Fossil tracks record 'oldest land-walkers'

By Jonathan Amos Science correspondent, BBC News

The oldest evidence of four-legged animals walking on land has been discovered in southeast Poland.

Rocks from a disused quarry record the "footprints" of unknown creatures that lived about 397 million years ago.

Scientists tell the journal Nature that the fossil trackways even retain the impressions left by the "toes" on the animals' feet.

The team says the find means that land vertebrates appeared millions of years earlier than previously supposed.

"This place has yielded what I consider to be some of the most exciting fossils I've ever encountered in my career as a palaeontologist," said team member Per Ahlberg from Uppsala University, Sweden.

"[They are] fossil of footprints that give us the earliest record of how our very distant ancestors moved out of the water and moved on to the land and took their first steps."

Numerous trackways have been identified in the Zachelmie Quarry in the Holy Cross Mountains.

They represent the movements of many animals as they scurried around what would have been a tropical muddy shoreline in the Middle Devonian Period of Earth history.

Slabs of carbonate rock are dappled with prints that range in size and detail.

Some indentations are obscured where successive animals have trampled over the same patch of ground; but others retain exquisite features of the pads and digits that made them.

The animals were probably crocodile-like in appearance and lived an amphibian-like existence (although those specific animal forms did not appear until many millions of years later).

The dimensions of the prints suggest some individuals were more than two metres long.

How one of the Devonian animals might have made the tracks

The Polish and Swedish scientific team analysed the trackway patterns to reconstruct how the ancient creatures would have moved their "hips", "elbows" and "knees".

This confirms that only true four-legged animals, or tetrapods, could have left the marks.

Theory holds that the first land creatures evolved from fish that had pairs of lobed fins. The precise timing of this transition has been a dynamic field of study in recent years.

The assumption of palaeontologists had been that there was a swift but stepwise transition between water and land.

The discovery changes the story of the emergence of vertebrates on land

Perhaps the most notable fossil in this story is an organism called Tikaalik roseae, an animal that had features intermediate between fish and tetrapods.

But Tiktaalik lived about 375 million years ago; and although there are slightly older transition fossils, the Zachelmie Quarry tetrapods break the neat and simple timeline.

"The discovery of undoubted trackways from the earliest period of the Eifelian - that is 397 million years ago - pushes back the divergence between fishes and the four-legged vertebrates by about 18 million years, if not probably more," commented Dr Philippe Janvier from the National Museum of Natural History, Paris, France.

"I suspect that now we can push the divergence back to the Emsian stage, maybe 400 million years ago. That's surprising, but this is what the fossil evidence tells us," the independent researcher told BBC News.

Another key surprise from the research is the recognition that these tetrapods lived in a marine environment, perhaps a coral lagoon.

The favoured origin before now for the emergence of tetrapods had been marshy environments, such as deltas or lakes where freshwater dominated.

The team behind the latest research said the new explanation made sense because it would have allowed marine ancestors of tetrapods gradually to acquire terrestrial competence while accessing a new and essentially untouched resource of food washed up with the tides.

"In the intertidal setting, you've got a smorgasbord laid out twice a day," said Dr Ahlberg.

"Every time the tide goes out, it leaves behind this drift-line of dead and moribund animals. All this was just left there for vertebrates - our ancestors - to emerge on to land and pick them off."

Jonathan.Amos-INTERNET@bbc.co.uk

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