Fossil fish from Tiktaalik’s time chose life in the water

Tiktaalik is an iconic fossil species of lobe-finned fish discovered by scientists in 2004, on Ellesmere Island in Nunavut, Arctic Canada. What set this extinct species apart was that, in addition to having typical fish characteristics like gills, scales and fins, it also had some features of tetrapods (four-legged vertebrates), like strong bones in the fins that would have enabled it to support its weight on land. Tiktaalik’s combination of features are thought to show the evolutionary transition between swimming fish and their descendants, the four-legged vertebrates.

Some people have jokingly admonished Tiktaalik for dragging itself out of the water and giving rise to four-legged land vertebrates. After all, if Tiktaalik had not done this, humanity would not be around today to endure the ravages of disease, war and modern life. And now it seems that a new fossil fish, found at the same time and in the same area as Tiktaalik, shows evidence of opting to return to living in open water, rather than taking the terrestrial option.

Scientists from the laboratory of Neil Shubin, PhD, who co-discovered Tiktaalik in 2004, now describe a fossil species that closely resembles Tiktaalik but has features that made it more suited to life in the water. The fossil remains include the fish’s partial upper and lower jaws, portions of its neck, and its scales. In addition, a complete pectoral fin is also present and this where the differences from Tiktaalik are obvious.

Qikiqtania wakei had a pectoral fin with a distinct humerus bone that lacked the ridges indicating muscles and joints that would have enabled it to walk on land. Instead, its upper arm was smooth and curved, more suited for a life of paddling underwater.  The uniqueness of the arm bones of Qikiqtania suggest that it returned to paddling the water after its ancestors began to use their appendages for walking. 

“At first we thought it could be a juvenile Tiktaalik, because it was smaller and maybe some of those processes hadn’t developed yet,” Shubin said. “But the humerus is smooth and boomerang shaped, and it doesn’t have the elements that would support it pushing up on land. It’s remarkably different and suggests something new.”

Shubin, who is the Robert R. Bensley Distinguished Service Professor of Organismal Biology and Anatomy at the University of Chicago, found the fossil days before Tiktaalik was discovered, at a site about one mile east on southern Ellesmere Island in the territory of Nunavut in northern Arctic Canada. The name Qikiatania comes from the Inuktitut word Qikiqtaaluk or Qikiqtani, the traditional name for the region where the fossil site is located. The species name wakei is in memory of the late David Wake, an eminent evolutionary biologist from the University of California at Berkeley.

Shubin and his field partner, Ted Daeschler from the Academy of Natural Sciences of Drexel University, collected the specimens from a quarry after spotting a few promising looking rocks with distinctive, white scales on the surface. But they sat in storage, mostly unexamined, while the team focused on preparing Tiktaalik.

It was fifteen years before postdoctoral researchers Justin Lemberg and Tom Stewart put one of the larger blocks of rock through a CT scanner and realized it contained what they thought was a fossilized pectoral fin. Unfortunately, it was too deep inside the rock to get a high-resolution image and they would have to remove parts of the block to get closer to the fin. And then came COVID, and laboratories were forced to close down. 

“We were trying to collect as much CT-data of the material as we could before the lockdown, and the very last piece we scanned was a large, unassuming block with only a few flecks of scales visible from the surface,” said Lemberg, who is now doing cultural resource management fieldwork in Southern California. “We could hardly believe it when the first, grainy images of a pectoral fin came into view. We knew we could collect a better scan of the block if we had the time, but that was March 13th, 2020, and the University shut down all non-essential operations the following week.”

The scientists were able trim pieces off the block and send it for another CT scan within a few months, and this time the resulting images were much clearer, revealing a nearly complete pectoral fin and upper limb, including the distinctive humerus bone. Their exciting results are published in the journal Nature.  

“That’s what blew our minds,” Shubin said. “This was by no means a fascinating block at first, but we realized during the COVID lockdown when we couldn’t get in the lab that the original scan wasn’t good enough and we needed to trim the block. And when we did, look at what happened. It gave us something exciting to work on during the pandemic. It’s a fabulous story.”

Qikiqtania is slightly older than Tiktaalik but not by much. The team’s analysis of its position on the tree of life places it, like Tiktaalik, adjacent to the earliest creatures known to have finger-like digits. But even though Qikiqtania’s distinct pectoral fin was more suited for swimming, it wasn’t entirely fish-like either. Its curved paddle shape was a distinct adaptation, different from the jointed, muscled legs or fan-shaped fins we see in tetrapods and fish respectively today.

Unfortunately for Qikiqtania, its own adaptation ultimately did not help it to survive in the evolutionary race – we do not see evidence of this type of pectoral fin adaptation in today’s fish. However, the find does help us to understand that there was a diversity of different forms around in ancient times and that most of them ended up becoming extinct. Animal evolution can’t be explained in terms of a straight line that connects prehistoric forms to living creatures today. The story of Qikiqtania shows that some animals stayed on a different path that ultimately didn’t work out. 

“Tiktaalik is often treated as a transitional animal because it’s easy to see the stepwise pattern of changes from life in the water to life on land. But we know that in evolution things aren’t always so simple,” said Stewart, who will be joining the faculty at Penn State University this summer. “We don’t often get glimpses into this part of vertebrate history. Now we’re starting to uncover that diversity and to get a sense of the ecology and unique adaptations of these animals. It’s more than simple 

—

By Katherine Bucko, Earth.com Staff Writer