photographs shown here, there are some typical ammonites collected in the Northern Limestone Alps. Except for Joannites sp., which is an important species in Middle to Upper Triassic rocks (the Anisian to Carnian stages), all these fossils are from the Jurassic. Starting with Amaltheus sp., this is a remarkably sculptured ammonite that reaches an adult diameter of about 6.5cm. The single ribs with sinuous shapes, as well as the depth of the fossil, are, to my mind, excellently displayed by the anaglyph. Harpoceras sp. is characterised by a very prominent aperture, which produces an astonishing effect in 3D photography. The ammonite has a maximum diameter of about 18cm and, therefore, can easily be found in the Jurassic rocks. Hildoceras sp. is a very impressive ammonite due to the occurrence of ribs with a sinuous shape on the outer flanks of the whorls and the weak development or complete lack of these ribs on the inner flanks.
The maximum diameter of this cephalopod is about 11cm. Pseudothurmannia sp. is an index fossil of the Upper Jurassic (Kimmeridgian and Tithonian) and Lower Cretaceous (Hauterive and Bareme). Eye-catching characteristics of this ammonite include the slightly bent ribs, which are only rudimentarily preserved in the example shown here, and the remarkably extended last whorl. The dimension of the last whorl and aperture of the remaining shell can be well seen by means of an anaglyphic photograph. Oppelia sp. represents a rather small but beautiful ammonite, which mainly occurred during the Middle Jurassic (Bajocian). In this example, the shape of the shell produced by the ribs and knobs is significantly amplified by the photographic technique introduced here.
I turn now to some more anaglyphs of ammonites that are pictured below in this article and which I have collected on my field trips to the Alps south of Salzburg. Grossouvria sp. is one of my favourites, due to its thickness, which allows a highly effective application of 3D photography. The ammonite occurred during the middle Jurassic (Callovian) and reached a diameter of 8cm. The Triassic ammonite, Joannites sp., is characterised by the smooth surface of its shell, regularly occurring grooves and a noticeable suture line. The diameter of the shell reaches a maximum of 6.5cm. Staufenia sp. represents an index fossil of the Middle Jurassic (upper Aalenian) and is marked by a rather flat, smooth shell with a significant and predominant last whorl. The shell has a maximum diameter of 8cm. The last fossil I want to discuss is Catacoeloceras sp., which mainly occurred in the Lower Jurassic (lower Toarcian) and has an oval-shaped aperture. It has linear ribs that are evenly distributed over the whole shell. The diameter of this fossil is in the order of 4cm to 5cm.
Where to find ammonites
There are numerous localities near Salzburg, where fossil ammonites can be collected. I will discuss four main sites that I have explored extensively in the past.
The first is a marble quarry on the northern flank of the Untersberg, about 10km southwest of Salzburg. The marble exposed in this quarry is fine-grained and of excellent quality, so that it has been used as a preferential building material for five centuries. Within the blocks, a wide range of Jurassic ammonites can be found (and also all of the ones described above). Unfortunately, the quarry is privately owned, which restricts access for collectors.
Another source of Triassic and Jurassic ammonites is the so-called Glasenbach-klamm, a very romantic and narrow gorge, which can be easily reached from the centre of Salzburg by bus. It is located about 1km south of the city and represents a favourite target for weekend trippers. Besides its high abundance of diverse fossils (including an Ichthyosaurus), the Glasenbachklamm is famous for its rich vegetation and aquatic fauna living in the brook that runs through the gorge. In a small, but very fine museum located in my home village of Elsbethen-Glasenbach, fossils collected from the Glasenbachklamm can be seen, and it also provides some interesting facts about the historical development of the village.
A third site capable of exploration is situated 15km south of Salzburg, near the lovely village of Adnet. The quarries there contain a well-known, white to red-coloured marble dating from the Jurassic. In this marble, which was used as a preferential building material in the past, huge amounts of marine fossils and, above all, ammonites have been found. In the village of Adnet itself, there is a small marble museum, where fossil treasures that have been collected and prepared over the last few centuries can be admired.
The last site I will mention is about 20km south of Salzburg in the so-called Taugltal, an impressive valley cutting through the limestone mountains from east to west. While at its origin the valley is a narrow gorge with the rocks arranged in layers that have partly undergone extensive folding, the other end of the valley increases in width. Fossils are mainly found in the layers of limestone, in between which are small layers of clay.
Lehmann, U. 1981. The Ammonites: Their Life and Their World. Cambridge University Press, 252 p.
Modern Microscopy Journal 6/2005 (www.modernmicroscopy.com).
Monks, N & Palmer, P. 2002. Fossil Ammonites. Smithsonian Books, 176 p.
Russ, J.C. & Russ, J.C. 2005. Assembling Stereo Anaglyph Images with Photoshop.
Sturm, R. 2005. Ammonites in 3D – Anaglyphs for the stereographic analysis of Mesozoic ammonites. Micscape, September 2005 (www.microscopy-uk.org.uk).
Upper image: Geological map of Central Europe with its main tectonometamorphic units. (The Northern Limestone Alps form the Northern margin of the Austroalpine nappes.)
Lower image: Geographic map of the county of Salzburg with the four main localities of discovery described in this work.