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Together, these fossils have overturned the old picture of the fish-tetrapod transition, which conjured up the image of creatures like the modern lungfish crawling out of water onto land. That picture certainly didn't fit Acanthostega, whose short, flimsy legs were ill equipped for terrestrial locomotion. Rather, according to University of Cambridge paleontologist Jennifer Clack, Acanthostega was an aquatic creature that used its limbs and lungs to make a living in water. And that scenario makes sense because it sets up conditions for natural selection--the force that powers evolution--to favor transitional life-forms like the fishapod, with its funny wrist and five digits encased in the webbing of a fin.
On land, observes Shubin's collaborator Ted Daeschler, chair of vertebrate zoology at Philadelphia's Academy of Natural Sciences, such an appendage would have been worse than useless. But it would have been more than adequate for propping the animal's head above the water so that it could survey its surroundings or for anchoring it underwater as it waited to ambush its prey. The advantage of being able to gulp air through lungs as well as gills would likewise have been immediate, given that the fishapod made its home in warm, shallow waters that were frequently rendered inhospitable by decaying vegetation.
The fishapod was among the pioneering organisms to take advantage of an ecological frontier--the marshy floodplains of large rivers--that opened between 410 million and 356 million years ago during the Devonian period, known as the Age of Fishes. Early in the Devonian, the continents were mostly masses of bare rock with just a fringe of plants "no taller than your ankle," as Daeschler puts it, growing along the wet margins of rivers and streams. By the late Devonian, however, thick vegetation had taken hold in marshes, fens and floodplains, and mosses, ferns and trees had coalesced into the world's first forests.
Those archaic plants prepared the way for the tetrapods. The plants created new aquatic habitats by stabilizing the banks of rivers and streams. They pumped oxygen into the atmosphere, making the earth habitable for large, air-breathing creatures. And they shed organic debris that formed the basis of a new food chain. Bacteria, fungi and small arthropods (the animal group that includes crustaceans and insects) moved in to feed on the debris; small fish moved in to eat the arthropods; bigger fish moved in to eat the small fish. Among them were the fishapod's lobe-finned ancestors, which found in the vegetation-clogged shallows abundant food and relative safety from predators.
Because characteristics like limb development are governed by powerful families of genes known as Hox genes, the fishapod's curious mix of features intrigues developmental biologists as much as it does paleontologists. Recent experiments on mice by University of Geneva geneticist Denis Duboule and his colleagues, for example, show that Hox genes control limb development in two stages. "Even though the same genes are involved," says Duboule, "separate processes govern the development of arms and legs and the development of hands and feet."