By William Broad www.smh.com.au

Scientists investigating an abandoned quarry in Canada have found what appear to be the oldest known footprints of terrestrial creatures - bug-like creatures 30 centimetres long that crawled from the sea and left tracks in sandy dunes.

The sandstone is between 480 and 500 million years old. Scientists believe the discovery region, just north of Lake Ontario outside Kingston, Ontario, was a sandy beach on a primordial sea.

Scientists say the find pushes back the colonisation of land by about 40 million years and puts it in or near the late Cambrian period, when the seas were starting to boil with large creatures.

The tracks were saved from oblivion when quarry crews shunned the sandstone as unsuitable for commercial use.

Dr Robert MacNaughton, a scientist at the Geological Survey of Canada who led the team, noted the quarry owner had preserved some of the trackways and used them "as ornaments on his lawn".

The find is described in the May issue of journal, Geology, by MacNaughton and five other Canadian and British scientists.

In the past 10 years or so, specialists studying old rocks have steadily pushed back the time when sea animals are believed to have first come ashore.

The date has gone from the Silurian period, which is generally accepted to have started 440 million years ago, to the Ordovician, which started 490 million years ago, and to the Cambrian, which started 544 million years ago. It was characterised by warm, shallow seas and the appearance of the first hard-shelled marine animals, such as trilobites.

The scientists said the Canadian find included more than 25 trackways that criss-cross an area roughly the size of a small basketball court. Some of the footprints are deep enough to cast shadows. Typically, the trackways have a central area where the body and tail made impressions as the animal moved forward and parallel areas where rows of legs left multiple footprints.

Trackways of several sizes indicate the presence of several individual animals over an extended period, the scientists wrote, implying a "group exodus from the water."

The New York Times