A spider-like 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 explained.
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.
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."
"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.