The study of the fossil appears in this week's issue of the journal Nature.
Until recently it was believed that frogs, salamanders, and caecilians—long wormlike amphibians—all shared a common ancestor.
(See a photo of a caecilian.)
The rediscovered fossil suggests that frogs and salamanders branched out from one set of ancestors while caecilians descended from another.
The creature's combination of frog- and salamander-like features further supports earlier studies suggesting that frogs and salamanders are more closely related to each other than to caecilians.
"This animal seems to show that frogs and salamanders are more closely related to each other than either are to these elongate caecilians," said Robert Carroll, a paleontologist at McGill University in Montreal, Canada, who was not involved in this study.
Carroll noted that some of Gerobatrachus's features crop up in older fossils, suggesting that the animal may be descended from an ancestral group that branched out into frogs and salamanders at an earlier point.
"I would say Gerobatrachus was a relic at least in relationship to the question of the ancestry and point of division between frogs and salamanders," Carroll said.
Bolt, the Field Museum expert, cautioned that it is difficult to say for sure whether this creature was itself a common ancestor of the two modern groups, given that there is only one known specimen of Gerobatrachus, and an incomplete one at that.
"At this point I would say it is by no means certain that this is representative of a common ancestor to frogs and salamanders, although it might be," Bolt said.
"The fact that something this remarkably like the modern amphibians is present [during this period] suggests that, really, you could have had an even earlier example of something that was ancestral to modern frogs and salamanders."
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