Tiktaalik: Iconic fossil's rear parts described

Tiktaalik artist's impression An artist's impression of Tiktaalik

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Scientists have finally managed to describe the back end of one of the key fossil finds of the past 10 years.

Known as Tiktaalik, the 375-million-year-old creature is considered pivotal because it has many features that look half-way between fish and land animals.

As such, it provides insight into life's evolutionary move from water into the terrestrial environment.

The first specimens to be detailed only had foreparts, but the new fossils now show important rear elements.

Chief among these, reported in the journal PNAS, is the pelvic bone and tail fin.

These give scientists a much better idea of how Tiktaalik propelled itself through the shallow waters that it occupied during Earth's Devonian Period.

Tiktaalik is what is termed a tetrapodomorph - a type of transitional vertebrate.

About 2.5m in length, it looked somewhat like a fish in that it had scales and fins with webbing; but it also had a flat head, and shoulder, forearm and wrist bones that echoed later, fully land-living, four-limbed animals.

The early analysis of Tiktaalik was based on fossils gathered in Canada's High Arctic, on Ellesmere Island in 2004. This assessment concentrated just on the creature's front end.

It is only now, after cleaning and preparing other specimens gathered at the same time as the original finds, that the team has been able to say something definitive about Tiktaalik's hind region.

In the new collection is a thick, powerful rear fin. But the big surprise is the pelvic girdle that would have supported its hind fins.

There are only limited impressions of these appendages in the fossil rock, but it is clear from the size of the pelvis that Tiktaalik's back fins, too, would have been big.

"The pelvis is as large as the shoulder girdle, and that's not what we would have expected in this finned stage in the fin-to-limb transition. We would have expected the pelvic fins to be smaller," explained Dr Ted Daeschler from the Academy of Natural Sciences of Drexel University, Philadelphia, US.

Writing in PNAS, the team says this means that locomotion based on a quartet of robust appendages was already being emphasised in fish fins long before land animals made the most of "four wheel drive".

In addition, the latest work has allowed the researchers to produce a new simulation of how Tiktaalik looked and moved through its environment.

The fins undoubtedly were employed as paddles to swim, but might also have been used in a leg-like way on occasions.

"Tiktaalik probably had the ability to use those fins as props to move along, using them to push along the shallow bottom, to work its way through plants; and, who knows, maybe it got out of the water briefly if it needed to move over to another watercourse," speculated Dr Daeschler.

"But in no way was it specialised for getting out of the water. It may have had some ability to do that, but everything about its reproduction, its sensory system, its hunting, its breathing - all these things tied it to the water," he told BBC News.

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos

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