Scientists have known since the last century that something was roaming the sandflats of what is now eastern North America 500 million years ago - 150 million years before our amphibian ancestors made the transition from sea to land.
They knew this because whatever it was left strange, fossilised tracks in the sand: two parallel ridges set several inches apart with a series of chevron-like furrows connecting them looking uncannily like the tyre-marks of a motorbike.
Two palaeontologists, Ellis Yochelson of the National Museum of Natural History in Washington DC and Mikhail Fedonkin of the Palaeontology Institute in Moscow, believe they now know what caused the tracks so many millions of years ago.
Working with Mary Parrish, the Washington museum's artist, they have constructed a sketch of the animal estimated to be the same size and shape as a human foot - an apt analogy considering the organism made the first apparently successful attempt to haul itself about on land.
The animal, called Climactichnites after the Latin word for ladder-like, has been known only from its tracks since they were discovered in 1860 by Sir William Logan, the first director of the Geological Survey of Canada.
Scientists were even unsure which way the creature was heading until they discovered, at the turn of the century, oval impressions at one end of the tracks and, 40 years later, two overlapping tracks.
These provided enough information to determine the direction of movement. But further details of the organism were scarce until Yochelson and Fedonkin teamed up several years ago.
Following an exhaustive investigation of Climactichnites fossils, the two researchers prepared a detailed sketch of the creature, due to be published next year by the Smithsonian Institution Press.
Our drawing is the first published preliminary sketch.
'It's like no other animal we know,' said Dr Yochelson. 'This was certainly one of the earliest attempts to get on to land, if not the first attempt.'
He said the sandflats at that time would have been rich in micro-organisms and Climactichnites appeared to survive by munching on sand for morsels of food.
Because it had no bones or hard parts, it crawled about by moving around its body fluids, much like an earthworm. It also secreted a sticky mucus to help it glide along, Dr Yochelson said.
The tracks, which survived long enough to become fossils, were made on damp sand, he said. Tracks on dry or wet sand would not have survived.
The two researchers believe the animal moved by gripping the sand with two muscular flaps on each side of its body - raising the two parallel ridges in the sand - and heaved itself along with flaps at the front, creating the chevron furrows.
Dr Yochelson said Climactichnites survived for between five and ten million years before becoming extinct. 'It was an experiment that worked remarkably well for a short time.'
However, no known descendants exist, and the experiment may have led to an evolutionary cul-de-sac.
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