Footprints are key to explosion of life

Fossilised trackways from the Grand Canyon, resembling little more than the idle doodles of some ancient stonecarver, are helping scientists trace out the emergence of complex life on earth.

The Cambrian explosion, which occurred 540 million years ago, saw the make-up of life change forever, with multicelled creatures evolving alongside microbes that had dominated for some 2 billion years.

Yet evidence of many of these pioneers is scarce, restricted to a handful of locations - for example in Canada, Greenland and China - where conditions preserved these marine animals' fragile forms.

A study by University of Bristol researcher Abby Lane has opened a new window on this era. She has been able to match faint footprints left in Cambrian rocks from the Grand Canyon with the feet of the earliest arthropods, the vast phyla of exoskeletoned animals that includes insects, spiders and crustaceans.

"This is a whole new wealth of information on the Cambrian explosion - we no longer have to rely on digging up these creatures," Lane said.

She used a computer modelling technique that was developed by her supervisor at Bristol, Simon Braddy, to calculate the probable trackways that would have been left by some of the long-extinct arthropods discovered in Canada's Burgess Shale. These animals were remarkably well preserved after catastrophic mud slides buried them.

Using Braddy's modelling technique, dubbed Locobug, Lane was able to generate trackways for a number of these early arthropods by entering details of their reconstructed body plans, such as the number, position and length of their limbs.

The computer program was able to match such data with the creatures' probable gaits and predict the trackways they would have left.

Lane then compared the predictions with real trackways recovered from the Grand Canyon, where the calm conditions had meant few bodies but many footprints survived.

In this way, she was able to identify the marks left by several of the Burgess Shale animals, including trackways comparable to the scalloped prints of the woodlouse-like Sidneyia inexpectans and the railway track prints of the crustacean Canadaspis perfecta .

Her research, described in the latest issue of Geoscientist , reveals the possible ways in which some of the first complex lifeforms were able to scurry across their shallow seabed homes and indicates how widely some of these creatures were able to colonise the planet.