麻豆传媒

An ancient fish that lived just before the first four-legged animals walked on land had a pointed forearm bone and a hinge-like elbow joint, adding to the growing evidence of trial and error in early arm formation.

The 350-million-year-old animal was one of several 鈥渇ishapods鈥� 鈥� creatures with fins that had evolved some limb-like features. It might have used its fins for actions other than swimming, such as moving along the bottom of rivers.

鈥淲e keep uncovering more and more variation in the shapes and geometries of these bones,鈥� says at Pennsylvania State University. 鈥淭he more we study the diversity, the more we realise there鈥檚 just a range of unusual forms of animals living in and exploring their ecologies.鈥�

In the past two decades, scientists have found fossils from dozens of distinct species of finned tetrapodomorphs, or fishapods. These animals lived in water and had fins that contained bones similar to those found in the limbs of today鈥檚 four-legged animals. The most iconic fishapod 鈥� Tiktaalik roseae 鈥� has been nicknamed the 鈥渃rawling fish鈥� because it could probably use its fin-limbs to crawl out of water, capturing an important step in the evolution of vertebrates.

But the story of how fish evolved into land animals is anything but a straight line, Stewart says. For instance, Qikiqtania wakei, which he and his colleagues described in 2022, was a fishapod that appears to have initially evolved weight-supporting fins but then evolved to be a full-time swimmer.

Now, the team has analysed another unusual fishapod. So far, the animal is known only from fossils of a single isolated fin that was discovered on Canada鈥檚 Ellesmere Island. The researchers CT-scanned the fossil and compared its arm-like bones with those of 16 other known fishapods. The isolated fin didn鈥檛 quite match any of them, suggesting it came from a distinct species.

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The radius 鈥� which, in humans, runs from the elbow to the thumb side of the wrist 鈥� was long and triangular, coming to a sharp point at its far end 鈥� where the wrist would be in a tetrapod. Other bones and tissue would have led from here to the fin鈥檚 tip, but the shape of the radius itself would have provided little support for pushing against the ground, says Stewart.

At the elbow end, the radius lacked the rounded knob seen in other fishapods, but instead was narrow and 鈥渟haped like a bean鈥�. The upper arm bone was missing, but the radius鈥檚 shape suggests the joint between them would have worked like a simple hinge that largely bent back and forth in a single plane. This is more restrictive than the elbows of other fishapods: they could twist and rotate their elbows, like we can, to provide greater flexibility of movement.

The fossil also contained the intermedium, a small bone seen in fishapods that scientists suspect eventually evolved into part of the wrist. In the fossil from the unknown fishapod, the intermedium was relatively longer than in Tiktaalik and its close relatives.

Exactly what the animal was doing with its unusual fins isn鈥檛 clear, says Stewart. The restricted movement of the elbow might have been a kind of experiment in evolving appendages that could have served this particular animal鈥檚 needs, in its environment. But this doesn鈥檛 necessarily mean the elbow joint was perfectly suited for its role.

鈥淲hen animals begin to explore new kinds of movements, they might be kind of awkward,鈥� he says. 鈥淭hey might not be the best at it, but they鈥檙e still good enough. It鈥檚 a matter of sufficiency, not optimality.鈥�

at New York Institute of Technology, Long Island, says she was excited to see the new specimen. 鈥淭he question of when and why the fin-limb and water-land transitions occurred is still very much up for debate,鈥� she says. 鈥淚 would not be surprised if this wide variation in radius shape represents different approaches to the problems and opportunities of this changing world.鈥�

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Importantly, the variations shouldn鈥檛 be interpreted as different attempts to evolve limbs for a future life on land, but rather to suit their own current environment, says at the University of California, Berkeley.

鈥淎 fish like this one might use its fins to 鈥榳alk鈥� in shallow water or push through vegetation,鈥� says Molnar, 鈥渁llowing it to escape large aquatic predators and prey on smaller fish and arthropods at the water鈥檚 edge.鈥�