Fingers and toes are characteristics of tetrapods. But were digits an adaptation to life on land, or did they appear long before the fateful transition was made? The old-fashioned view is that digits would have evolved from the finny feet of the first fish to struggle landwards. This picture was overturned in the late 1980s by Jennifer Clack and Michael Coates at the University of Cambridge. They were working on the intriguing and very primitive fossil amphibian Acanthostega, from 360-m.y.-old Devonian rocks in Greenland. Clack and Coates's discoveries of fish-like internal gills and flat, paddle-like limbs clearly incapable of supporting the animal's weight out of water, showed that Acanthostega lived and died in the water. Yet it had distinct digits, 8 per limb. Although no fish has digits, it could be that digits evolved as a way of making fins better for getting around the shallow, weed-choked pools in which Acanthostega lived. Eventually, the water got so shallow that it gave out completely, and these fingered fish became amphibians.
Support for this view comes from research in the 8 January 1998 Nature from Neil Shubin of the University of Pennsylvania and Ted Daeschler of the Academy of Natural Sciences, Philadelphia. For some years, Shubin and Daeschler have been working on Devonian rocks exposed in a road cutting in Northern Pennsylvania. Early finds included remains of a primitive amphibian, Hynerpeton. Their latest discovery is the pectoral fin of an extinct fish called a rhizodont: a fin that appears to have structures very like digits.
Tetrapods and rhizodonts both belong to the sarcopterygians, or lobe-finned fishes. Apart from tetrapods, the only other members of the group alive today are the lungfishes and the coelacanth. Many other forms existed, but have become extinct. Alive or dead, all lobe-finned fishes are distinguished by having fins supported by distinct bony axes. All lobe-finned fishes have bones corresponding to upper arm and leg bones, but only in tetrapods have these axes been extended to produce hands and feet. The limbs of all other lobe-fins terminate with fishy fin-rays.
Rhizodonts are one of the several extinct groups of lobe-finned fishes. Like the earliest tetrapods, they inhabited fairly shallow, weedy freshwater or brackish environments. Some of them became very large (several metres long) and must have been formidable predators.
The shovel-like rhizodont fin discovered by Shubin and Daeschler is about 20 cm across. It has a series of highly-developed bones in the 'hand' portion of the bone, between the powerful axial radius and ulna, and a fringe of radials. These intermediate bones are digit-like, though they are closely pressed together, and they could not have been spread or separated like tetrapod digits.
It is clear that the immediate ancestors of tetrapods did not have digits of any kind, and that rhizodonts are rather distantly related to tetrapods themselves. This means that the digit-like structures in rhizodonts evolved completely independently from those in tetrapods, presumably as convergent solutions to similar problems, possibly related to swimming in shallow, weed-strewn water. And yet rhizodonts are undoubtedly fishes. The discovery is strong evidence in favour of the argument that tetrapods evolved digits long before they left the water. It also suggests that, had things taken a slightly different turn, it could have been rhizodonts that made it on to land, leaving tetrapods in the water.
©Macmillan Magazines Ltd 1998 - NATURE NEWS SERVICE
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