Joe Shimmin and Stephen Day (UK)
Picture yourself strolling through lush, green woodland, on an Earth unspoiled by man and yet to witness the rise of the dinosaurs. You’d be forgiven for feeling at peace with the world, even slightly euphoric – that is until you stumbled across the giant Arthropleura, a millipede relation as long as a park bench. This encounter might make even the most enthusiastic creepy-crawly hater think twice before squashing the bug in front of them under foot!
I (JS) had a slightly less dramatic (but still very exciting) experience involving the creature while on a recent fossil hunting trip to Crail in Fife. On investigating some sandstone ledges that ran across the shore to the south-west of this pretty little fishing village in western Scotland, my eyes were drawn to what could only be a huge set of fossil tracks in the rock. The stratum in which they had been preserved also contained plant remains such as Stigmaria roots, as well as sections of tree trunks and branches.
I took numerous photographs of the track, which measured about 3m long by 30cm wide and also of other, similar tracks nearby, in the hope that someone might be able to identify what kind of creature had created them. My guess was that it was some sort of amphibian, but I wasn’t sure. All I knew was the thing that made them was BIG.
Happily, when I posted a picture on the UK Fossils Discussion Board, (www.discussfossils.com), the tracks were quickly identified by Fiona Jennings and Steve Day as being some quite well-known examples belonging to Arthropleura. Given the interest shown in these on the forum, Steve suggested that we might co-write an article on this ancient giant for Deposits magazine.
Geological and geographical range
Arthropleura predominantly lived in the ‘coal swamp’ biome, across the Euramerican minor supercontinent of the Carboniferous period. The creature first appeared during the Namurian age of the first half of this geological period and is thought to have died out some time in the Stephanian at the end of the period. Significant body and trace fossils have been found in Britain, France, Germany, Holland, the United States and Canada. Although around ten different species of Arthropleura have been described from the Upper Carboniferous of Europe, the classification of the Arthropleurids is still very much an open debate at the subphylum and class levels.
The most abundant finds of Arthropleura have been made within the Saar River basin, with the best, most complete body fossil of the creature being found by Guthörl in 1934 in a coal mine near Saarbrüchen. Recently, further fine German specimens have been found near Chemnitz and Piesberg.
Trace fossils (including the tracks encountered by JS in summer 2007) show that the creature also roamed the area that is now Fife in Scotland, with tracks occurring between Boarhills and St. Monans, near St. Andrews. The geology of this stretch of coast belongs to the Anstruther Beds that form part of the Dinantian Strathclyde Group. There are a variety of rock types within this group including thin coals, lagoonal mudstones and laminated siltstones and shales.
Tracks also occur in the Tynemouth Creek Formation of Gardner Creek Bridge, on the outskirts of New Brunswick in Nova Scotia, Canada. The formation sequence containing these trace fossils consists of red sandstones, interbedded with red and green siltstones. American examples have been reported from numerous states including Illinois, Ohio, Pennsylvania and New Mexico.
To determine the size of our creature, one needs to study its tracks, as complete fossil specimens of an adult animal have not yet been discovered. To date, the widest set found is 48cm across, with track widths into the high 30cms being relatively abundant.
From the trace fossils that have been found across Arthropleura’s geographical range, along with fragmentary body fossils, it has been determined that the creature would have grown to a staggering size, in the region of three to seven feet long. Consequently, this makes it the largest invertebrate ever to have inhabited the Earth’s land surface.
As a result of studies of Arthropleura’s mouthparts, it was initially believed that it was a carnivore that fed on smaller invertebrates. However, within one juvenile British specimen were found the preserved remains of its last meal. Inside the stomach were carbonised fragments of vegetable debris. This matter consisted mainly of partially decomposed Carboniferous lycopod (for example, Lepidodendron and Sigillaria) pieces. However, until another preserved gut is found, it will remain unclear whether the creature was purely a herbivore or actually omnivorous.
Other terrestrial invertebrate giants
Arthropleura was not the only terrestrial invertebrate giant of its time. There was the dragonfly-like Meganeura from the Carboniferous and Meganeuropsis from the Permian, the latter having a wingspan of up to 75cm and being the largest insect of all time. There were also scorpions that grew to 70cm in length, as well as large mayflies, silverfish and spiders.
Contemporary ‘titans’ include giant millipedes that can reach nearly 30cm long, the Amazonian giant centipede (Scolopendra gigantean) that grows up to 33cm and the Titan beetle (Titanus giganteus) that can measure nearly 17cm in length, not including its antennae. The Titan stick insect (Acrophylla titan) can be up to 50cm in length. However, the largest land invertebrate today is the coconut crab (Birgus latro) whose carapace can be as much as 40cm wide and which has a leg span of up to a metre. This is a substantial beast – but compared to the monsters of the past, the giant terrestrial invertebrate contenders of today look quite puny!
Why did they grow so large?
Below are a few theories as to why Arthropleura and some other Carboniferous and Permian terrestrial invertebrates grew so large.
(a) Atmospheric oxygen content
The Earth’s atmosphere during Arthropleura’s reign is thought to have had an oxygen content of about 36% (compared to 21% today). This, the highest atmospheric oxygen content in the history of the planet, was largely due to an explosion in the number of vascular plants during this time. The available land surface for them to inhabit had also increased, due to drops in sea level during the Devonian. In the Carboniferous, photosynthesis within the new, massive floral population led to a steady decrease in carbon dioxide as plants used carbon within their structures, releasing oxygen as a waste product. Also, periodically throughout this time, substantial portions of the vegetation became buried and locked away to become coal. We all know that coal is a fossil fuel – and many people are worried about releasing this fossil carbon into our present-day atmosphere. The flip side of this is that a huge quantity of carbon was removed from circulation, and hence the atmosphere, during the Carboniferous. There is some speculation that there was a lag between the development of lignified bark material in the new vascular plants and the evolution of bacteria to break it down upon death – thus augmenting coal accumulation, but this theory is not widely accepted.
Atmospheric oxygen content can be directly related to how large a terrestrial arthropod may grow, before it is crushed by its own weight. These animals ‘breathe’ through passive air diffusion into their body tissues. They have pores in their exoskeletons called spiracles that form tubes into their bodies. These spiracles then branch off into smaller air tubes called tracheas. Current atmospheric oxygen levels limit both how far the spiracles can enter the body while still being effective and also how far diffusion of oxygen from the tracheas can penetrate into body tissues (about 1mm in present day arthropods). Higher oxygen levels in the past meant that the gas could be carried further into the body and also diffuse deeper into tissues. A given unit volume of air would also be able to ‘feed’ a larger quantity of tissue. Another benefit of increased oxygen diffusion into tissues was that an invertebrate’s internal composition could be sufficiently dense to be able to support itself. If Arthropleura existed today, its tracheas would have to form a much more extensive network, causing the creature to be less dense and therefore liable to collapse.
However, spiracles have not yet been identified on Arthropleura body fossils and some researchers believe that certain structures on its legs can be interpreted as gills. Current thinking sways more towards the passive diffusion method of respiration for our creature, but gills have not been wholly discounted.
If the creature did have gills it would mean that it was aquatic rather than land based and would have used water pressure to help support its bulk. However, apart from the questionable structures on the legs, there is little evidence to support this aquatic habitat; comparisons with modern invertebrates indicate that its form was adapted well for burrowing through leaf litter on land. It fed on decaying terrestrial plants and its tracks were left on the land surface, not underwater.
(c) Air pressure
It has been suggested that air pressure could have been higher during the Carboniferous and Permian periods. An increase in air pressure would have lent support to the larger terrestrial invertebrates, in the same way that water supports some marine life that would be crushed by its own weight if it lived on land. It would also allow for a greater network of tracheas before the animal became ‘too hollow’ and would help to force oxygen both into the body of the creature through the spiracles and further into the tissues from the tracheas – all an aid to gigantism in terrestrial invertebrates.
(d) Absence of large predators and abundance of food
Put quite simply, there was no shortage of food for Arthropleura to eat in the Carboniferous forests. Their floors would have been littered with great quantities of decomposing vegetable debris, through which the beast sifted to find its meals. At the same time, there was a shortage of large, vertebrate predators, from which our beast would have been poorly equipped to run or hide, given its size and weight and also its need to spend much of its time feeding.
One website’s answer to the question of why the giant terrestrial invertebrates of the past were able to exist is that there was simply less gravity on the planet back then. Since gravity is related to an object’s mass, this theory is basically saying that there was less of the Earth during these times. Quite where some of it had gone is not broached on the site however!
Arthropleura has found itself the subject of a variety of modern interpretations. The trackways in Crail featured in David Attenborough’s excellent series Lost Worlds Vanished Lives and the ancient myriapod itself appeared in the BBC series Walking with Monsters and ITV’s Prehistoric Park in which Nigel Marvin discusses the Crail trackways (as well as wrestling with the beast). These latter two programs show the huge invertebrate with the ability to rise up and look a grown man in the face. It is also portrayed as an animal that would only act aggressively if provoked by a predator. Recently the creature starred in another show, ITV’s Primeval. However, this was a less literal incarnation, as it was given a venomous bite and had its length increased to six metres!
Arthropleura was not built to be a ‘geological survivor’. It was too reliant on the specific ecological conditions that prevailed during its reign, and indeed, was a product of them. Only in such a lush environment as the Carboniferous swamps, with their enriched oxygen content and lack of large vertebrate predators was the creature safe. Sadly, when conditions changed it would have been among the first animals to die out. Big is not always best!
The authors would like to thank Werner Kraus of RWTH Aachen University in Germany for his help while writing this article.
Briggs, Derek and Rolfe, Ian. 1983. A Giant Arthropod Trackway from the Lower Mississippian of Pennsylvania. Journal of Paleontology 57, 2, 377-390.
Rolfe, W. D. and Ingham, J. K. 1967. Limb structure, affinity and diet of the Carboniferous ‘centipede’ Arthropleura. Scottish Journal of Geology 3, 118-124.
Pearson, P. N. 1992. Walking traces of the giant myriapod Arthropleura from the Strathclyde Group (Lower Carboniferous) of Fife. Scottish Journal of Geology 28, 2, 127-133.
Proctor, C. J. 1998. Arthropleurids from the Westphalian D of Writhlington Geological Nature Reserve, Somerset. Proceedings of the Geologists Association 109, 2 93-98.