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Key clue to amphibian evolution found in Petrified Forest

By Nicholas Gerbis
Published: Friday, January 27, 2023 - 1:03pm

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Ben T. Kligman.
Petrified Forest National Park paleontology intern William Reyes excavating fossils at the ‘Thunderstorm Ridge’ fossil site.

For years, debate has raged about where a group of snakelike animals fit in the amphibian family tree.

Now, an analysis in the journal Nature of fossils from Arizona’s Petrified Forest National Park may hold the answer.

A lack of fossil remains often hinders attempts to reconstruct the family tree of caecilians —– limbless, bullet-headed, wriggly amphibians that live in soil or streambeds an eat mollusks, termites, worms, small snakes and other amphibians. They also have small, sensitive tentacles, located between their nostrils and their eyes.

At 220 million years old, the fossilized remains extracted from the Petrified Forest’s portion of the Chinle geological formation are 35 million years older than previous finds.

They show the amphibians’ ancestors lacked their telltale tentacles and retained some features of an extinct group of four-legged amphibians called dissorophoid temnospondyls.

This suggests they bridged a gap between modern caecilians and those ancient, extinct amphibians and strengthens the argument that they should be clustered with frogs and salamanders in the group Lissamphibia.

Andrey Atuchin, the National Park Service, and the Petrified Forest Museum Association. (Artists website:
Artistic reconstruction of extinct caecilian ancestor Funcusvermis gilmorei (foreground) and the crocodile relative Acaenasuchus geoffreyi (background) in the tropical forest of Petrified Forest National Park about 220 million years ago.

Ben T. Kligman.
Petrified Forest National Park interns (Elliott Armour Smith and William Reyes) excavating fossils at the ‘Thunderstorm Ridge’ fossil site.
Ben T. Kligman.
Microscopic photograph of a lower jaw from an extinct caecilian, Funcusvermis gilmorei, soon after it was recovered during microscopic sorting of sediment from the Thunderstorm Ridge fossil site in the Petrified Forest National Park Paleontology Lab.
Ben T. Kligman.
A coprolite (fossilized poop) collected from Thunderstorm Ridge.
Ben T. Kligman.
Photograph of the holotypic (reference) specimen of Funcusvermis gilmorei (a right lower jaw) soon after it was first collected.