Evolution News - Tiktaalik Just Keeps on Transitioning!

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.

New clues emerge in how early tetrapods learned to live—and eat—on land

Tiktaalik is one of those embarrassing fossils that drive Creationists bonkers because they annoyingly keep violating so many dogmas that it makes it difficult to fool even the scientifically illiterate simpletons who normally comprise the marks of Creationist frauds.

For one thing, it was discovered exactly where biologists, using the TOE, predicted it would be found in rocks of the right age and type for a species transitional between a fish and a terrestrial tetrapod.

Then back in January, 2014, a team from Chicago University, which included Dr Neil Shubin, who was one of the biologists who predicted where the fossil they named Tiktaalik would be found, showed how the hind limbs of terrestrial tetrapods had evolved out of the hind fins of a fish like Tiktaalik.

And now essentially the same team have shown how Tiktaalik roseae's skull shows the transition between feeding in water, where suction is important and feeding on land where biting is the main method of catching prey.

Animation of CT-based cranial elements, showing relative motions of the different regions of the skull of Tiktaalik roseae during cranial kinesis.

Credit: Justin Lemberg (University of Chicago)

The problem which these transitional species faced was that, whereas water is a perfect medium for sucking prey into your mouth, this doesn't work on land, so prey must be caught, held and taken into the mouth - an altogether different mode of feeding. This presented the emerging tetrapods with a potentially bigger problem than transitioning from fins to limbs - a process that could well have begun in water.

The Chicago University press release explains:

T. roseae, a creature whose flat skull is reminiscent of an alligator, is a species that lived "right at the cusp of the transition from life in water to life on land," said senior author Neil Shubin, PhD, the Robert R. Bensley Distinguished Service Professor of Organismal Biology and Anatomy at UChicago. Studying its fossilized remains can provide new insights into how key traits for life on land originally evolved.

"Water is different from air, being much denser and more viscous," said first author Justin Lemberg, PhD, a postdoctoral researcher at UChicago. "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?"

Most aquatic vertebrate species use suction feeding to help pull prey into their mouths. To create suction, many species of fish can expand their skulls laterally to expand their mouths and produce negative pressure. This movement of the skull bones relative to one another is called cranial kinesis.

"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."

The research team used advanced new computed tomography (CT) analysis to conduct a detailed examination of the morphology of the T. roseae skull. This allowed them to identify key new traits that had not been seen with other techniques, including sliding joints that would have allowed for the necessary cranial kinesis for the animal to expand its skull laterally to create suction.

"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."

CT scan animation of how an alligator gar catches its prey.

Credit: Justin B. Lemberg et al.

Investigators noted distinct similarities between T. roseae and earlier work analyzing the skulls of alligator gar, a "living fossil" species previously thought to only use lateral snapping motions to capture prey. In a 2019 study, Lemberg et. al. found that gar use lateral snapping and suction synergistically while feeding, thanks to unique sliding joints in their skulls that help create suction while biting.

These similarities led the researchers to believe that T. roseae may have fed in the same way, indicating that this adaptation likely evolved long ago, before animals ever colonized 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.

Beyond teaching us about the evolution of our distant, fishy ancestors, better understanding of the biology and behavior of creatures like T. roseae can provide new insights into our own anatomy and development.

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

The tragedy is that Creationists who have been fooled into believing magic is the best explanation for the way things are, miss out on the pleasure of discovering what it really was that made us the way we have turned out, and understanding the connectedness of all living things and how we fit into the pattern. The search for simple answers because science is hard and learning takes effort, makes them easy prey to frauds who sell them these simplistic solutions and so deprive them of the pleasure of discovering the truth, while deluding themselves into believing they know it anyway.

The team's findings were published open access yesterday in PNAS:

Significance

The water-to-land transition is a major event in vertebrate history, involving significant changes to feeding structures and mechanics. In water, fish often use suction-feeding to capture prey, but this feeding strategy is not possible on land. Therefore, it has been traditionally believed that the invasion of land involved a shift from suction-based prey capture to mechanisms based on biting and snapping. Computed tomography analysis of Tiktaalik roseae, a key intermediate in tetrapod evolution, compared with extant analogs (gars and polypterids), reveals a rigid skull, capable of biting, with joint morphologies suggestive of cranial kinesis and suction generation. An intermediate condition that utilizes both feeding strategies helps explain some of the key morphological changes in cranial anatomy during the water-to-land transition.

Abstract

Changes to feeding structures are a fundamental component of the vertebrate transition from water to land. Classically, this event has been characterized as a shift from an aquatic, suction-based mode of prey capture involving cranial kinesis to a biting-based feeding system utilizing a rigid skull capable of capturing prey on land. Here we show that a key intermediate, Tiktaalik roseae, was capable of cranial kinesis despite significant restructuring of the skull to facilitate biting and snapping. Lateral sliding joints between the cheek and dermal skull roof, as well as independent mobility between the hyomandibula and palatoquadrate, enable the suspensorium of T. roseae to expand laterally in a manner similar to modern alligator gars and polypterids. This movement can expand the spiracular and opercular cavities during feeding and respiration, which would direct fluid through the feeding apparatus. Detailed analysis of the sutural morphology of T. roseae suggests that the ability to laterally expand the cheek and palate was maintained during the fish-to-tetrapod transition, implying that limited cranial kinesis was plesiomorphic to the earliest limbed vertebrates. Furthermore, recent kinematic studies of feeding in gars demonstrate that prey capture with lateral snapping can synergistically combine both biting and suction, rather than trading off one for the other. A “gar-like” stage in early tetrapod evolution might have been an important intermediate step in the evolution of terrestrial feeding systems by maintaining suction-generation capabilities while simultaneously elaborating a mechanism for biting-based prey capture.

Lemberg, Justin B.; Daeschler, Edward B.; Shubin, Neil H.
The feeding system of Tiktaalik roseae: an intermediate between suction feeding and biting
Proceedings of the National Academy of Sciences, February 16, 2021 118 (7) e2016421118; doi: 10.1073/pnas.2016421118

Copyright: © 2021 The authors. Published by The National Academy of Sciences.
Open access Reprinted under a Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND 4.0).

Tiktaalik continues to refute creationist dogma without really trying, simply by being what it was - a species that was part-way to becoming a terrestrial tetrapod, the foundation of all non-aquatic vertebrates.

As I said in my book, What Makes You So Special?:

From that unpromising beginning were to evolve the fastest running animal, the fastest flying animal, birds that can soar on the wind and sing symphonies, three forms of powered flight, echolocation at least twice, a brain capable of doing calculus without realising it, and an ape that can go to the moon.

But first, it had to learn to walk on land and breathe air.

I could have added, "... and learn to catch its food".

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