Evolution: A fish that evolved to stand up on land went back to living in water

A fossil from 385 million years ago named Qikiqtania wakei shows that a descendant of early land animals lost its adaptations for land and became a more efficient swimmer

By Christa Lesté-Lasserre

A scaly, finned creature that lived in water 385 million years ago descended from four-legged land animals, in a clear example of a “backward” step in evolution.

Some fish grew legs and developed the ability to stand nearly 400 million years ago, starting the evolutionary lineage of four-legged animals called tetrapods. But the smooth limb bones of the newly described Qikiqtania wakei fossil couldn’t have supported the muscles needed for standing, meaning the “fishapod” had evolved back into a swimming, full-time water dweller, says Neil Shubin at the University of Chicago.

“The ancestors of Qikiqtania were already taking those steps [out of water], but this was a creature that said, ‘I’m not doing that, I like the water better!’” he says. “It was, basically, the relative that went back.”

Shubin and his colleagues unearthed the scaly Qikiqtania fossil embedded in rock in the Canadian Arctic Archipelago in 2004, at the same time and in nearly the same place as their discovery of Tiktaalik roseae, a fishapod representing the evolutionary transition from fish to amphibian.

Scans of Qikiqtania – which was probably about 75 centimetres long when it was alive – revealed parts of the upper and lower jaw that suggest it could feed by sucking in prey, like Tiktaalik did, says Shubin. It also had fangs and teeth that were “like steak knives”, indicating the animal could bite as well.

Qikiqtania is clearly a close relative of Tiktaalik, but the humerus bone in its pectoral fin is relatively smooth and boomerang-shaped, which would have made it unlikely to walk or even stand. This meant that it was neither a fish that had never walked, nor a fishapod that was currently walking.

“It had lost all these processes and shelves on the humerus and all these places where muscles would attach that would have allowed it to push up,” says Shubin.

Instead of reverting to the body of its more primitive water-dwelling ancestors, Qikiqtania had evolved to become an even more efficient swimmer in open waters, with fins somewhat like ping-pong paddles. “What’s interesting about it is that it went back and became super specialised,” says Shubin.

The team named the animal for Qikiqtani, the Indigenous word for the region where the fossil was found, and for David Wake, an evolutionary biologist at the University of California at Berkeley who died in 2021.

The findings highlight the fact that evolution isn’t always a straightforward process, says Shubin. “This is a vivid example of not only just going back, but going back in a whole new way,” he says. “And it’s a true surprise when you see it in such a dramatic fashion.”

Journal reference: Nature, DOI: 10.1038/s41586-022-04990-w