Devonian Four-Limbed Creature

Devonian Four-Limbed Creature Reveals Insights into How Vertebrate Hand Evolved from Fish Fins

A 1.57-m (5.2-foot) long specimen of Elpistostege watsoni from the Devonian period of Canada has yielded the missing evolutionary link in the fish to tetrapod transition.

An artist’s reconstruction of Elpistostege watsoni fossil found in Miguasha, Canada. Image credit: Katrina Kenny.

The evolution of fishes into tetrapods — four-legged vertebrates of which humans belong — was one of the most significant events in the history of life.

Vertebrates were then able to leave the water and conquer land.

In order to complete this transition — one of the most significant changes was the evolution of hands and feet.

In order to understand the evolution from a fish fin to a tetrapod limb, paleontologists study the fossils of lobe-finned fish and tetrapods from the Middle and Upper Devonian (393-359 million years ago) known as elpistostegalians. These include the well-known Tiktaalik roseae from Arctic Canada, known only from incomplete specimens.

“We announce the discovery of a complete specimen of a tetrapod-like fish, called Elpistostege watsoni, which reveals extraordinary new information about the evolution of the vertebrate hand,” said Flinders University’s Professor John Long, senior author of the study.

Professor Long and colleagues examined the 380-million-year-old specimen using the high-energy computed tomography.

The skeleton of Elpistostege watsoni’s pectoral fin revealed the presence of a humerus (arm), radius and ulna (forearm), rows of carpus (wrist) and phalanges organized in digits (fingers).

“This is the first time that we have unequivocally discovered fingers locked in a fin with fin-rays in any known fish,” Professor Long said.

“The articulating digits in the fin are like the finger bones found in the hands of most animals.”

“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.”

“Over the past decade, fossils informing the fish-to-tetrapod transition have helped to better understand anatomical transformations associated with breathing, hearing, and feeding, as the habitat changed from water to land on Earth,” said lead author Dr. Richard Cloutier, a researcher at the Universite du Quebec a Rimouski.

“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. 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. Elpistostege watsoni is not necessarily our ancestor, but it is closest we can get to a true ‘transitional fossil,’ an intermediate between fishes and tetrapods.”

The study is published in the journal Nature.

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R. Cloutier et al. Elpistostege and the origin of the vertebrate hand. Nature, published online March 18, 2020; doi: 10.1038/s41586-020-2100-8