Dean R Lomax (UK)
A ‘big’ discovery
In 2002, a wonderful discovery of a 9.7m-long trackway (ichnofossil) with the tracemaker (a horseshoe crab) preserved was made in a quarry near the village of Wintershof, north of the town of Eichstätt in Bavaria, Southern Germany (Fig. 1). Specifically, the specimen was collected from the Solnhofen Lithographic Limestones, Eichstätt Formation (Solnhofen Group), Hybonotum Zone, Riedense Subzone from the Late Jurassic (Tithonian). Fossils from Eichstätt are often confusingly thought to be from the area of ‘Solnhofen’, with the Solnhofen area being a world renowned Lagerstätte. Many exceptionally well-preserved fossil specimens have been collected from that area, including the famous fossils of Archaeopteryx.
However, many people are unaware that there are numerous localities that surround the area of Solnhofen, which yield many of the fossils from this famous geological unit. It has been suggested that several of the fossils found within the rocks originally laid down in the Solnhofen lagoons (which were part of an archipelago) are the result of mass storm events, during which organisms from the nearby Tethys Ocean were thrust into these anoxic lagoons during heavy storms. In many cases this was the beginning of the end.
The horseshoe crab that produced this trackway was identified as Mesolimulus walchi. The taxon is fairly well recorded within the limestones of the area and is the only species described. Most specimens of M. walchi from this area represent juveniles, including the specimen discussed in this article. On discovery, the large block (allocated specimen number: WDC CSG-233) was carefully removed in many pieces and painstakingly placed back together in the laboratory. Despite the name ‘horseshoe crab’, they are not actually ‘crabs’ and are, in fact, limulids, in the order Xiphosura. Fossilised horseshoe crabs have been documented within rocks of Late Ordovician age (roughly 450Ma) to the present day, where a few species still survive. Their general morphotype (body plan) has remained fairly similar.
Death tracks and ichnotaxonomy
Generally within the fossil record, remains of trackways are well represented, found across the world and range from the Palaeozoic through to the Cenozoic. Yet, finding trackways and tracemakers preserved together is rare. Despite this, some geological units (such as the Solnhofen limestone) have indeed yielded several mortichnial examples. A mortichnia is the ‘death march’ – or final walk – of a living creature, captured in the rock, with the tracemaker preserved. In fact, within the Solnhofen areas, a variety of arthropods have been recorded alongside their traces; this has also included horseshoe crabs of the same taxon. However, such trackways are far less extensive, with only short tracks recorded.
Trace fossils facilitate our understanding of the palaeoecology of the prehistoric world. Such fossils, including the specimen discussed in this article, provide palaeontologists with a wealth of information on an animal’s behaviour at a specific time, which subsequently allows a unique glimpse into the behaviour and ecology of the animal just before death. Like bones and skeletons, when studying trace fossils, palaeontologists catalogue and describe them, with such elements as, for example, a footprint or a trackway, receiving a scientific binomial name. In this way, scientists talk about ichnotaxa, including ichnogenus (plural: ichnogenera) and ichnospecies, and follow the same taxonomic conventions as are used in the binomial systems used to name species. As an example, using the trackway discussed in this article and based on the morphology of the traces and the style of track present, it was identified as the ichnotaxon Kouphichnium, a name given to limulid tracks where the tracemaker is not available for true determination.
Numerous species have been assigned to this ichnotaxon and, even if the tracemaker is preserved with its tracks, the trackway is still identified as an ichnotaxon. At least four different types of limulid track types have been described. They include: walking, crawling, dying and ploughing. Both the crawling and dying style of tracks are represented by the specimen discussed here. Having the tracemaker preserved alongside a specific type of trace can help significantly with the identifications of individual trackways and their variations, where the tracemaker is not preserved.
A beautiful fossil, but a sad ending
The well-preserved trackway consists of five large, individual slabs, made up of numerous smaller portions (labelled individually A–E, as seen in Fig. 2).
Slab A is the longest portion – here the limulid is first introduced into the environment. An unusually disturbed surface, reminiscent of trampling, is present. This surface is about 23cm wide and includes numerous pedal prints (feet), leg imprints and telson (tail) marks, along with circular depressions. The latter probably represent the area where the limulid had first landed on the substrate dorsal side up, that is, on its prosoma (‘head’). The entire section indicates an initial struggle, with perhaps the horseshoe crab struggling to firmly right itself (Fig. 3).
On doing so, the limulid then began its long death march, with the general walking patterns of the specimen changing throughout the course of the trackway (Fig. 4). The numerous pedal prints and the consistent use of the telson are seen all the way along the trackway. The initial ‘march’ begins with roughly a straight track, although meandering curves are seen throughout.
The track then changes several times with sudden, abrupt angular turns, which result in either a straight track (in another direction) or total body turn of 90o (which is seen at two points: Fig. 2A and 2B, also see Fig. 5). Within the track, where there is a break between the telson drag marks (that is, where the continuous line stops and starts at another angle), it demonstrates that the limulid has changed direction and the telson has been lifted, thereby beginning drag marks at a separate angle for the track to continue. This walking style is seen throughout the slab, especially through A to C.
However, as the limulid progresses, at certain points of the track, the telson drag marks become much shorter, erratic and discontinuous. As an example, midway through slab D and to the end of the track, this becomes very prominent. The consistent presence of the telson and interchanging short/long leg imprints is a strong indicator of a crawling or dying limulid.
In some instances, deeper and more erratic leg imprints may demonstrate that the limulid had attempted to ‘push off’ the substrate, but had little power or energy to do so. Therefore, this change in the track style suggests that the limulid had begun to struggle under the anoxic bottom waters of the lagoon, eventually succumbing to the environment by asphyxiation. Seen at the very end of the slab is the animal that had walked for 9.7m – a 12.7cm-long juvenile horseshoe crab (Fig. 6). The nature of this lagoonal environment (anoxic and hypersaline) prevented scavenging and resulted in the fine preservation of the limulid and undisturbed trackway.
This is one of the most remarkable ichnofossil specimens in the fossil record. The specimen captures the entire ‘life cycle’ – from beginning to end – of a mortichnia, currently regarded the world’s longest and most extensive example. However, one must spare a thought for the little limulid. Despite representing a fantastic specimen – one that scientists are able learn and understand lots from – it does represent one moment in Earth’s history where an animal was flung into a toxic lagoon, struggled to breath, attempted to escape and eventually marched into oblivion. You cannot help but feel some sadness for an animal that lost its life in such a scenario.
I would like to thank Chris Racay for co-authoring the scientific publication about the specimen, and Dr Burkhard Pohl and the Wyoming Dinosaur Center for allowing access to the fossil, enabling the study. Thanks also to Dr Adam Smith and Nobumichi Tamura for creating the fantastic illustrations of the trackway.
About the author
Dean Lomax is a palaeontologist from Doncaster, England. Dean works on palaeontological projects across the world, especially in Europe and the USA. He researches fossils and writes books, articles and peer reviewed scientific papers. At the time this article was writing, he was a Visiting Scientist at The University of Manchester in the UK and an Honorary Research Associate at Doncaster Museum & Art Gallery.
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