Video: Oldest tetrapod footprint
Evidence that four-legged vertebrates walked on Earth some 10 million years earlier than previously believed could force a radical rethink of where they evolved, as well as when.
Tetrapod footprints dating back 397 million years have been discovered in the Świętokrzyskie mountains in southern Poland in what was, at the time they were made, a seashore. All previous fossil evidence for these earliest known four-limbed vertebrates has been found in river deltas and lakes.
"Our discovery suggests that the current scientific consensus is mistaken not only about when the first tetrapods evolved, but also about where they evolved," says Grzegorz Niedźwiedzki of the Department of Palaeobiology and Evolution at the University of Warsaw in Poland, who discovered the footprints in 2002 in an old quarry near the town of Kielce.
The footprints are 18 million years older than the earliest known examples of fossilised tetrapod bones. They are roughly 15 centimetres wide, suggesting that the creatures that left them were 2.5 metres long.
Until now, it was generally thought that tetrapods diverged from the group of large, lobe-finned fish called the elpistostegalians – which includes the famous Tiktaalik – around 387 million years ago.
"It was assumed that tetrapods evolved in river deltas and lakes, partly because all previous fossil evidence has been found in these environments," says Jenny Clack, curator of vertebrate palaeontology at the University Museum of Zoology in Cambridge, UK. "This research suggests that the first tetrapods that crawled onto land were, in fact, living in shallow seas."
Imprinted in mud
The creatures left the footprints as they tramped through mud in what was at the time an intertidal marine lagoon, south of the equator, along the south coast of the supercontinent called Laurasia.
Why might tetrapods have evolved to crawl onto land from the sea rather than from rivers or lakes? One hypothesis put forward by Niedźwiedzki is that the seashore provided a tempting new source of food: stranded fish, served up twice daily by the tides.
However, Ted Daeschler of the Academy of Natural Sciences in Philadelphia, Pennsylvania, warns that fossilised footprints can be misleading. "With all due respect to the scientists involved in this study, there may be other explanations for these suggestive tracks and traces," he says.
Clack, however, believes a reassessment of the consensus view on tetrapod evolution might be in order. "The evidence that these footprints were made by bona fide tetrapods is convincing," she says.
Journal reference: Nature, DOI: 10.1038/nature08623
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