Science1 publisher3 min readPublished
CT scans of a single Ellesmere Island fin show an elbow that bent in one plane only
The 350-million-year-old radius is triangular and pointed where the wrist would be, and the upper arm bone is missing. It matches none of the 16 fishapods that Thomas Stewart's team at Penn State compared it against.
The Scientist · Science desk
What happened
- The animal lived 350 million years ago, just before the first four-legged animals walked on land, and its fin carried an elbow joint that worked like a hinge.
- Its entire fossil record is one isolated fin, found on Canada's Ellesmere Island, with no other bones from the same animal recovered.
- Researchers CT-scanned the fin and compared its arm-like bones with 16 other known fishapods, and the fit was poor enough that they read it as a distinct species.
- The radius is long and triangular and tapers to a sharp point where a tetrapod's wrist would sit, a shape Stewart says would have given little support for pushing against the ground.
- The fin also preserves the intermedium, a small bone suspected of becoming part of the tetrapod wrist, and here it is relatively longer than in Tiktaalik and its close relatives.
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Why it matters
- capability One CT-scanned element measured against a reference set of 16 species was enough to call a new form, so unassociated fins can now carry taxonomic weight without a skull or a skeleton to back them.
- contradiction Stewart describes the restricted elbow as a kind of experiment in appendages, while Swartz warns against reading any of these forms as attempts at limbs for land. Both readings fit the same bone, and a single fin cannot settle which is right.
- precedent With dozens of fishapod species described in twenty years and this one added from a lone fin, the working expectation becomes that new finds widen the range of elbow geometries; they do not fill in a sequence toward land.
A hinge joint involves two bones, and this fossil preserves one of them [12]. The elbow end of the radius is narrow and, in Stewart's description, "shaped like a bean", without the rounded knob that other fishapods carry [11]. From that contour the team infers a joint that bent back and forth largely in a single plane [12]. Other fishapods could twist and rotate the elbow much as people can [13].
The comparison set is 16 previously known species plus this one, so 17 fins in total [22]. For the new animal the count is one [8]. That single fin cannot show how much a radius varied within a species, and the species call rests on how poorly this bone matches the others [9]. New Scientist's report does not name the journal where the analysis appeared, and the animal has no species name in it [23].
Stewart said the story of how fish evolved into land animals is anything but a straight line [21]. Qikiqtania wakei, which he and colleagues described in 2022, appears to have first evolved weight-supporting fins and then become a full-time swimmer [7]. Fossils of dozens of distinct finned tetrapodomorph species have been found in the past two decades [5].
"We keep uncovering more and more variation in the shapes and geometries of these bones," Stewart said [3]. What this animal did with its fins is not clear, he said, and the restricted elbow may have been an experiment that served its needs without being well suited to the job [15]. "When animals begin to explore new kinds of movements, they might be kind of awkward," he said. "They might not be the best at it, but they're still good enough. It's a matter of sufficiency, not optimality." [16]
In my view the fin supports a claim about variation. It does not support one about function. A radius that would have given little purchase against the ground and an elbow with a single axis of motion are consistent with several possible uses [10][12], and the alternative on offer is a hypothesis about habitat, not a measurement.
Brian Swartz at the University of California, Berkeley said the variation should not be read as separate attempts to evolve limbs for a future life on land, but as fits to the environments the animals were living in [19]. Julia Molnar at the New York Institute of Technology said the question of when and why the fin-limb and water-land transitions occurred "is still very much up for debate" [17]. She said a fish like this one might have used its fins to walk in shallow water or push through vegetation, letting it escape large aquatic predators and prey on smaller fish and arthropods at the water's edge [20].
What to watch
- A formal description giving the Ellesmere fin a species name and a position in the fishapod family tree.
- A second specimen, above all a humerus, which would test the single-plane elbow now inferred from radius shape alone.
- Comparative work testing whether radius shape across described fishapods tracks habitat or ancestry.