About 374 million years ago, life on Earth began to transition out of the world's oceans to walk on land. This gave rise to the tetrapods, or four-limbed vertebrates, that included dinosaurs, land animals and eventually humans. Scientists consider this transition from water to land, and animals acquiring hands and feet, to be one of the most significant events in the history of life on Earth.
But the fossil record about the evolutionary step between marine and land life is sparse. Researchers have focused their efforts on tetrapod-like fish, called elpistostegalians, that lived between 359 and 393 million years ago during the Middle and Late Devonian periods.
Until now, they had never found the complete skeleton of the pectoral fin, also known as the fore-fin. But researchers have discovered one of the most complete elpistostegalian fossils yet: a 5-foot-long fossilized fish in Miguasha, Quebec.
CT scans of the skeleton revealed at least two skeletal digits that resembled fingers, as well as three more potential ones. They also found an arm, elbow, forearm and wrist attached to the finger-like digits.
All of them were still contained within a fin ray, or webbed flipper-like appendage, but the researchers believe it's the missing link between fish fins and vertebrate hands.
The study published Wednesday in the journal Nature.
"Today we announce in the journal Nature our discovery of a complete specimen of a tetrapod-like fish, called Elpistostege, which reveals extraordinary new information about the evolution of the vertebrate hand," said John Long, study author and Strategic Professor in Palaeontology at Flinders University in Australia.
"This is the first time that we have unequivocally discovered fingers locked in a fin with fin-rays in any known fish. The articulating digits in the fin are like the finger bones found in the hands of most animals."
To date, this skeletal arrangement is the most similar to previously found tetrapods. And the fact that it was located in the fore-fin suggests it was more like a hand.
This appendage would have aided fish as they explored shallow water habits during the Late Devonian period.
"The origin of digits relates to developing the capability for the fish to support its weight in shallow water or for short trips out on land," said Richard Cloutier, study co-author and professor at the Universite du Quebec a Rimouski. "The increased number of small bones in the fin allows more planes of flexibility to spread out its weight through the fin."
"The other features the study revealed concerning the structure of the upper arm bone or humerus, which also shows features present that are shared with early amphibians," Cloutier said. "Elpistostege is not necessarily our ancestor, but it is [the] closest we can get to a true 'transitional fossil', an intermediate between fishes and tetrapods."
Previously, researchers have studied these tetrapod-like fish to better understand how creatures adapted to breathing, hearing and eating on land as they emerged from the water.
Elpistostege watsoni, as the fish has been dubbed, would have been the largest predator dominating Quebec's shallow marine and estuary habitat 380 million years ago. Sharp fangs helped it snack on other large fish, whose fossils were found in the same area.
Fragments of fossils belonging to this fish were initially found in Quebec in 1938. At the time, they only found a portion of the skull's roof and assumed it was a tetrapod.
Another piece of the puzzle was found in 1985, revealing it was a lobe-finned fish. Lobe-finned fish differ from others because their fleshy fins connect to the body through a single bone.
And the most complete fossil, which was part of the this study, was originally found in 2010. CT scans of the fossil, followed by detailed analysis of its backbone and fin, occurred after the discovery. The researchers partnered with colleagues from other institutions to continue CT scans, which revealed the digits found in the fin. Their work was finally completed in 2019.
Their years of analysis revealed that this was the most evolutionary fish of its kind.
"This finding pushes back the origin of digits in vertebrates to the fish level, and tells us that the patterning for the vertebrate hand was first developed deep in evolution, just before fishes left the water," Long said.
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2020-03-18 16:29:08Z
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