A full version of this article appears in the magazine.

Ancient life is often considered the preserve of the palaeontologist – a different animal from the birdwatcher, the botanist or the marine biologist, being more closely affiliated with geology. Although the palaeontological subject is inanimate and dead, it was once a living creature. Piecing together the behaviour of an extinct animal, from the fossil evidence it left behind after it died, employs the same disciplines as walking along the tideline and working out when the dogfish laid their eggs, or what wading birds were feeding in the mud an hour ago.

 

The significance of Wales in geological history

Wales plays a significant part in the classification of geological time, and many of the subdivisions of the palaeozoic epoch – basically, life before the dinosaurs – are named after places in Wales. British geologists mapping the palaeozoic in the early 19th century were inspired by Welsh names, so the first three major epochs, the Cambrian, Ordovician and Silurian, were derived from the names of Welsh mountains or tribes.

 

The Cambrian detective

At Cwm Graianog in Gwynedd strange patterns can be seen on the rocks littering the floor of the cwm: they look like the tracks and burrows of marine organisms. On the west face of the cwm the apparent tidal ripple marks on the cliff face are exactly that; the marks in the rock that look like worm burrows are worm burrows. These are called trace fossils – the fossilised behaviour of life, rather than life itself. But there are also some long linear marks in the rock that look like pleated rope (cruziana). What do they belong to? A crab? Some kind of snail?

 

The answer involves an extraordinary piece of detective work. The rocks can be confirmed as Cambrian because the finer mudstones in the cwm occasionally contain the shells of a primitive brachiopod called Lingullela davisi. Lingulid brachiopods can still be found today in the Pacific, typically in estuarine environments, and are still harvested for food in Japan, but this particular species is indicative of the late Cambrian, so it acts as a geological clocking-in card: wherever it occurs crowded on the bedding planes of the hard flagstone, it provides an indicator of the late Cambrian. However, it is so similar to the modern day forms that it is probably safe to assume it lived in similar conditions.

 

The Cambrian contained no fish, no crabs, and not many gastropods, starfish or sea urchins. It did contain curious arthropods called trilobites: marine creatures that looked a bit like woodlice. In fact, they were the dominant life form in the Cambrian. Could they be the creatures responsible for the pleated tracks at Cwm Graianog, and if so, where are their remains?

 

The Graianog rock is fairly coarse sandstone, indicative of a turbulent shallow-water environment, perhaps similar to the beach at Newborough on Anglesey. It is possible to walk a long way on the beach at Newborough and see very little direct evidence of life. Shells and seaweed are all cast up into a narrow strandline; the rest of the beach is apparently sterile, although if the tide hasn’t wiped it clean it may contain worm cast and tracks. So perhaps the ancient beach at Cwm Graianog, preserved as a snapshot in all its detail, may have looked like Newborough (and somewhere inside the mountain, there is a fossil strandline with the shells of the creatures that lived there).

 

It was the trilobite wot done it

There may be no trilobite remains at Cwm Graianog but further south, near Trawsfynydd, rocks of similar age were laid down in muddier conditions, probably in deeper water. We know they’re the same age, because Lingullela davisi can be found there too. There were no currents to disseminate the remains: an animal lay where it fell until the muddy sediments buried and entombed it. Millions of years later those sediments, turned to rock, are exposed by the course of a river, or by the construction of a road, and the creatures in them are revealed, often in exquisite detail.

 

They might look like woodlice, but they got that way from a different evolutionary starting point, and any similarity is purely coincidental. Their internal organisation was radical: for example, the eggs were probably incubated in the swollen lobe on the head (called the glabella) as in modern Horseshoe crabs, their closest living relative. Legs aren’t preserved, even in the fine mudstones. What is seen in a typical trilobite fossil is only the carapace, because the limbs were not encased in the same material. In effect, the animal was soft-bodied, and wore its shell like a tortoise.

 

Without evidence of legs, it isn’t possible to directly match the fossil trilobites with the Cwm Graianog tracks, but the rope-like Cruziana follow a pattern: the organism adopted a systematic approach, following wide loops to avoid repeatedly retracing its own footsteps. A feeding animal sifting through the detritus on the modern sea floor would adopt similar strategies to avoid sampling the same sand twice, so Cruziana look like feeding trails. Periodically, there are strange, heart-shaped marks, sometimes associated with the Cruziana tracks. In the best preserved cases, these heart-shaped marks (called Rusophycus) look almost like fossil trilobites, adding weight to the theory that trilobites caused them. If so, then the animal must’ve buried itself in the mud at this point, pushing the sediment outwards and over its back, as modern-day crabs do.

 

If there is still doubt, then observations from around the world confirm that Cruziana and Rusophycus are the tracks of trilobites. Whole specimens at the end of Cruziana trails have been found in America, and there are quite amazing Rusophycus from Australia, with every segment and leg movement picked out by the fine sediments that filled the burrow afterwards.

 

Trilobites became extinct during the cataclysmic extinction in the Permian, 290 million years ago. They left no descendants, and they are as alien to the world of today as if they had come from another planet. Yet at Cwm Graianog and Trywyn llech-y-doll, trace fossils offer an extraordinary insight into the life of these creatures and the period in which they lived; a period which enshrines Wales in the language of the geology for the whole world.

 

Richard Birch is an ecologist with Capita Symonds consultancy.

 

Back