The finding tells researchers that limbs were a long time in the making, says evolutionary biologist Neil Shubin of the University of Chicago and The Field Museum in Chicago. "This is showing that complex structures have a history and they're assembled over time—they don't happen in one fell swoop," he says.
Polyodon hails from an ancient line of fish known as ray fins that includes minnows, salmon and other common fish. Roughly 415 million years ago, the ray-finned fish split from the lobe-finned kind—the latter of which would branch off into lungfish, amphibians and, finally, mammals. As an early representative of ray-finned fish, Polyodon offers a window into what its more "lobey" precursors were like.
Shubin and his University of Chicago co-workers studied a fleshy nub at the base of each of Polyodon's rear fins, each of which contains a fanlike array of bones [see image above] that corresponds to the leg bones of four-limbed animals, called tetrapods. The team analyzed these areas for the activity of a telltale group of genes called Hox, which are active during the embryonic development of all known animals.
The Hox family divides embryos into a series of regions that will each morph into a different body section, such as the tail, belly or head, based on its particular combination of active Hox genes.
Tetrapods show one pattern of Hox activity during late limb development, as their digits are forming, but the modern ray-finned fish show a different pattern, which has left researchers wondering how tetrapods arrived at theirs, says team member and evolutionary biologist Marcus Davis.
Lo and behold, Polyodon's nub had the same pattern of Hox activity as did the tetrapod limb, meaning that modern ray-finned fish must have lost this ancient genetic program, the researchers report in a paper published online today by Nature.
Shubin says the result fits with his group's discovery last year of a half-fish, half-tetrapod fossil called Tiktaalik, which had a palmlike bone structure in its flipperlike limbs.
The Hox finding explains how Tiktaalik produced its complicated limb structure, and shows that the jump to tetrapods was not so large, says vertebrate paleontologist Jennifer Clack of the University Museum of Zoology in Cambridge, England. "It takes us quite a big step forward," she says, in understanding the shift from fins to feet.