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 “fishapods” – 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.
These animals lived in water and had fins that contained bones similar to those found in the limbs of today’s four-legged animals. The most iconic fishapod – Tiktaalik roseae – has been nicknamed the “crawling fish” 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’s 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’t quite match any of them, suggesting it came from a distinct species. The radius – which, in humans, runs from the elbow to the thumb side of the wrist – was long and triangular, coming to a sharp point at its far end – where the wrist would be in a tetrapod.
Other bones and tissue would have led from here to the fin’s 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 “shaped like a bean”. The upper arm bone was missing, but the radius’s 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.
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