"Frog-amander" Fossil May Be Amphibian Missing Link

Anne Casselman
for National Geographic News
May 21, 2008

A new fossil find may be an evolutionary missing link in the amphibian family tree, scientists say.

The 290-million-year-old fossil was first collected in Texas by a paleontologist with the Smithsonian Institution in the mid-1990s. It was rediscovered in the collections of the National Museum of Natural History in Washington, D.C., in 2004.

"It had an overall amphibian gestalt. … You know, kind of a froggy slamander-y sort of look," said Jason Anderson, a comparative biologist at the University of Calgary, Canada, who led a new analysis of the fossil.

"But also I recognized some of the archaic features too, and I thought that this would be a critical piece of evidence in trying to work out the origins of modern amphibians."

Dubbed Gerobatrachus hottoni, the animal looked somewhat like a salamander with a stubby tail and froglike ears.

"So it's kind of a frogamander, if you will," said Anderson.

Filling in Fossil Gap

Gerobatrachus fits into a noted gap in the fossil record of amphibians—between one of the groups hypothesized to be ancestors of modern amphibians, called temnospondyls, and the earliest frogs and salamanders.

(See related photo: "Pre-Dino Amphibian Body Casts Found" [October 30, 2007].)

"It pretty convincingly settles the question [that the] frog and salamander shared origins from the same fossil group," Anderson said.

John Bolt, curator for fossil amphibians and reptiles at The Field Museum in Chicago Illinois, who was not involved in the study, said, "The most astonishing thing to me about this study is that this animal is far more froglike than I would ever have expected from its age."

"Nothing this nonprimitive has ever been described from this age. It's just amazing."

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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