Dead fish breathes new life into the evolutionary origin of fins and limbs

A trove of fossils in China, unearthed in rock dating back some 436 million years, have revealed for the first time that the mysterious galeaspids, a jawless freshwater fish, possessed paired fins.

The discovery, by an international team, led by Min Zhu of the Institute of Vertebrate Palaeontology and Palaeoanthropology, Bejiing and Professor Philip Donoghue from the University of Bristol's School of Earth Sciences, shows the primitive condition of paired fins before they separated into pectoral and pelvic fins, the forerunner to arms and legs.

Until now, the only surviving fossils of galeaspids were heads, but these new fossils originating in the rocks of Hunan Province and Chongqing and named Tujiaaspis after the indigenous Tujia people who live in this region, contain their whole bodies.

Theories abound on the evolutionary beginnings of vertebrate fins and limbs -- the evolutionary precursors of arms and legs -- mostly based on comparative embryology. There is a rich fossil record, but early vertebrates either had fins or they didn't. There was little evidence for their gradual evolution.

First author Zhikun Gai, a University of Bristol alumnus, said: "The anatomy of galeaspids has been something of a mystery since they were first discovered more than half a century ago. Tens of thousands of fossils are known from China and Vietnam, but almost all of them are just heads -- nothing has been known about the rest of their bodies -- until now.

"The new fossils are spectacular, preserving the whole body for the first time and revealing that these animals possessed paired fins that extended continuously, all the way from the back of the head to the very tip of the tail. This is a great surprise since galeaspids have been thought to lack paired fins altogether."

Corresponding author Professor Donoghue said: "Tujiaaspis breathes new life into a century old hypothesis for the evolution of paired fins, through differentiation of pectoral (arms) and pelvic (legs) fins over evolutionary time from a continuous head-to-tail fin precursor.

"This 'fin-fold' hypothesis has been very popular but it has lacked any supporting evidence until now. The discovery to Tujiaaspis resurrects the fin-fold hypothesis and reconciles it with contemporary data on the genetic controls on the embryonic development of fins in living vertebrates."

Corresponding author Min Zhu of VPP, Beijing, added "Tujiaaspis shows the primitive condition for paired fins first evolved. Later groups, like the jawless osteostracans show the first evidence for the separation of muscular pectoral fins, retaining long pelvic fins that reduced to the short muscular fins in jawed vertebrates, such as in groups like placoderms and sharks. Nevertheless, we can see vestiges of elongate fin-folds in the embryos of living jawed fishes, which can be experimentally manipulated to reproduce them. The key question is why did fins first evolve in this way?"

Bristol's Dr Humberto Ferron used computational engineering approaches to simulate the behaviour of models of Tujiaaspis with and without the paired fins. The co-author said: "The paired fins of Tujiaaspis act as hydrofoils, passively generating lift for the fish without any muscular input from the fins themselves. The lateral fin-folds of Tujiaaspis allowed it to swim more efficiently."

Co-author Dr Joseph Keating at Bristol modelled the evolution of paired fins. He said: "Fossil jawless vertebrates display a dizzying array of fin types, which has provoked extensive debate about the evolution of paired fins.

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Engineers uncover the secrets of fish fins

Peer into any fishbowl, and you'll see that pet goldfish and guppies have nimble fins. With a few flicks of these appendages, aquarium swimmers can turn in circles, dive deep down or even bob to the surface.

New research led by the University of Colorado Boulder has uncovered the engineering secrets behind what makes fish fins so strong yet flexible. The team's insights could one day lead to new designs for robotic surgical tools or even airplane wings that change their shape with the push of a button.

The researchers published their results Aug. 11 in the journal Science Robotics.

Francois Barthelat, senior author of the study, noted that fins are remarkable because they can achieve feats of dexterity even though they don't contain a single muscle. (Fish move these structures by twitching sets of muscles located at the base of the fins).

"If you look at a fin, you'll see that it's made of many stiff 'rays,'" said Barthelat, professor in the Paul M. Rady Department of Mechanical Engineering. "Each of those rays can be manipulated individually just like your fingers, but there are 20 or 30 of them in each fin."

In their latest research, Barthelat and his colleagues drew on a range of approaches, including computer simulations and 3D-printed materials, to dive deep into the biomechanics of these agile structures. They report that the key to fish fins may lie in their unique design. Each ray in a fin is made up of multiple segments of a hard material that stack on top of much softer collagen, making them the perfect balance between bouncy and stiff.

"You get this dual capability where fins can morph, and yet they're still quite stiff when they push water," he said.

Armor and airplanes

Barthelat is no stranger to looking into aquariums. He previously studied how fish scales can help engineers to design better body armor for humans, and how seashells might inspire tougher glasses.

Fins may be just as useful. When it comes to engineering, Barthelat explained, materials that are both stiff and flexible are a hot commodity. Airplane designers, for example, have long been interested in developing wings that can morph on command, giving planes more ability to maneuver while still keeping them in the air.

"Airplanes do this now, to some extent, when they drop their flaps," Barthelat said. "But that's in a rigid way. A wing made out of morphing materials, in contrast, could change its shape more radically and in a continuous manner, much like a bird."

To understand how ordinary run-of-the-mill goldfish achieve similar feats every day, take a close look at these structures under the microscope. Each of the rays in a fin has a layered structure, a bit like a bakery éclair: The spikes include two layers of stiff and mineralized materials called hemitrichs that surround an inner layer of spongy collagen.

But, Barthelat said, those layers of hemitrichs aren't solid. They're divided into segments, as if someone had cut up the éclair into bite-sized pieces.

"Until recently, the function of those segments hadn't been clear," he said.

Swimming, flying and walking

The engineer and his team decided to use computer simulations to examine the mechanical properties of fins. They discovered that those segments can make all the difference.

Pretend for a moment, Barthelat explained, that fish fins are made up entirely of collagen. They could bend easily, but wouldn't give fish much traction in the water because hydrodynamic forces would collapse them. Rays made up of solid, non-segmented hemitrichs, in contrast, would have the opposite problem -- they'd be way too stiff.

"All of the segments, essentially, create these tiny hinges along the ray," Barthelat said. "When you try to compress or pull on those bony layers, they have a very high stiffness. This is critical for the ray to resist and produce hydrodynamic forces that push on water. But if you try to bend individual bony layers, they're very compliant, and that part is critical for the rays to deform easily from the base muscles."

The researchers further tested the theory by using a 3D printer to produce model fish fins made from plastic, some with those hinges built in and some without. The idea panned out: The team found that the segmented design provided better combinations of stiffness and morphing capabilities.

Barthelat added that he and his colleagues have only scratched the surface of the wide diversity of fins in the fish world. Flying fish, for example, deploy their fins to glide above the water, while mudskippers use their fins like legs to walk on land.

"We like to pick up where the biologists and zoologists have left off, using our background in the mechanics of materials to further our understanding of the amazing properties of the natural world," Barthelat said.

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From fins to limbs

 When tetrapods (four-limbed vertebrates) began to move from water to land roughly 390 million years ago it set in motion the rise of lizards, birds, mammals, and all land animals that exist today, including humans and some aquatic vertebrates such as whales and dolphins.

The earliest tetrapods originated from their fish ancestors in the Devonian period and are more than twice as old as the oldest dinosaur fossils. They resembled a cross between a giant salamander and a crocodile and were about 1-2 meters long, had gills, webbed feet and tail fins, and were still heavily tied to water. Their short arms and legs had up to eight digits on each hand and foot and they were probably ambush predators, lurking in shallow water waiting for prey to come near.

Scientists know how the fins of fish transformed into the limbs of tetrapods, but controversies remain about where and how the earliest tetrapods used their limbs. And, while many hypotheses have been proposed, very few studies have rigorously tested them using the fossil record.

In a paper published January 22 in Science Advances an international team of researchers examined three-dimensional digital models of the bones, joints, and muscles of the fins and limbs of two extinct early tetrapods and a closely related fossil fish to reveal how function of the forelimb changed as fins evolved into limbs. The research led by Julia Molnar, Assistant Professor at New York Institute of Technology College of Osteopathic Medicine and Stephanie Pierce, Thomas D. Cabot Associate Professor of Organismic and Evolutionary Biology at Harvard University, discovered three distinct functional stages in the transition from fins to limbs, and that these early tetrapods had a very distinct pattern of muscle leverage that didn't look like a fish fin or modern tetrapod limbs.

To reconstruct how limbs of the earliest known tetrapods functioned, Molnar, Pierce and co-authors John Hutchinson (Royal Veterinary College), Rui Diogo (Howard University), and Jennifer Clack (University of Cambridge) first needed to figure out what muscles were present in the fossil animals. A challenging task as muscles are not preserved in fossils, and the muscles of modern fish fins are completely different from those of tetrapod limbs. The team spent several years trying to answer the question, how exactly did the few simple muscles of a fin become dozens of muscles that perform all sorts of functions in a tetrapod limb?

"Determining what muscles were present in a 360-million-year-old fossil took many years of work just to get to the point where we could begin to build very complicated musculoskeletal models," said Pierce. "We needed to know how many muscles were present in the fossil animals and where they attached to on the bones so we could test how they functioned."

They built three-dimensional musculoskeletal models of the pectoral fin in Eusthenopteron (a fish closely related to tetrapods that lived during the Late Devonian period about 385 million years ago) and the forelimbs of two early tetrapods, Acanthostega (365 million years old living towards the end of the Late Devonian period) and Pederpes (348-347 million years old living during the early Carboniferous period). For comparison, they also built similar models of the pectoral fins of living fishes (coelacanth, lungfish) and forelimbs of living tetrapods (salamander, lizard).

To determine how the fins and limbs worked, the researchers used computational software originally developed to study human locomotion. This technique had been used recently to study locomotion in the ancestors of humans and also dinosaurs like T. rex, but never in something as old as an early tetrapod.

Manipulating the models in the software, the team were able to measure two functional traits: the joint's maximum range of motion and the muscles' ability to move the fin or limb joints. The two measurements would reveal trade-offs in the locomotor system and allow the researchers to test hypotheses of function in extinct animals.

The team found the forelimbs of all terrestrial tetrapods passed through three distinct functional stages: a "benthic fish" stage that resembled modern lungfish, an "early tetrapod" stage unlike any extinct animal, and a "crown tetrapod" stage with characteristics of both lizards and salamanders.

"The fin from Eusthenopteron had a pattern that was reminiscent of the lungfish, which is one of the closest living relatives of tetrapods," said Pierce. "But the early tetrapod limbs showed more similarities to each other than either fish or modern tetrapods."

"That was perhaps the most surprising," said Molnar. "I thought Pederpes, and possibly Acanthostega, would fall pretty well within the range of modern tetrapods. But they formed their own distinct cluster that didn't look like a modern tetrapod limb or a fish fin. They were not smack dab in the middle but had their own collection of characteristics that probably reflected their unique environment and behaviors."

The results showed that early tetrapod limbs were more adapted for propulsion rather than weight bearing. In the water, animals use their limbs for propulsion to move themselves forward or backward allowing the water to support their body weight. Moving on land, however, requires the animal act against gravity and push downward with their limbs to support their body mass.

This doesn't mean that early tetrapods were incapable of moving on land, but rather they didn't move like a modern-day living tetrapod. Their means of locomotion was probably unique to these animals that were still very much tied to the water, but were also venturing onto land, where there were many opportunities for vertebrate animals but little competition or fear from predators.

"These results are exciting as they independently support a study I published last year using completely different fossils and methods," said Pierce. "That study, which focused on the upper arm bone, indicated that early tetrapods had some capacity for land movement but that they may not have been very good at it."

The researchers are closer to reconstructing the evolution of terrestrial locomotion, but more work is needed. They plan to next model the hind limb to investigate how all four limbs worked together. It has been suggested that early tetrapods were using their forelimbs for propulsion, but modern tetrapods get most of their propulsive power from the hind limb.

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