Planet Earth Online

Fossil recipe rewritten

25 November 2008

The Burgess Shale in the Canadian Rockies is one of the best-preserved fossil deposits in the world. Until now, palaeontologists have failed to find a truly plausible explanation for why the eyes, guts and gills of some of the planet's first animals were so perfectly preserved here, but not elsewhere.

But new findings on how fossils form, published in the journal Geology this month, may make it easier to locate other sites with exceptionally preserved fossils.

Normally, only the hard parts of animals are fossilised - the bones, teeth or shells. But the Burgess Shale contains abundant fossilised remains of soft-bodied animals - filter feeders, worms, scavengers and predators - as well as the organs of hard-bodied animals - trilobites and primitive shrimp-like creatures. These animals thrived on the seafloor 500 million years ago, shortly after an event known as the Cambrian Explosion - widely believed to be the dawn of animals on Earth.

'There's been a lot of debate as to why we only see exceptional preservation in the Burgess Shale and a few other deposits', says Dr Phil Wilby from the British Geological Survey, one of the paper's authors.

Previous theories speculated that immediately after their death, a clay 'template' shrouded the creatures, halting their decay.

'The received wisdom was that the best preserved fossils should not be found in rocks that have undergone deep burial and mountain building, where they would have been subjected to high temperatures and huge forces' said Wilby.

'We now show how this is not the case.'

The Burgess Shale, discovered by Charles Doolittle Walcott in 1909 high in the Rocky Mountains, was the bottom of a shallow sea half a billion years ago.

The creatures in the shale lived around the foot of a submerged limestone escarpment. Every so often a large submarine mudslide smothered the area in sediment, killing the animals instantly.

This was essential for their preservation because it stopped predators devouring the carcasses and prevented bacteria from rotting the flesh.

Earlier research suggested that at this stage, clay precipitated on the creatures, creating a permanent imprint of their bodies. As the sediment was buried deeper in the Earth and eventually forced upwards as a part of a mountain range many millions of years later, the hard clay protected their outlines, giving the world immaculate specimens.

But, Wilby and colleagues, lead-author Dr Alex Page, Dr Sarah Gabbott, and Dr Jan Zalasiewicz from the universities of Cambridge and Leicester have shown that the clay fossilisation process happened much later. It is the deep burial and high temperature - reaching 300ºC - that is the recipe for success.

Quick kill, then bake until done

'It is true that the delicate anatomy of these fossils is picked out by clays, but these formed long after the animals' initial smothering - millions of years later and at great depth and temperature,' said Wilby.

Page said, 'This provides a whole new theory for how fossils form.'

Wilby added, 'We are not entirely clear about how this happens. It could be that lighter organic chemicals in the fossils were driven off at high temperature and pressure. This is what happens during oil formation. The process may act as a catalyst to precipitate the clay.'

'Or it could be that by driving off these volatile chemicals a space for the clay is created.'

'What is interesting is that it had previously been thought that a process this harsh would destroy such fossils, but it turns out that it is crucial to preserving their outlines.'

So the recipe for fossilising soft body parts seems to be: a quick kill, seal the carcass in sediment to stop it rotting, push it deep underground for millions of years, then bake at 300ºC until done.

Keywords: Biodiversity, Geology, North America,

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