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X-rays extract 'virtual harvestmen' from French fossils

By Jonathan Amos
Science correspondent, BBC News

media captionA spiny body might have deterred predators

They may not be the most attractive creatures on Earth but there is no doubting their success.

Harvestmen, or Opiliones as scientists would call them, have existed for hundreds of millions of years.

What is fascinating about them is how little they have changed in that time.

This is confirmed by new research published in Nature Communications that has produced some exquisite 3D renderings of two fossil harvestmen from France.

The specimens are just over 300 million years old - from the Carboniferous Period.

Many of their features were buried in their host rocks and difficult to study.

So, Dr Russell Garwood, who is currently based at London's Natural History Museum, put the fossils in a computed tomography (CT) scanner.

This machine is able to generate three-dimensional models of objects from a series of two-dimensional X-ray images - more than 3,000 images in the case of these two specimens.

The result is a virtual extraction of the harvestmen and a fabulous means to study the creatures' anatomy.

Both arachnids are shown to belong to taxonomic groups that still exist today.

image captionAn impression of what A. scolos (top) and M. cronos might have looked like in life. The study will help unravel the evolutionary relationships between different creatures

Ameticos scolos, from the sub-order Dyspnoi, displays two spine-like structures on its 9mm-long body - something that might have deterred any predator trying to swallow it.

The slightly smaller Macrogyion cronos, from the sub-order Eupnoi, is distinguished here by its long legs, one of which has a big curve. Modern relatives have a very similar appearance and use these looped structures to grab on to leaf parts as they move through foliage.

"We can't actually say scientifically why harvestmen have changed so little through Earth history, but basically everything else around on land at this time in the Carboniferous was in a very primitive form," explained Dr Garwood.

"These creatures, on the other hand, were pretty much as they appear now all the way back then. They are the exception in that sense," he told BBC News.

Being so slight and spindly, it is not really surprising that ancient harvestmen have a relatively poor record. Only around 33 fossilised species have been discovered so far, and for some of those the quality of preservation is not brilliant.

That makes the CT scanning technique all the more valuable, enabling scientists to capture every last detail - even of those anatomical aspects hidden inside the encapsulating rock.

The other key advantage, of course, is that scanning like this is non-destructive; the fossil does not need to be split open, which risks damaging the very features a researcher wants to see.

"With the Dyspnoi specimen, all I could see from the rock were the two spines sticking out - quite unprepossessing," recalls Dr Garwood. "With the other one, I could see a bit of the underside and then one nice long leg, and that was it."

Although currently based in the computed tomography lab at the NHM, Dr Garwood carried out his research while at Imperial College London.

media captionLong and leggy: M. cronos would have been adept at moving through vegetation

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