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Last Updated: Thursday, 21 October, 2004, 10:45 GMT 11:45 UK
Early sea spider flashes pincers
A virtual reconstruction revealed the fossil sea spider's delicate features

Scientists have painstakingly recreated an ancient sea spider from the faint imprint it left when it fossilised around 425 million years ago.

The virtual reconstruction sheds new light on a debate about whether sea spiders sit in the same biological family as true spiders and scorpions.

The presence of pincers suggests it does, a UK-US team claims in Nature.

This has previously been difficult to determine because the animals' delicate bodies do not fossilise very well.

Although you might use the word reconstruct, what you've got is a real animal. It's not part of someone's imagination
Professor David Siveter, University of Leicester
Living sea spiders are small marine arthropods with slender, segmented bodies, a proboscis and four pairs of spindly legs.

The fossil sea spider Haliestes dasos lived in an ancient sea that covered the area now known as Herefordshire, England, during the Silurian Period.

An eruption that carpeted the area in volcanic ash was responsible for the sea spider's spectacular, three-dimensional preservation.

Owing to this fortuitous event, Haliestes avoided being flattened or decomposing, leaving a perfect record of its form in the Herefordshire rocks.

But in order to bring that form to life, scientists from the Universities of Oxford and Leicester, UK, and Yale in the US had to destroy the fossil itself.

A close shave

"The rock itself is quite calcareous so we can't extract the fossil from the rocks using any mechanical or chemical techniques," co-author Professor David Siveter of the University of Leicester told BBC News.

"The composition of the fossil as it's preserved and the composition of the rock matrix in which it sits is similar. So there's very little difference to play on. We can't, for example, use acid etching techniques."

Sea spider, Nature
Pincers shed light on sea spiders' place in the evolutionary tree
The researchers used a machine that shaves off layers of the fossil at very fine intervals of between 20 and 30 microns. To get an idea of just how fine this is, one micron is equivalent one-thousandth of a millimetre.

After the machine grinds off a new slice of the fossil, the team took a digital photograph of the fossil.

"We capture all of the morphology of the fossil digitally and then we use computer techniques to reconstruct the animal.

"Although you might use the word reconstruct, what you've got is a real animal. It's not part of someone's imagination."

Pincer action

Sea spiders are so rarely preserved that there are only a handful of specimens in the fossil record. Haliestes is a complete specimen and an adult, which can tell scientists a lot about where ancient and modern sea spiders sit in the tree of life.

One theory had proposed that the pycnogonids (the technical name for the sea spiders) were only distantly related to the chelicerates - the biological grouping which includes true spiders, scorpions and horseshoe crabs.

But the presence of chelicera - or pincers - is exclusive to the chelicerates, say the researchers. This, they claim, suggests the pycnogonids are full members of that group.

A 2001 study in Nature combined genetic data with physical characteristics to reconstruct a family tree of arthropods - the umbrella grouping that covers sea spiders and chelicerates as well as crustaceans and insects.

This study supported a more distant relationship between pycnogonids and chelicerates.

Dr Ward Wheeler, curator of invertebrate zoology at the American Museum of Natural History in New York and co-author of the 2001 study, praised the latest research but was cautious in drawing conclusions about the creatures' evolutionary relationships.

"Pycnogonids have features that would lead to kinship with chelicerates, like their [pincers] of course," Dr Wheeler told BBC News.

"Certain DNA features are very much like those of spiders and horseshoe crabs. But there are other features that are not.

"In order to get a more stable, robust result, we're going to have to get an analysis that includes extinct biological lineages as well as living ones and includes morphological and molecular information."

The name Haliestes dasos means "hairy-rump soothsayer of the sea".

Ancient fossil penis discovered
05 Dec 03  |  Science/Nature
Virtual imaging brings fossils to life
03 Apr 01  |  Science/Nature

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