This week's Friday Fossil is Arthropleura.

"Calm down, dear!!"

I'm about to pop off to the isle of Arran, the poster-child of British Geology. Not many countries can boast a handy island that has displays almost its entire geological history, from precambrian to recent, along one continuous shoreline.

But ask any british geology student (they will almost certainly have been to Arran) for just one thing that they remember from Arran, and chances are it will be the trackway of the giant myriapod, Arthropleura (it was for me, anyway). Yes, there are some pretty cute granites there and some cool volcanics, but nothing is more likely to lodge itself in your memory than the tracks of a metre long myriapod, like the tracks of a miniature tank wandering over a delta plain, 320 million years ago.

One of the truly remarkable things about this trace fossil in Arran is the shear wealth of information that we can get from it. We do from time to time find body fossils of Arthropleura, but they are never complete and, because they rarely have legs, tell us nothing about how this animal actually lived.

OK, so it had tubercles on its carapace. Meh.

But things that we can glean from the one trace fossil in Arran are that

- it had 23 pairs of legs
- its legs were about 10cm long and swung through an angle of 60 degrees, and had a stride length of 22cm
- its precise gait was its legs moving in 3 metachronal waves, with 45% of the appendages being in contact with the ground at any one time, and there was a high-geared forestroke-backstroke ratio of 5.5-4.5
(in traces found in more verdant beds, this is found to be more powerful, presumably because it needs to thrash through vegetation).

Impressive, right? This makes it the largest terrestrial arthropods to have ever lived. Much as I would love to see David Attenborough hushedly observing Arthropleuras in the wild, magnificent massive myriapods such as these could only live in the carboniferous, and if we tried to resurrect them today, they simply wouldn't be able to breathe. Because of their high metabolic demands, arthropods must rely on bringing air in very close proximity to their muscles, and when you get very big, it becomes a long distance to bring all of that air and a lot of body tissue to feed with oxygen. Only with the 30-35% oxygen levels that we had in the carboniferous would giant arthropods like these be able to supply their muscles with enough oxygen.

As cool as it would be to create Them!, giant insects such as these would be confined to their bed like an emphysema patient; too heavy to be able to support their own weight and not efficient enough at breathing to be able to live without pure oxygen on tap.

I love trace fossils like this because, rather than representing some unlucky beastie that got killed and buried quickly, after making the fossil these elusive trace-fossil makers went off on their own business and didn't meet such a perilous death, living out their happy lives free from our knowledge. I've even seen traces with two tracks where one wanders towards the other track, the two combine for a bit, then go about their separate ways. Paleocopulation, perhaps?

Another reason why I like them is that you can lie on side (the beds are now inclined at 30 degrees), put your boots on the bedding surface and just image, for a moment, that you are walking on a Carboniferous delta, with this arthropleura scuttling alongside you, dragonflies as big as seagulls wheeling in the air and the sound of pelycosaurs snorting from the Lycopod forests. Although, with all that oxygen, I'd have to be wary where I threw my fag end.

I'm going to be in Arran for 2 weeks, so unless you catch me before I leave, I won't be able to reply to your comments until I get back - sorry about that.