Ancient arachnid 'walks again'

Animation courtesy of Manchester University and Natural History Museum, Berlin

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The dead walk again: an arachnid that lived 410 million years ago has crawled back into the virtual world.

The creature's remains were so well preserved in fossil form that scientists could see all its leg joints, allowing them to recreate its likely gait using computer graphics.

Known as a trigonotarbid, the animal was one of the first predators on land.

Its prey were probably early flightless insects and other invertebrates, which it would run down and jump on.

"We know quite a bit about how it lived," said Russell Garwood, a palaeontologist with the University of Manchester, UK.

"We can see from its mouth parts that it pre-orally digested its prey - something that most arachnids do - because it has a special filtering plate in its mouth. So, that makes us fairly sure it vomited digestive enzymes on to its prey and then sucked up liquid food," he told BBC News.

The trigonotarbid specimens studied by Dr Garwood and colleagues are just a few millimetres in length.

They were unearthed in Scotland, near the Aberdeenshire town of Rhynie. Its translucent Early Devonian chert sediments are renowned for their exquisite fossils.

Chert slice The 3D model is built from extremely thin slices that were cut through the Scottish chert

The team used a collection held at the Natural History Museum in London that have actually been prepared since the 1920s. The rock had been cut into extremely fine slices just a few tens of microns thick, making it possible to construct 3D models of the arachnids, much like a doctor might do with the X-ray slices obtained in a CAT scan.

"We could see the articulation points in the legs," explained Dr Garwood.

"Between each part of the leg, there are darker pieces where they join, and that allowed us to work out the range of movement.

"We then compared that with the gaits of modern spiders, which are probably a good analogy because they have similar leg proportions. The software enabled us to see the centre of mass and find a gait that worked. If it's too far back compared to the legs, the posterior drags on the ground. The trigonotarbid is an alternating tetrapod, meaning there are four feet on the ground at any one time."

The trigonotarbids were extremely successful, appearing in the fossil record until about 290 million years ago, by which time larger animals would have been making a meal of them.

Jason Dunlop, from the Natural History Museum in Berlin, was a co-author on the research. He said: "These [trigonotarbid] fossils are unusually well preserved. During my PhD, I could build up a pretty good idea of their appearance in life.

"This new study has gone further and shows us how they probably walked. For me, what's really exciting here is that scientists themselves can make these animations now, without needing the technical wizardry (and immense costs) of a Jurassic-Park style film. When I started working on fossil arachnids, we were happy if we could manage a sketch of what they used to look like. Now, they run across our computer screens."

The work is part of a special collection of papers on 3D visualisations of fossils published in the Journal of Paleontology.

Trigonotarbid Modern computing provides powerful tools to study the past

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos

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