Ancient crustacean raises new questions

The tiny fossil is less than half a millimetre long

By BBC News Online's Ivan Noble

This tiny half-billion-year-old fossil throws up some tricky questions about how and when life evolved on Earth.

It is a really wonderful fossil

Mark Williams

It is a crustacean - a relative of modern day lobsters and shrimps - and it is less than half a millimetre long.

"It is the oldest complete crustacean ever found. We found it in rocks in Shropshire which are 511 million years old," co-finder Mark Williams, of the British Geological Survey, told BBC News Online.

The find adds fuel to the debate about why so many species found as fossils seem to turn up all at the same time.

Record of evolution

Looking back at the fossil record, a large range of species begins to appear at the beginning of the Cambrian Period, 545 million years ago.

Older fossils tend to be of very small organisms like bacteria. There are two main theories which try to explain this.

One says that there was a "Cambrian Explosion": a period of rapid evolution during which a huge number of species developed.

The other says that evolution proceeded at a much slower pace but that few of the creatures that were around before the Cambrian were not preserved.

They may have been soft-bodied, which would make fossilisation very rare, or the geological conditions may simply not have been right to preserve anything.

Because this crustacean is so old, yet so advanced, it lends weight to the second theory.

'Virtually mummified'

Dr Williams described the fossil as a "career find".

"It is a really wonderful fossil. Most fossils only have their skeletons preserved, but this one is virtually mummified, with legs and soft parts preserved.

"Because the soft tissues are so soft, there must have been a very special kind of preservation. It was probably fossilised immediately after death," he said.

The unusually complete fossil bears all the distinguishing feeding appendages of a crustacean, including an extra antenna and a jaw.

Dr Williams and his colleagues David Siveter and Dieter Waloszek describe the crustacean in the journal Science.