Petrified velvet worms from 425 million BC reveal true ecology of the distant past (11/28/2007)

Fossilized velvet worm, preserved in calcium phosphate, with possible colour banding. The image on the left shows the specimen is covered in glycerol, and illuminated with polarized light; the other side of the rock slab preserving the specimen is shown on the right, photographed using under low angle lighting.

University of Leicester Geologist Dr Mark Purnell, with Canadian colleagues, reported, in the journal Geology, a new, exceptionally preserved deposit of fossils in 425 million year old Silurian rocks in Ontario.

The fossils include complete fish (only the second place on earth where whole fish of this age have been found), various shrimp and worm like creatures, including velvet-worms, which look (in Dr Purnell's words) "rather like a dozen headless Michelin men dancing a conga".

The velvet worms were deflated slightly by a little early rotting, but within days of dying these animals had been transformed to the mineral calcium phosphate. This preserved them as beautiful petrified fossils, showing the wonderful detail of their bodies, including coloured stripes.

This Canadian deposit is unusual even for sites of exceptional preservation because it also includes normal shelly fossils. From this it is possible to be sure that the conditions in which all the animals were living were not much different to normal nearshore seas of the Silurian period.

Dr Purnell commented: "It provides us with our best view of what lived together in such environments 425 million years ago, and our best information for understanding how life on earth at that time was different to today.

"If people think of a fossil, they will undoubtedly be thinking of something with a hard skeleton or shell of some sort, and it is true that the vast majority of fossil are what in today's world we call sea shells. But imagine trying to understand the biodiversity and ecology of a submarine seaside ecosystem with only the remains of sea shells to go on.

"All the variety of worms that crawl over and into the sand would be unknown, as would all the shrimpy things that scurry over the surface. We would have only a very partial view of the real biological picture.

"This is what palaeontologists are faced with when they try to reconstruct the history and past ecology of life on Earth, because everything without a shell very quickly, within hours or days, rots away to nothing, leaving no trace that it ever existed."

Fortunately, there are a few special rock deposits scattered around the world that preserve fossilised traces of those things that normally rot away. These are known to palaeontologists as sites of exceptional preservation, but they are, Dr Purnell says, tricky to interpret precisely because they are exceptional.

"They require very unusual environmental conditions in order to slow down the decomposition of soft tissues, such as muscle and skin, and rapidly transform them into geologically stable minerals that will survive as fossils for millions of years.

"The difficulty for geologists has been that if the conditions are exceptionally unusual, is that also true of the preserved fauna or is it a more typical example? That is something our latest find has helped resolve."

Note: This story has been adapted from a news release issued by the University of Leicester

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