Visiting for myself the fossil trackways of Arthropleura on Arran

Hello and welcome to Palaeofauna as well as my first blog post! I am establishing this blog in order to describe findings, experiences, events and interpretations appropriate to my passion for palaeontology as well as to my seemingly never-ending quest to accurately reconstruct extinct life. I already have lots of article ideas I would like to talk about whenever I’m not too busy, but to start with I wanted to cover a time (back in August 2017) when I decided to visit for myself upclose the fossil trackways of a giant centipede/millipede called Arthropleura at a remote location at Laggan Harbour, Isle of Arran, Argyllshire, Scotland.

Figure 1: Here I am with the fossil trackway (Diplichnites cuithensis) exposed on an outcrop of Carboniferous sandstone that may have formed in a swampy deltaic environment. I couldn't get any closer due to safety concerns. Located at Laggan somewhere between the mouth of the Sannox burn and Lochranza, Isle of Arran. Fossil trackways indicated by rings. Scale bar on close up images = approx. 30 cm.

It took about 5 hours to find that fascinating fossil trail since arriving at Lochranza on the island by ferry. Going on a full-circle hike that afternoon, I trekked up and down steep hills and then later looked carefully while walking along the low-lying coast. I almost gave up looking for it after a long search, but then there it was - two long, parallel rows of small imprints preserved on a bedding plane of 320 million year old Carboniferous channel-filled sandstone (hence the ichnogenus name Diplichnites). They were around 1 foot in width. 

I remembered the area where it was located from after the beginning of Prehistoric Park Episode 5, where the brilliant wildlife TV presenter Nigel Marven was planning on bringing back giant Carboniferous bugs.  I was fortunate enough to witness this particular trace fossil in person as it is the type specimen of the ichnospecies D. cuithensis, associated with the giant myriapod Arthropleura (meaning “jointed ribs”), the largest terrestrial invertebrate of all time. A large number of small, regularly-spaced imprints show that they were indeed made by the appendages of an arthropod and are consistent with Arthopleura, which could reach maximum lengths of over 2 m long (so nowhere near as big as the one in Primeval, which was portrayed as being 6 m long). Any body fossils are still yet to be found on Arran, though. 

Figure 2: My old (2015) reconstruction of Arthropleura armata in dorsal view. Height of human figure = approx. 1.8 m.

Analysis of the size and stride length of the trail on Arran suggests an individual approximately 1 m long with a flattened body consisting of 23 pairs of appendages each roughly 10 cm long. This number of appendages was determined by distances between two succeeding imprints of the same limb, i.e. within a stride length. (Briggs et al., 1979)

Other examples of Diplichnites are also known from the United States and in Nova Scotia, Canada. Each segment of the body of Arthropleura supported a pair of appendages underneath, which is why Arthropleura is considered a myriapod (centipedes, millipedes and kin). The segments of Arthropleura would typically disarticulate upon death, so only isolated armour segments have so far been found as body fossils. Possible stomach contents have been found preserved in body fossils of Arthropleura elsewhere in Scotland, showing that it was entirely herbivorous, feeding on plant matter such as lycopods (tree-like club mosses), which were the main element of the coal forest vegetation. The diet of many modern day millipedes consists of dead plant matter, so it is plausible to assume that Arthropleura had the same kind of diet. (Rolfe and Ingham, 1967)

A detailed study of the known body fossils by Kraus and Brauckmann (2003) have demonstrated that the bulky-looking body armor is just a few millimeters thick and, unlike crustaceans, not fortified by calcium carbonate. Bearing in mind their size, adult Arthropleura likely would have had no enemies in the Carboniferous coal forests and consequently, no need for reinforced body armour.

Figure 3: My reconstructions of other examples of giant invertebrates from the Carboniferous: (a) Meganeura brongniarti, one of the largest flying insects to have lived (with a 70 cm wingspan), known from fossils found in England & France. (b) Pulmonoscorpius kirktonensis, a giant metre long scorpion known from fossils found at the East Kirkton Quarry in West Lothian, Scotland. Scale bars = 15 cm 

The maximum body size of insects and other arthropods has fluctuated throughout geological time as a result of changes in atmospheric composition, but oxygen levels were at their highest during the Carboniferous period (30-35 % back 300 million years ago compared to 21% today) so insects and other invertebrates became very large as a result. Maybe this was to help protect them from oxygen poisoning that threatens the larvae of insects living in oxygen-rich water. Larger larvae are less prone to oxygen poisoning as their relatively small surface area lowers the amount of oxygen their bodies absorb.

Finally, why exactly did Arthropleura and these other giant bugs go extinct? It is most likely due to an increasingly cooler, drier climate as the Permian dawned, brought about by a short glacial period. This ultimately led to the disappearance of the coal forests and in turn a drop in oxygen levels. 

And with that, I'm not sure what else I can say really, other than how cool it would have been to encounter these animals alive and to thank you very much for reading through my first post! Have a nice day!

Best regards,

Will Toosey

References:

Briggs, D. E. G., Rolfe, W. D. I., Brannan, J. 1979. A giant myriapod trail from the Namurian of Arran, Scotland. Palaeontology, 22, 273-291.

Kraus, O., Brauckmann, C. 2003. Fossil giants and surviving dwarfs. Arthropleurida and Pselaphognatha (Atelocerata, Diplopoda): Characters, phylogenetic relationships and construction: Verhandlungen des naturwissenschaftlichen Vereins in Hamburg, 40, 5-50.

Rolfe, W. D. I., Ingham, J. K. 1967.  Limb structure, affinity and diet of the Carboniferous  ‘centipede’ Arthropleura. Scottish Journal of Geology, 3, 118-124.