Fool’s gold holds fossil treasure | University of Oxford
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Fool’s gold holds fossil treasure

Pete Wilton

They float like tiny jewels encased in stone: most are only a few millimetres or centimetres long but full of incredible detail – boasting tiny tentacles, eyes, legs, and forceps-like pincers.

‘Many of them are entirely soft-bodied, they have no right to be preserved over 525 million years. They should really have quickly decomposed on the sea bottom, or have been eaten, or destroyed over millions years of earth history’ explains Derek Siveter of Oxford University’s Museum of Natural History and the Department of Earth Sciences.

These are the Chengjiang fossils from Yunnan, China, on display at the Oxford University Museum of Natural History, in the first major exhibition of these fossils outside China.

The Chengjiang fossils, first unearthed in 1984, are particularly important because they open up a window onto one of the most important events in the history of life: the so-called Cambrian 'explosion' when most of the major animal groups we know today first appeared in the fossil record.

The big question has always been: where did this dazzling variety of animals come from? Did they suddenly develop or were earlier examples of complex animals simply never preserved?

These particular fossils have survived because of an amazing stroke of luck: ‘The preservation of all the soft tissues is a result of the organic materials of these animal bodies being replaced by ‘fool’s gold’ – iron pyrites. It means their forms have been captured for posterity,’ Derek tells me.

At first glance they may not be as impressive as the Museum’s dinosaur skeletons or stuffed dodos but, get your nose close enough to the glass cases, and you’ll enter a different world.  

More than 100 species have been recognised from these golden-coloured fossils:

There are examples of groups that include velvet worms (lobopodians) that Derek describes as ‘worms with legs’, 'lamp-shells' (brachiopods) looking like balloons (the body) on the end of a piece of string (the soft, fleshy, tethering stalk), and ancient arthropods, some of them the ancestors of modern horseshoe crabs with armoured bodies and forceps-like appendages – probably for grabbing on to prey – and others probably representing forms that we would identify as crustaceans (this group today includes crabs, lobsters and shrimps).

‘The collection, significantly, also includes the earliest example from the fossil record of what is generally agreed to be a vertebrate,’ Derek reveals.

But as well as these familiar-looking creatures are others that might go unnoticed but are nevertheless vital scientific finds. One example is the sea gooseberry (ctenophore), which looks like a cross between a soft fruit and a diving bell.

There are some 100 species of sea gooseberry floating in today’s oceans and the Chengjiang fossils show examples of these creatures, some with branched tentacles, which like their modern cousins probably captured their dinner by engulfing it.

Derek comments: ‘Here we have examples of the sea gooseberry. They are exceptionally rare as fossils, numbering just a handful of specimens from just a very few localities anywhere on Earth, and the Chengjiang examples are the oldest.’

‘These miraculously-preserved examples give us rare insights into the very early nature of the sea gooseberry and help determine the timing of the origin of this animal group and its place within the tree of life.’

The exceptional preservation of these fossils (in what researchers call a lagerstatte) makes it possible to examine the remains of soft-bodied animals in intricate detail, and such preservation can often give us a much better idea of how they might have behaved:

The wonderful ‘daisy chain’ fossil creatures, that we reported on back in 2008, are one example of this: shrimp-like animals that have congregated together perhaps for reproduction, or more likely as part of some Cambrian migration through the ancient seas.

If you expect life from 525m years ago to look rather primitive then you’re in for a surprise: these fossils reveal fully-formed, finely-adapted animals that seem as sophisticated as many of their relatives we know today – something we wouldn’t be able to say but for their extraordinary preservation.

‘It’s an amazing treasure, it's the Chinese equivalent of the famous Burgess Shale fossils from North America or, put another way, the material is every bit as wondrous palaeontologically as the Terracotta Warriors are archaeologically,’ adds Derek.

The exhibition, ‘Exceptional Fossils from Chengjiang, China: Early Animal Life’, is on display at the OU Museum of Natural History 18 May-14 November 2010. 

The specimens displayed are from the Key Laboratory for Palaeobiology, Yunnan University.