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New Clues Emerge in How Early Tetrapods Learned to Live — and Eat — on Land

New evidence from scientists at the University of Chicago shows exactly how the tiktaalik roseae, one of the first semi-aquatic organisms, fed itself using a similar feeding mechanism to modern predatory fish, filling in another blank about early water-to-land evolution.

New evidence from scientists at the University of Chicago shows exactly how the tiktaalik roseae, one of the first semi-aquatic organisms, fed itself using a similar feeding mechanism to modern predatory fish, filling in another blank about early water-to-land evolution.

Side-by-side comparison of Tiktaalik (top) and alligator gar (bottom) showing similarly shaped snouts that may suggest convergence in feeding strategies. (Credit: Justin Lemberg, University of Chicago)

(CN) — A study released Monday has yielded fascinating new knowledge about the tiktaalik roseae, one of the first vertebrates to develop four limbs and move onto land, including how it fed and ultimately thrived on land. 

In the study published in the journal Proceedings of the National Academy of Sciences, the researchers explore how their findings change what the scientific community knows about this ancient animal.

The extinct tiktaalik was a strange looking, semi-aquatic creature that answered several important evolutionary questions. It is known as a transition form, a term given to fossils mid-adaptation that carry traits both from the organism before them and their subsequent evolutionary descendants. 

Lead author Neil Shubin, professor of organismal biology and anatomy at the University of Chicago said this is likely because it emerged "right at the cusp of the transition from life in water to life on land." 

Having lived mainly in shallow waters, the tiktaalik was used to standing to catch its prey and was well suited to be semi-aquatic.

It had a distinct alligator-like head and scales and gills from the aquatic world it came from. The animal’s fins were mid-adaptation to becoming legs, as they had thin fish-like bones that would have allowed them to move through the water, but not so much that they could swim like other amphibians.

However, it also had thicker bones around its neck, wrist, shoulders and ribs similar to most four-legged land dwellers. Because it had fewer bones in its head and stronger muscles in its neck, the tiktaalik would have been able to lie in wait in shallow waters and snatch up prey with its mouth.

The reason the tiktaalik crawled onto land remains a debate, though it may have been a matter of survival. It was a fairly large predator, but there were much larger fish in the deeper waters which would have encouraged it to live in the shallows. Also, there were smaller fish and other prey along the water’s edge providing plenty of food resources.

The authors were curious as to how the tiktaaliks fared once they made it onto land. Would their feeding habits and mechanisms stay the same or did they have to find a new way to eat their dinners?

"Water is different from air, being much denser and more viscous," said first author Justin Lemberg, a postdoctoral researcher at the university. "This would have created unique problems for animals that were moving out of water and onto land for the very first time, including challenges in locomotion, reproduction, maintaining homeostasis and sensory processing and, of course, feeding. If you can't feed yourself on land, how can you colonize it?"

The question soon became: Did the tiktaalik consume prey via suction or biting? Most aquatic animals used the former to capture food using a technique called cranial kinesis, where they expand their skulls and mouths to pull food in. However, this would have been quite impossible to catch prey on land, suggesting the tiktaalik must have found a better way.

"Suction feeding is ineffective on land, because it no longer works from a distance and it's hard to create the pressure seal needed to draw something in," said Lemberg. "So terrestrial vertebrates had to turn to other methods to capture prey. But the fossil evidence for how this happened is ambiguous, much more so than the transition from fin to limb. We wanted to look specifically at the sutures in the T. roseae skull, where the bones fit together, to see if they could tell us how the skull was being used."

To investigate this, the team performed a CT scan on the skull of a tiktaalik fossil and found some telling new features. 

"We discovered tiktaalik in 2004 and at the time, prepared it with the classical methods, removing rock from the fossil grain by grain," said Shubin. "By the time Justin joined the project, we had access to this CT scanning technology, which lets us see the skull in 3D, taking each part out individually to see its shape and motion. Using the CT analysis transformed how we were able to think about the skull."

The scan showed that the tiktaalik did in fact possess the sliding joints needed for suction feeding, and it also contained similar features to that of the modern alligator gar, a fearsome predatory freshwater fish.

In a separate 2019 study, researchers found the gar uses both a suction technique as well as a biting motion to catch its prey using similar sliding joints found in the tiktaalik skull. The authors believe this finding indicates that the ancient animal must have developed this trait long before it reached land.

"The thing that really stuns me is that every innovation, every invention used by tetrapods on land, originally appeared in some form in fish, including lungs, appendages, and now, feeding," said Shubin.

"The neat thing about the water-to-land transition is that it's deeply personal to us," Lemberg said. "How did we get to where we are now, and what are some of the evolutionary quirks we've adapted to get here?"

For example, Lemberg noted how the tiktaalik’s strange mid-transition features could be seen in the hyomandibula, a bone found in the cranium. In fish, this bone is quite large and assists with breathing underwater, but in land-based animals the bone was so small that it has evolved into stapes, a bone found in our inner ears. 

In the tiktaalik, this bone, among others, had been changed over time to land somewhere in between the two extremes, showing once again how it was an important stepping stone in evolution.

"Those three bones in Tiktaalik are what we use to hear sound," Lemberg said. "A little bit of cranial kinesis that's maintained in modern mammals!"

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