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| Elpistostege watsoni |
The fossil record has receipts.
Meet Elpistostege watsoni, a Late Devonian fish from what is now Québec whose fins contained an extraordinary arrangement of bones: the beginnings of a hand, still wearing its swimming costume.
Roughly 380 million years ago, this formidable predator inhabited shallow coastal and estuarine waters. At 1.57 metres long, the remarkably complete specimen studied by palaeontologists was considerably more fish than we would want investigating our ankles. Its sharp teeth suggest that smaller fishes had good reason to keep their distance.
Today, its remains help us explore one of the great transformations in vertebrate history: how fins became limbs.
The crucial evidence comes from Miguasha, on the Gaspé Peninsula, within the traditional territory of the Mi’gmaq. Here, the rocks of the Escuminac Formation preserve an exceptional window into the Devonian world.
From the outside, Elpistostege was unmistakably fishy. Inside its pectoral fins, however, evolution had been getting interesting.
Using high-energy computed tomography, researchers examined the skeleton concealed within the fossil. They found the familiar foundations of our own forelimbs: a humerus, followed by a radius and ulna, then increasingly intricate arrangements of smaller bones. Farther out were rows interpreted as digits and possible digits.
Finger bones. Inside a fin.
The study, published in Nature in 2020 by Richard Cloutier and colleagues, provided compelling evidence that the skeletal beginnings of fingers evolved before the fin rays disappeared. A hand-like arrangement was already taking shape while the appendage remained a fin.
This was no tiny human hand tucked into a fish sleeve. The bones formed part of a working aquatic appendage, with a structure different from our own. Nevertheless, the evolutionary connection is wonderfully tangible.
We can follow the family resemblance right down our arms.
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| Tiktaalik roseae |
Discovered on Ellesmere Island in the Canadian Arctic in 2004, Tiktaalik combined scales and fin rays with a broad, flattened skull, a mobile neck and robust internal fin bones. Its shoulder, elbow and wrist-like joints could help support its body against a surface.
A fish capable of propping itself up has a certain attitude. We can almost picture it resting on a fin, surveying the shallows as though disappointed with the service.
Yet neither animal was rehearsing for an inevitable future on land.
Evolution has no itinerary. Structures that later proved useful for terrestrial life first evolved in animals living in water. Supporting the body, manoeuvring through shallows and interacting with the bottom offered immediate practical advantages long before anyone needed to climb a tree or open a stubborn jar.
Nor should we arrange these fossils into a tidy procession, with Tiktaalik handing the evolutionary baton to Elpistostege, followed by something obligingly growing feet. They represent branches close to the origin of limbed vertebrates. Their anatomy reveals stages in the assembly of the tetrapod body plan; it does not establish either species as our direct ancestor. Studies of these fins show how limbs could emerge through the modification of structures already present in fishes.
That is what makes these fossils so satisfying. The transition becomes a sequence of anatomical changes we can investigate, bone by bone. Flipping through the literature, we have become remarkably accomplished with our fish fingers.
References & further reading
- Cloutier, R. et al. (2020). Elpistostege and the origin of the vertebrate hand. Nature 579, 549–554.
- Shubin, N. H., Daeschler, E. B. & Jenkins, F. A. Jr. (2006). The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb. Nature 440, 764–771.
- Flinders University (2020). Ancient fossil reveals origin of human hand.
