A complete Elpistostege watsoni fish fossil found in Miguasha, Canada, has shed new light on how vertebrate hands could have evolved from fish fins.
E. watsoni was the largest predator living in a shallow marine to the estuarine habitat of Quebec during the Late Devonian period about 380 million years ago.
An international team of palaeontologists report in the journal Nature that the fossil could provide the missing evolutionary link in the transition from marine to land animals.
The 1.57-metre-long specimen shows the full arm (pectoral fin) skeleton for the first time in any elpistostegalian fish.
With high energy CT-scans, the skeleton showed a humerus (arm), radius and ulna (forearm), rows of carpus (wrist) and phalanges organised 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. The articulating digits in the fin are like the finger bones found in the hands of most animals,” says senior author John Long from Flinders University, Australia.
“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.”
The evolution of fishes into tetrapods – four-legged vertebrates including humans – was one of the most significant events in the history of life.
Vertebrates (back-boned animals) were then able to leave the water and conquer land. To complete this transition, one of the most significant changes was the evolution of hands and feet.
Co-author Richard Cloutier from Université du Québec à Rimouski, Canada, says over the past decade, fossils informing the fish-to-tetrapod transition have helped further understanding of anatomical transformations associated with breathing, hearing, and feeding, as Earth’s habitat changed from water to land.
“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 fossil also showed the structure of the upper arm bone or humerus, features that are shared with early amphibians.
“Elpistostege is not necessarily our ancestor,” says Cloutier, “but it is closest we can get to a true ‘transitional fossil’, an intermediate between fishes and tetrapods.”