Ancient Walking Fish May Have Walked on All Fours

A fossilized pelvis shows the fish had functioning rear “legs”

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In addition to its limb-like front fins, Tiktaalik had large, mobile rear fins that it used to push itself around in the water. Shubin et al.

A full decade ago, paleontologists digging around in the Canadian High Arctic discovered something amazing—a fish whose front fins seemed made for walking. The ancient fish, which lived 375 million years ago, looked a lot like a fish and a bit like a crocodile: it had a flat, broad head, a long, slender body and front fins that the researchers described as “morphologically and functionally transitional between a fin and a limb.”

After analyzing the bones, the scientists, led by Neil Shubin, suggested that the fish, Tiktaalik roseae, could prop itself up on its front fins to help it catch its prey in the river waters of what is now Canada's Ellesmere Island.

When the first Tiktaalik roseae fossil was found in 2004, however, large parts of the organism were missing, including its hindquarters. But now, Shubin and his colleagues are back with a new Tiktaalik fossil. This time they've got a preserved pelvis, and more surprises for the evolution of four-legged propulsion.

In addition to its limb-like front fins, Tiktaalik also had large, mobile rear fins that it used to push itself around in the water. Postmedia News' Margaret Munro reports that, according to the new study, the fish's pelvis was "much bigger than expected"—and indicates that Earth's organisms may have started doing something like walking on four legs much earlier than scientists had thought:

“It looks like this shift actually began to happen in fish, not in limbed animals,” team leader Neil Shubin, at the University of Chicago, says in a summary of the findings....

“This is an amazing pelvis, particularly the hip socket, which is very different from anything that we knew of in the lineage leading up to limbed vertebrates,” co-author Edward Daeschler said in a summary of the findings.

The finding that Tiktaalik had useful back limbs, too, certainly sounds like would have given the fish some advantages, says Jonathan Amos for the BBC:

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.

Animals like Tiktaalik roseae can be tricky to think about, and it's easy to misrepresent them as animals that so desperately wanted to walk on land. But, of course, says Berkeley's Understand Evolution, that's not how evolution works:

Tiktaalik was specialized for life in shallow water, propping itself up on the bottom and snapping up prey. The adaptations it had for this lifestyle ended up providing the stepping stones for vertebrates to climb onto dry land — but of course, Tiktaalik was not "aiming" to evolve features for land-living. Tiktaalik was simply well-adapted for its own lifestyle and later on, many of these features ended up being co-opted for a new terrestrial lifestyle.

There was already some contention over the timing of the arrival of four-limbed motion, with some researchers in 2010 suggesting they'd found track marks from a four-legged animal that predated even Tiktaalik. But aside from that, the new discovery can help answer one of the persistent questions in evolution: what good is half a leg?