When fins became limbs
The origin of digits in land vertebrates is hotly debated.
The tiny bones in the tip of the pectoral fins of fishes such as Elpistostege are called “radial” bones. When radials form a series of rows, like digits, they are essentially the same as fingers in tetrapods.
The only difference is that, in these advanced fishes, the digits are still locked within the fin, and not yet free moving like human fingers.
Our recently uncovered Elpistostege specimen reveals the presence of a humerus (arm), radius and ulna (forearm), rows of carpal bones (wrist) and smaller bones organised in discrete rows.
We believe this is the first evidence of digit bones found in a fish fin with fin-rays (the bony rays that support the fin). This suggests the fingers of vertebrates, including of human hands, first evolved as rows of digit bones in the fins of Elpistostegalian fishes.
What’s the evolutionary advantage?
From an evolutionary perspective, rows of digit bones in prehistoric fish fins would have provided flexibility for the fin to more effectively bear weight.
This could have been useful when Elpistostege was either plodding along in the shallows, or trying to move out of water onto land. Eventually, the increased use of such fins would have lead to the loss of fin-rays and the emergence of digits in rows, forming a larger surface area for the limb to grip the land surface.
Our specimen shows many features not known before, and will form the basis of a series of future papers describing in detail its skull, and other aspects of its body skeleton.
Elpistostege blurs the line between fish and vertebrates capable of living on land. It’s not necessarily our ancestor, but it’s now the closest example we have of a “transitional fossil”, closing the gap between fish and tetrapods.
The full picture
The first Elpistostege fossil, a skull fragment, was found in the late 1930s. It was thought to belong to an early amphibian. In the mid 1980s the front half of the skull was found, and was confirmed to be an advanced lobe-finned fish.
Our new, complete specimen was discovered in the fossil-rich cliffs of the Miguasha National Park, a UNESCO World Heritage site in Eastern Canada. Miguasha is considered one of the best sites to study fish fossils from the Devonian period (known as the “Age of Fish”), as it contains a very large number of lobe-finned fish fossils, in an exceptional state of preservation.