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Stone baked fossils

Tom Feilden | 07:48 UK time, Tuesday, 25 November 2008

High above the tree line in the Rocky mountains of British Columbia, on a ridge running between two jagged peaks, a thin strip of slate grey rock breaks the surface.

Walcott QuarryTo the untrained eye this nondescript seam of sedimentary rock hardly rates a second look, but to palaeontologists, it's worth its weight in gold. The Burgess Shale contains some of the oldest and best preserved fossils anywhere in the world, and offers a unique insight into the blossoming of marine animal life known as the Cambrian explosion some 500 million years ago.

All the more remarkable because the Burgess Shale really shouldn't be there. Everything we know about the geophysical processes involved in mountain formation tells us the intense heat and pressure generated as the rocky mountains were thrust up should have destroyed these fragile deposits long ago.

Burgess Shale fossilInstead we're treated to a fantastic array of spiny, bug-eyed and armour plated monsters preserved in the kind of exquisite detail that even includes imprints of their soft fleshy parts - the eyes and internal organs - that almost never survive the process of fossilisation.

So what happened? Why are the Burgess Shale fossils there at all?

To answer that question, Dr Alex Page, at Cambridge University, and Dr Jan Zalasiewicz, at the University of Leicester, have painstakingly re-examined hundreds of Burgess Shale fossils, but this time paying much closer attention to the geophysical processes that were occurring as the shallow seabed these animals inhabited was sucked down into the earth's crust and then thrust back up again to form a ridge in the Rocky mountains.

They've shown that as the delicate organic tissues of these fossilised animals were heated deep in the earth's crust they became the site for clay mineral formation. These new minerals picked out the intricate detail of gills, guts and eyes we see today, enhancing their preservation.

As Alex Page says: "Far from cremating these unique specimens the processes that thrust up the rocky mountains actually baked the Burgess Shale fossils in."



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