National Geographic Today
What creature first crawled out of the prehistoric swamps to conquer the land? The question has long puzzled paleontologists because the transitional species seems to have lived during a mysterious 30-million-year gap in the fossil record called Romer's Gap.
Now a researcher in Britain has found a very rare fossil of a short, squat crocodile-like creature that she believes provides a stepping stone between our aquatic ancestors and the first four-legged land dwellers.
"This really is one of a find," said Jennifer Clack, a paleontologist at the University Museum of Zoology Cambridge, United Kingdom. "It may even be the first five-toed foot," she added, "but I can't swear to that yet."
Pederpes probably came from a shallow-water environmenta lagoon or coastal flatthat was vulnerable to sudden increases in salinity as water levels rose and fell. Romer believed such an environment would have favored the evolution of limbs enabling land travel, to allow the animal a broader area to feed. The discovery is published in the July 4 issue of the journal Nature.
Clack feels that the early evolution of hands and feet occurred in relationship to strictly aquatic locomotion, somewhat in the manner of seals.
The fossil was originally discovered in 1971 in a stream valley that cuts through moorland about three kilometers from Dumbarton, Scotland, and lay incorrectly classified in the Hunterian Museum, in Glasgow. The skeleton is almost complete, just missing a tail, and is thought to date back to 348 to 344 million years agothe heart of Romer's Gap. The Gap is named for Alfred Sherwood Romer, an American paleontologist and a prolific writer of textbooks in the 1950s and 1960s, who first recognized the lack of fossils from this 30-million-year period.
The hint that Clack's new species, called Pederpes finneyae, might provide a missing link between the swimmers and the landlubbers lies in the bone structure of the hind leg.
During the late Devonian period, about 365 million years ago, tetrapods had paddle-like feet for swimming that pointed back or to the side, said Clack.
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