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Rare frog fossil sheds light on a gap in Arizona's geological record

Adam Marsh and Michelle Stocker had been out all day in the Petrified Forest trying to beat the heat as they, broke rocks and sifted through the pieces, when they made a major discovery last May.

“What we saw under the magnifying glass was something that looked a lot like a fossil frog pelvis,” said Marsh, the lead paleontologist at Petrified Forest National Park in northern Arizona where the discovery was made.

The tiny pelvic bone is no larger than a pinky nail. Fossilized fragments also were found at a nearby ranch and a stone quarry near St. Johns.

Researchers don’t know whether the frogs underwent metamorphosis like modern frogs, Marsh said. Because so few fragments were discovered, it’s hard to say what the frogs actually looked like – or whether they hopped.

Marsh said the fossils reveal an important fact: They had found a frog that proved frogs  survived a mass extinction in the Triassic period.

Stocker, an assistant professor at Virginia Tech, said their discovery fills in a gap in the timeline for the Triassic. Paleontologist only had records from the early Triassic that are about 250 million years old.

“The purpose of it (finding fossils) is more that we’re trying to understand biodiversity on earth,” Stocker said. “And you can’t just look at the animals that are alive today.”

The biggest challenge was finding the fossil in the first place, she said.

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“We look for bones and jaws and teeth and things like that that might be preserved in those layers that way,” Stocker said. “This part of the frog, that hip bone that we found, is really distinctive for the group.”

Stocker and Marsh found the fossil by washing the rocks through metal screens, then using a microscope to determine what was in the rock.

The fragments were discovered in the Chinle Formation, rocks that were deposited more than 200 million years ago, putting the frogs in the late Triassic period, a time when the area supported rivers and lakes rather than the desert seen today.

“So they’re (the frogs) living in somewhat wet environments,” Marsh said. “In the Triassic here in Arizona, all the rocks are deposited by rivers, lakes, streams.”

Sharks, reptiles and other creatures lived along with the frogs, he said.

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Andrey Atuchin

An artist’s take on what a Triassic period frog may have looked like. A pelvic bone of a frog from that period was discovered last May by paleontologists, who plan to return to Petrified Forest National Park in June to search for more of these fossils.