Footprints in sands of time

August 2, 1996

If history is a mostly false account of events mostly unimportant, then the history of life, as told by the fossil record, is an account mostly fragmentary of body parts mostly insignificant. Living creatures, being almost entirely squishy, do not fossilise well. Bones, teeth and shells are their messengers. Most of the species that have ever lived have left no trace of their existence; most of the rest we know only in fragments.

Under exceptional conditions, however, soft body parts are fossilised. Very occasionally entire communities of squishy prehistoric creatures are preserved. For scientists trying to trace the genealogy of prehistoric life these soft-bodied fossils are invaluable, like photographs of long-forgotten family members. Soft-bodied fauna, although rare, form a series of snapshots of evolution that have contributed enormously to our knowledge of the history of life on earth.

Two weeks ago a team of palaeontologists from the universities of Leicester, Bristol and Oxford announced, in the journal Nature, the discovery in Herefordshire of a deposit of superbly preserved soft-bodied fossils. The deposit, which is 425 million years old, is a near-complete record of a lower Silurian marine ecosystem, the only one of its kind in the world.

The fossils shed some light onto what has been, until now, a shadowy 100 million year period of evolutionary history. "This is a find of global importance," says David Siveter of Leicester University, one of the scientists who announced the discovery. "It fills an important gap in the history of life."

During the lower Silurian period, the Welsh borderlands formed a subtropical seabed off the west coast of the continental shelf that now makes up much of central England. Land was on the verge of colonisation and the sea was teeming with life.

The Herefordshire find offers the first glimpse of some strange Silurian marine life. Strangest of all are a handful of peculiar beasts that do not fit into established evolutionary groups. They could be representatives of long-extinct lineages that emerged when multicellular life first evolved 570 million years ago. Even the most abundant animal, a 4-millimetre-long shrimp-like creature with jointed legs and stalky eyes, is somewhat enigmatic (see photograph). It is an arthropod, a member of the group that includes insects, crabs and spiders, but it does not belong in a known group.

There are some familiar faces: whelks, trilobites, graptolites (extinct tube-shaped animals), microscopic plankton and bristled worms. The finds are a particularly fine discovery. Silurian soft-bodied worm fossils are rare, in fact only a single intact specimen has been found before now. The Herefordshire worms fill a gap of 140 million years in our knowledge of worm evolution. They are so finely preserved that internal organs are visible.

The discovery of the fauna owes something to fortune. The first fossils on the site were dug up about three years ago by a minerologist, Bob King, then curator of geology at Leicester University. Dr King thought the fossils "interesting" but did not realise their full significance. He returned a few specimens to the university, which found their way onto a pile of rocks awaiting attention. There they languished, unnoticed, for a couple of years.

"We often collect fossils in the field that we never really get round to looking at," explains Dr Siveter, whose brother Derek, from Oxford University Museum, is also part of the team.

It took the appointment of a new curator, Roy Clemence, to rescue the fossils from obscurity. Dr Clemence realised that they could be something special and invited a team of experts to take a look. The team combined the Siveter brothers' knowledge of Silurian fossils with the expertise of Derek Briggs of Bristol University, who worked on the famous soft-bodied fossils of the Burgess Shale in British Colombia. "The more we looked, the more interested we became," said Dr David Siveter. "What we were looking at didn't shout out, oh yes, its x, y or z."

They went out and collected more - 378 specimens in total. The three are now co-leaders of a research project that plans to spend three years studying the fossils. Amateur collectors, though, will have to wait their turn. The location of the site is being kept secret until the research project is over.

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