Peter Gong

Friday, May 10, 2019

Rare frog fossil sheds light on a gap in Arizona geologic record

PETRIFIED FOREST – Adam Marsh and Michelle Stocker had been out all day in the hot weather, breaking rocks and sifting through the pieces, when they made a major discovery last May.

Paleontologists discovered fragments of fossilized frogs in the Chinle Formation at Petrified Forest National Park and the surrounding area. These rocks were deposited more than 200 million years ago, putting the fossils in the late Triassic period, a time when this area supported rivers and lakes rather than the desert seen today. (Photo by Peter Gong/Cronkite News)

“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 were also found at a nearby ranch and a stone quarry near St. Johns, Arizona.

Marsh said they can’t say whether the frogs went through metamorphosis like modern frogs – or whether they hopped. And because so few fragments were discovered it’s hard to say what the frog actually looked like.

Marsh said the fossil itself does reveal an important fact – frogs survived a mass extinction in the Triassic period.

Stocker , an assistant professor at Virginia Tech, said the biggest challenge was finding the fossil in the first place.

“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 fossils 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 fossils in the late Triassic period, a time when the area supported rivers and lakes rather than the desert seen today.

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“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.

Stocker 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.”

Marsh and Stocker plan to return to the park in early June to hunt for more fossils.


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