Published online 6 January 2010 | Nature | doi:10.1038/news.2010.1

News

Discovery pushes back date of first four-legged animal

But controversy surrounds 400-million-year-old fossilized tracks.

tracksCould these be the tracks of the first four-legged land animal?Grzegorz Niedzwiedzki

The oldest known tracks of a four-limbed land animal could rewrite part of vertebrate evolution.

Some prints, showing individual digits, were found in limestone slabs unearthed in a quarry near Zachełmie, Poland, dated to about 395 million years ago — more than 18 million years before tetrapods were thought to have evolved.

The tracks suggest that the animals that made them were up to 2.5 metres long and had a footpad up to 26 centimetres wide, although most prints were about 15 centimetres wide, reports a team of Polish and Swedish scientists in Nature this week1. This would mean that large, land-roaming tetrapods would have coexisted for 10 million years with the elpistostegids — including Tiktaalik roseae, which lived 375 million years ago — a group thought to mark the transition of from fish to land-roaming animals (see 'The fish that crawled out of the water').

“There is still a small chance these may be something else masquerading as tetrapod footprints.”

Jennifer Clack
University Museum of Zoology, University of Cambridge, UK

"The discovery of the Zachełmie footprints substantially changes the context for future research on the origin of tetrapods," writes the team, which includes Grzegorz Niedźwiedzki, a fossil-footprint specialist from Warsaw University, and Per Ahlberg, a palaeontologist from Uppsala University in Sweden (see video and podcast).

But the claim will not be accepted by all. "I am sure this paper will come under heavy fire," says Philippe Janvier of the National Museum of Natural History in Paris (see P. Janvier & G. Clément Nature 463, 40–41; 2009).

No bones

The find is not supported by fossil bones at the site, and palaeontologists familiar with the discovery say they have reservations about the tracks, because they may have been made by some natural process.

"There is still a small chance these may be something else masquerading as tetrapod footprints," says Jennifer Clack, palaeontology curator at the University Museum of Zoology, University of Cambridge, UK. "One of the startling things is the size of some of these isolated footprints," she adds. "You'd expect a smaller animal would more easily become terrestrial."

Edward Daeschler, head of vertebrate zoology at the Academy of Natural Sciences in Philadelphia, Pennsylvania, and part of the team that described Tiktaalik roseae2,3, agrees. "Trace evidence is not enough for me to change my mind about accepted theories on tetrapod evolution," he says.

Janvier says that "there is a risk" that natural processes formed the tracks, adding that the dating of the limestone formations bearing the tracks was "unambiguous".

Ahlberg insists the tracks are genuine prints. "You can see anatomical details consistent with a footprint, including sediments displaced by a foot coming down," he says. "There is no way these could be formed by a natural process."

Thrusting forward

The Zachełmie quarry is in the Świętokrzyskie Mountains (Holy Cross Mountains) in southern Poland. It is a well-known sequence of marine Middle Devonian formations. These formations are from the southern coast of what was once the supercontinent Laurasia, created when the earlier supercontinent Pangaea split up. The authors think that the new tetrapod lived in an intertidal or lagoon environment.

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The team says that the tracks bear the hallmarks of an animal capable of walking by thrusting its arms and legs forward. They note the fossil structure of transitional species such as Tiktaalik or Panderichthys4 — another elpistostegid — would make such walking motion "impossible".

The questions surrounding the discovery are only likely to be solved by exploring more of the intertidal rock formations from the Middle and Early Devonian periods to find fossil bones of the creature that made the tracks. 

  • References

    1. Niedźwiedzki, G., Szrek, P., Narkiewicz, K., Narkiewicz, M. & Ahlberg, P. E. Nature 463, 43-48 (2010). | Article
    2. Daeschler, E. B., Shubin, N. H. & Jenkins, F. A. Jr Nature 440, 757-763 (2006). | Article | PubMed | ChemPort |
    3. Shubin, N. H., Daeschler, E. B. & Jenkins, F. A. Jr Nature 440, 764-771 (2006). | Article | PubMed | ChemPort |
    4. Boisvert, C. A., Mark-Kurik, E. & Ahlberg, P. E. Nature 456, 636-638 (2008). | Article | PubMed | ChemPort |

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  • #61281

    These features allowed the scientists to deduct that the Ichthyologist likely moved about by dragging themselves like modern day seals. I think maybe even in a more important discovery, the researchers also identified hitherto unknown skeletal feature, like a string of bones extending on the chest of the extinct beast.

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