A model of Jeanerpeton on the floor of the Carboniferous forest at the Field Museum’s Evolving Planet exhibition. Credit: Allison Sefcovic

A misidentified museum fossil has revealed Jeanerpeton mazonensis, a distinct amphibian ancestor that lived near present-day Chicago 309 million years ago.

Jeanerpeton mazonensis, an early ancestor of modern amphibians, looked like a salamander with the squat, chubby build of a frog. Its fossil had been sitting in the Field Museum’s collection under another animal’s name until researchers recognized it as a distinct species.

The animal lived 309 million years ago in a swampy river delta near present-day Chicago, long before the dinosaurs. Its newly published description in the Journal of Vertebrate Paleontology, led by student researchers Payton Kohlberg and Allison Sefcovic, adds another species to the picture of life preserved at Illinois’ Mazon Creek fossil site.

Allison Sefcovic and Payton Kohlberg hold epoxy sculptures of their new species, Jeanerpeton mazonensis. Credit: Arjan Mann

Mistaken for Amphibamus

An amateur collector found the specimen many years ago and donated it to the museum, where it was initially classified as Amphibamus grandiceps, a species well documented in the region. Fossils that appear related but have not been studied closely sometimes end up grouped into a catchall category that paleontologists call a “waste bin taxon.”

“What we’re finding from the Field Museum’s collection are a bunch of things that historically were grouped together into this waste bin taxon known as Amphibamus grandiceps,” says Cam So, a study co-author and postdoctoral researcher at the Field Museum. “Jeanerpeton is one of those species. And now, we’ve found enough evidence to support it being an anatomically distinct animal. With this publication, we officially pull Jeanerpeton out of this waste bin taxon.”

Mazon Creek’s unusually detailed fossils

Recognizing the variety of animals in these collections helps researchers reconstruct an ecosystem where early land-dwelling creatures lived alongside giant insects and unusual swimming animals. During the Carboniferous period, sediment accumulating in the Mazon Creek river delta quickly buried dead plants and animals, creating favorable conditions for fossilization. The resulting deposits became one of the world’s richest fossil records of the period, preserving soft tissues and fine anatomical details that usually disappear.

Photo and illustration of the fossil of Jeanerpeton mazonensis. Credit: Allison Sefcovic

“What’s really cool about Mazon Creek is that it preserves all of these unusual details—things like toe pads and body outlines,” says Sefcovic. “In the fossil of Jeanerpeton, we can see some of the scales on its back and tiny ‘belly ribs’ called gastralia. You can even see the eye impressions! It’s so cool to see.”

The findings point to a 309-million-year-old origin for some characteristic traits seen in modern frogs and salamanders. They also give researchers another animal to examine as they continue documenting the biodiversity of ancient Mazon Creek.

Two Jeans behind Jeanerpeton’s name

For Kohlberg and Sefcovic, describing the species offered a chance to honor two people with a name in common. Both researchers’ mothers are named Jean, inspiring Jeanerpeton mazonensis, which means “Jean’s crawler from Mazon Creek.”

The two co-lead authors were members of the Field Museum’s 2025 cohort of Women in Science Interns, a program that encourages young women to pursue careers in science and museums. Along with So, they work in the lab group of Arjan Mann, the museum’s assistant curator of fossil fishes and early tetrapods, or four-limbed vertebrates, in the Negaunee Integrative Research Center.

“Before working in the Mann Lab, I didn’t know much about paleontology,” says Kohlberg. “My experience at the Field and in the Women in Science program has made me more confident in my abilities to try something new. It is scary to take on a project when you feel like you don’t know as much as you should, but I learned with my practical experience that it is possible to overcome that fear with hard work.”

Reference: “Jeanerpeton mazonensis, gen. et sp. nov.: a new, exceptionally preserved, late Carboniferous amphibamiform from the Mazon Creek Lagerstätte, Illinois” by Allison J. Sefcovic, Payton J. Kohlberg, Calvin So and Arjan Mann, 24 September 2026, Journal of Vertebrate Paleontology.
DOI: 10.1080/02724634.2026.2698799

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