These fins were made for walking… and then swimming

One of the biggest myths about evolution is that it is an inexorable march of progress. In fact, evolution is not a linear track but a branching tree. New species do not emerge as part of a long-term goal; They adapt to new opportunities in their environment.

On Wednesday, paleontologists unveiled a fossil that has proven to be an effective antidote to the myth of progress. It was a fish that lived about 375 million years ago, when our ancestors were scaly creatures vaguely resembling giant eels, walking across the mudflats with four limbs, complete with elbows, knees, wrists, and ankles. The newly discovered fossil called Qikiqtania wakei belonged to this lineage.

But its anatomy suggests that its ancestors, unlike ours, did not continue moving on land. Instead, they gave up walking to swim again.

“We think of evolution in directed terms,” ​​said Neil Shubin, a paleobiologist at the University of Chicago. “That is not the case here. Some species land and some actually return to the water.”

In 2004, Dr. Shubin and his colleagues made a headline-grabbing discovery while searching for fossils in Nunavut, an arctic territory of Canada. They discovered a large 375-million-year-old fish closely related to land vertebrates. Its most striking resemblance was its four leg-like flippers.

The creature’s two front flippers had bones corresponding to our humerus, radius, ulna, and wrist bones. The combination allowed the fish, which they named Tiktaalik, to walk on tidal flats and at the bottom of swamps.

The importance of tiktaalik became apparent when scientists placed it on an evolutionary tree along with terrestrial vertebrates — known as tetrapods — and other tetrapod-like fish. Using these branches, the scientists were able to see step by step how the tetrapod body developed. Fish first developed the long bones in their legs and later added wrists and ankles. Fingers and toes emerged even later.

Now have dr. Shubin and his colleagues added another branch to our evolution tree with a fossil they unknowingly discovered in Nunavut even before they found Tiktaalik.

The team first traveled to Nunavut in 1998, drawn by rocks that looked like they might contain fossils dating back to the earliest tetrapods. But field season after field season ended in disappointment.

When the researchers returned in 2004, they found something promising on a small hill next to their tents. “One day I was having lunch at this spot and I looked down and saw a field of white scales on dark rock,” said Dr. Shubin.

The scales had a distinctive diamond-like pattern found only in fish closely related to tetrapods. Near the dark rock, Dr. Shubin a fish jaw fossil. And next to it was a rock the size of a Frisbee with bone-like spots on its surface.

dr Shubin packed everything in a bag to take back to his lab, but four days later, researchers discovered the first fossils of Tiktaalik elsewhere, a mile from camp. They immediately recognized it as revolutionary, and when they returned to Chicago, Dr. Shubin’s lunchtime find forgotten.

“That was essentially in a drawer because we were focused like a laser beam on Tiktaalik,” said Dr. Shubin.

In subsequent field seasons, researchers found at least 10 specimens of Tiktaalik, some young and some adults. They were able to chart the animal’s growth over its lifetime to a nine-foot-long beast.

The fossils allowed scientists to reconstruct the walking style of Tiktaalik, a four-wheel drive fish version. They discovered that the animals hunted fish by biting with their long fangs and sucking them down their throats.

In 2019, researchers turned their attention back to Frisbee Rock. The University of Chicago had purchased a CT scanner specifically designed to create high-resolution images of fossils, even when they’re still in rock. After scanning the jaw and scales, Thomas Stewart, a postdoctoral fellow in Dr. Shubin’s lab, finally getting around to scanning the stone.

To his amazement, it contained a fairly complete fin. Although similar to Tiktaalik’s fin, it had some key differences that characterized a second species of tetrapod-like fish in Nunavut.

“You could have knocked me over with a feather,” said Dr. Shubin.

In normal times, researchers would have frantically gathered in their lab to understand their discovery. But dr Stewart discovered the hidden fin on March 13, 2020. Within days, the scientists were locked out of their lab as the pandemic shut down the university.

Only in June 2020 could they get back in, and then only individually. They managed to cut away part of the rock so they could better scan the bones inside. The researchers then brooded over the images for months.

“This became our pandemic lockdown project,” said Dr. Shubin. “It kept us sane when the world wasn’t.”

The scientists dubbed the fossil Qikiqtania (pronounced kick-kick-TAN-ee-ya) after the Inuktitut names for the region where it was found, Qikiqtaaluk and Qikiqtani. The second part of his name, wakei, honors David Wake, an evolutionary biologist at the University of California, Berkeley, who was a mentor to Dr. Shubin was and died last year.

Careful comparison of its anatomy confirmed that Qikiqtania was closely related to tetrapods and is possibly Tiktaalik’s closest known relative. But after Qikiqtania branched off from Tiktaalik, its development took a strikingly different path. For one thing, it got a lot smaller, probably only being about 30 inches long.

An even more dramatic change occurred with Qikiqtania’s fins.

In Tiktaalik and other tetrapod-like fish, the humerus had knobs and ridges to which powerful walking muscles were anchored. But Qikiqtania had a smooth humerus that offered little support for the muscles.

The researchers found another striking difference in the elbow. Tiktaalik leaned on his elbows while walking, bending his limbs at a 90-degree angle into a push-up position. Qikiqtania’s elbow was locked, fin stretched out in a straight line.

“It’s not a flexible limb — it’s like a paddle,” said Dr. Shubin.

Qikiqtania also had a larger ray fan at the end of its fin, which may have helped it swim, said Dr. Shubin. It would not have provided any assistance with walking.

dr Shubin theorized that Qikiqtania abandoned the running habit its ancestors had recently developed and instead chose to swim in open water, somewhat like a modern paddlefish.

To understand Qikiqtania’s remarkable evolutionary shift, Dr. Shubin pointed to tetrapods returning to the water millions of years later. For example, about 50 million years ago, land mammals adapted to aquatic animals, which eventually became whales and dolphins. The discovery of Qikiqtania indicated that some of our ancient relatives had almost given up walking by the time walking evolved.

But Qikiqtania didn’t return to the water by simply returning to the bodies of its swimming ancestors. It probably took advantage of the new bite-and-suck attack that tetrapod-like fish have evolved. “Not only are they returning to the water, but they’re doing so in new ways,” said Dr. Shubin.

“It’s great to see how we’re filling in the tree of life here,” said Stephanie Pierce, a paleobiologist at Harvard University who wasn’t involved with the new study. “Any new fossil that can help us understand what happens in the early stages of evolution of the tetrapod body plan is incredibly important because we have so few fossils documenting this period.”

Nonetheless, said Dr. Pierce that more fossils from Qikiqtania were needed to help Dr. to test Shubin’s hypothesis. For example, it was unclear to her whether the tetrapod’s fin was sticking out as a rigid paddle.

“It’s a great specimen and it raises a lot of questions that I’d like to explore,” she said.

dr Shubin and his colleagues are taking a new look at some of their Tiktaalik fossils to see if they actually came from Qikiqtania. They also wonder if a mysterious tetrapod-like fossil discovered in Scotland in the 1990s could also belong to Qikiqtania’s lineage.

Next year, Dr. Shubin and his colleagues are conducting an expedition back to Nunavut for the first time in nine years. You intend to go to Dr. Returning to Shubin’s midday place in 2014 and digging for more fossils. It’s possible they’ll find more tetrapod-like fish that have evolved their own weird adaptations.

“I feel like a kid in a candy store waiting to come back in,” said Dr. Shubin.

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