In The Field

Burgess Shale Centenary: Yet more decay

The annoying thing about fossils is trying to work out what the heck the creature looked like before it was trapped in a mudslide, lost some limbs, got squashed flat, and was chemically altered by millennia of burial. This is not easy. And as Robert Sansom of the University of Leicester points out, it’s made extra difficult by the fact that some discriminating features used to identify these creatures decay faster than others. That introduces a bias in how organisms are classified, he warns. His group is doing lab tests of decay rates of different bits and pieces of animals to sort these biases out.

The lucky thing is that Burgess Shale fossils and others from the same time period around the world are strangely well-preserved. I had assumed that this was just a lucky accident of some Cambrian beasts being swallowed by a mudslide, and Walcott finding the result. But it seems to be more complicated than that. The preservation of organic carbon from these beasties is a highly unusual phenomenon, and is very rare (possibly absent?) in the fossil record for animals of younger eras. Why is this? No one knows for sure. But it might have been a combination of those animals being swamped with fine-grained clay that kept the oxygen out, and the oceans being low in sulphate, which stopped other bacteria from eating up the remains. An intriguing thought, with evidence to support it from Emma Hammarlund of the University of Southern Denmark.

Posted on behalf of Nicola Jones

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