Ancient `froggie’ fossil is missing link

An undated artist’s rendering of Gerobatrachus hottoni, the 275-million-year-old amphibian that


Canadian researchers believe is the missing link between hopping frogs and walking salamanders.

Canadian team discovers the grandaddy of frogs, salamanders that predates the reign of dinosaurs

May 22, 2008 04:30 AM

Sheryl Ubelacker


It likely spent its days on the edge of a quiet, brackish pond, lunging at mayflies for lunch and taking to the water to elude predators.

Part-frog, part-salamander, Gerbatrachus hottoni lived about 275 million years ago, and the only fossil of the creature ever found has shed new light on just how ancient amphibians evolved into these two separate species.

Canadian scientists who undertook the painstaking work of exposing and analyzing the fossilized remains of the prehistoric animal say Gerbatrachus hottoni (Hotton’s elder frog) is the missing link, the common ancestor that lived before frogs and salamanders hopped or walked down their separate evolutionary paths.

They say their research, published in today’s issue of the journal Nature, settles a long and hot debate in scientific circles as to just how these species evolved.

“This fossil is the most like the modern amphibian that you find and it’s from incredibly ancient times,” said principal investigator Jason Anderson, an assistant professor of veterinary anatomy at the University of Calgary who specializes in vertebrate paleontology.

“So what this does is provide conclusive evidence that frogs and salamanders have an origin among one particular group of extinct fossil amphibians,” he said yesterday from Calgary. “This fossil falls right into a gap in the fossil record between one archaic group of amphibians and the earliest examples of the modern amphibians, frogs and salamanders.”

The fossilized remains were discovered in 1995 in the scrubland of north-central Texas by the late Nicholas Hotton of the Smithsonian Institution, the man for whom the long-extinct creature is named.

Hotton, who died two years later, knew he had made a significant find, said Anderson.

“With a slip of paper found with the specimen and in his handwriting is the nickname `Froggie.’ So he recognized the specimen for what it was immediately after he found it.”

The slab of silt stone bearing the 12-centimetre-long creature’s imprint had languished in the Smithsonian’s collection for some time before a U.S. colleague brought it to Anderson’s attention; he jumped at the chance to create a team to partially raise it from its rocky grave.

Co-author Robert Reisz, a biology professor who heads a vertebrate paleontology research lab at the University of Toronto, came on board to lay bare the creature’s skeleton, delicately chipping away at the chalk-like rock in which it was embedded.

Working on the tiny teeth – less than 1 millimetre in size – was especially challenging, said Reisz.

“This is not an easy specimen to study,” he said in an interview. “We don’t extract (the fossilized skeleton) because it would fall apart; basically we expose.”

Froggie, as it turned out, combined features of both frog and salamander.

“It’s got a great big froggie ear and it’s reduced the number of vertebrae in its back … but like salamanders, it shares a particular fusion of some ankle bones,” said Anderson.

Anderson said the fossil dates to a period about 50 million years before the dinosaurs first walked the Earth.

The researchers believe the discovery is important not just for science, but also for the general public.

“This is an important stage in the evolution of life that exists on Earth today, and being able to understand that, knowing a little more of the history of how we got to where we are at present is deeply satisfying on a sort of intellectual level,” said Anderson.


University of Toronto, Latest News

Discovery made at U of T Mississauga lab reveals the origins of frogs and salamanders ( May 21, 2008 )

How did frogs and salamanders evolve? This question, long-debated by biologists, has been answered by a team of researchers that includes U of T Mississauga Professor Robert Reisz and research assistant Diane Scott. Their discovery, based on the study of a 275-million-year-old fossilized amphibian, is outlined in the latest issue of the international research journal, Nature.

Research assistant Diane Scott
(left) and professor Robert Reisz

“Previous discussions focused on three competing hypotheses on the origins of frogs and salamanders, but our discovery proves that they evolved from one ancient amphibian group called temnospondyls,” says Reisz, who is one of the authors of the article, to be published on May 22.

Their findings are based on their study of an ancient fossil of a temnospondyl named Gerobatrachus hottoni (meaning Hotton’s elder frog), which exhibits a mixture of frog and salamander features. It has two fused bones in the ankle, a characteristic normally only seen in salamanders, but also a wide skull and very large eardrum, similar to a frog’s. Its backbone is anatomically half-way between that of modern frogs and salamanders, and more primitive amphibians.

Gerobatrachus hottoni’s morphology, or anatomical form, also provides clues that have helped the researchers better pinpoint the timeframe when frogs and salamanders evolved into two distinct groups, says Reisz. “With this new data, our best estimate indicates that frogs and salamanders separated from each other sometime between 240 and 275 million years ago,

The skeleton of Gerobatrachus

With painstaking attention to detail, Scott exposed the amphibian’s 12-centimetre skeleton by chipping away the rock in which it was embedded. Working on the tiny teeth—less than one millimetre in size—was especially challenging, but also rewarding, says Reisz; when revealed, they were seen to be similar in structure to a frog’s and salamander’s.

Other members of the research team who studied the fossil also have a U of T Mississauga connection. The lead author of the Nature article, Jason Anderson, now a professor at the University of Calgary’s Faculty of Veterinary Medicine, was a postdoctoral student at U of T Mississauga who studied with Reisz. Nadia Fröbisch, currently completing a PhD at McGill, is planning to work on postdoctoral research on campus this fall.

The fossil was first discovered in Texas in 1995 by a group of Smithsonian Institution scientists led by Nicholas Hotton, after whom the specimen was named. They took the fossil back to Washington, where it remained unstudied until Anderson and a colleague borrowed it and brought it to Reisz’s vertebrate palaeontology research lab where they were able to examine it in detail.

Learning about the evolutionary history of frogs and salamanders is important, says Reisz, “because they are our barometers of environmental degradation. They are very sensitive to pollutants.”

“It is bittersweet to learn about frog origins in this Year of the Frog, dedicated to informing the public about the current global decline of amphibians,” adds Anderson. “Hopefully we won’t ever learn about their extinction.”

By Olena Wawryshyn


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