Modern amphibians -- frogs, salamanders and earthworm-like caecilians -- have been a bit slippery about divulging their evolutionary ancestry. Gaps in the fossil record showing the transformation of one form into another have led to a lot of scientific debate.
The fossil Gerobatrachus hottoni or elderly frog, described in the journal Nature, may help set the record straight.
"It's a missing link that falls right between where the fossil record of the extinct form and the fossil record for the modern form begins," said Jason Anderson of the University of Calgary, who led the study.
"It's a perfect little frogamander," he said.
Gerobatrachus has a mixture of frog and salamander features, with fused ankle bones as seen only in salamanders, a wide, frog-like skull, and a backbone that resembles a mix of the two.
The fossil suggests that modern amphibians may have come from two groups, with frogs and salamanders related to an ancient amphibian known as a temnospondyl, and worm-like caecilians more closely related to the lepospondyls, another group of ancient amphibians.
"Frogs and salamanders share a common ancestor that is fairly removed from the origin of caecilians," Anderson said.
Gerobatrachus hottoni was discovered in Texas in 1995 by a group from the Smithsonian Institution that included the late Nicholas Hotton, for whom the fossil is named.
Anderson's team painstakingly removed layers of rock to reveal the anatomy of the skeleton.
"The fossil itself is almost perfectly complete," Anderson said.
"It died on its back. Its legs and arms were curled up on its belly and it's that part that weathered away."
While scientific opinion moves slowly, Anderson thinks the find will confirm the prevailing opinion that frogs and salamanders share a more modern ancestor.
"I think they (scientists) will be very happy with this as a resolution," he said.
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