Researchers have found the first small finger-like bones in the fins of a fish that lived 380 million years ago, about 15 million years before the first four-footed creatures, called tetrapods, clambered onto the land.
The finding upends the most recent theory of the evolution of digits: The need to adapt to swampy marshlands and terra firma, the theory went, is what drove the gradual shift through natural selection from fish fins suitable only for swimming to weight-bearing limbs with articulated joints. The study, however, reveals that rudimentary fingers were already present inside the fins of the shallow-water Panderichthys, a transitional species that was nonetheless more fish than tetrapod [The Daily Telegraph].
The first Panderichthys fossils were found in the 1990s, but the specimens were embedded in clay and difficult to study. To get around this difficulty, researchers ran the fossil through a CT scanner at a hospital. The image shows stubby bones at the end of the fin skeleton clearly arrayed like four fingers, called distal radials. There are no joints, and the bones are quite short, but there could be no doubt as to what they were. “This was the key piece of the puzzle that confirms that rudimentary fingers were already present in the ancestors of tetrapods,” said lead author Catherine Boisvert [AFP].
In the study, published in Nature [subscription required], researchers argue that the primitive finger bones may have helped the Panderichthys navigate its muddy environment. The broad fins would have made for stronger supports for the fish to lean on rather than for all-out swimming. “It was probably using its front fins as supports to be able to look up, kind of doing push-ups at the bottom of the river looking outside with its eyes,” Boisvert said, adding that the fish’s eyes were on the top of its skull and thus probably good for looking above the mud for fish food [LiveScience].
Image: Wikimedia Commons