Two moms named Jean inspired the name of a 309-million-year-old fossil

When your daughter is a budding scientist, gift giving can take a whole new form. Think fewer flowers and more new prehistoric species named in your honor. Jeanerpeton mazonensis (aka “Jean’s crawler from Mazon Creek”) is an early ancestor of today’s living amphibians, whose genus name Jeanerpeton is for two human moms named Jean. This salamander-like creature lived in a swampy river delta about 309 million years ago in present-day Chicagoland. The early amphibian is described in a study published today in the Journal of Vertebrate Paleontology, and is an example of how some of the classic traits seen in today’s living frogs and salamanders evolved. 

J. mazonensis (also called Jeanerpeton) is a squat frog-like amphibian that once lived in and around Mazon Creek, an area about 60 miles southeast of  Chicago and teeming with fossils. Mazon Creek was a swampy river delta during the Carboniferous period, where sediment quickly deposited on dead animals and plants. This combination of quick deposits and plenty of organic material is the perfect recipe for fossilization. These fossils preserve records of soft tissues and fine details normally lost. 

“What’s really cool about Mazon Creek is that it preserves all of these unusual details—things like toe pads and body outlines,” Allison Sefcovic, an undergraduate student at the University of Michigan, study co-author and a 2025 Women in Science intern at Chicago’s Field Museum, said in a statement. “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.” 

An amateaur fossil collector found the specimen the team used in this study and donated it to the Field Museum. It was initially classified as another species named Amphibamus grandiceps that had a well-established presence in the region. Poorly-understood specimens like these can sometimes be lumped together in a “waste bin taxon,” a metaphor that scientists use to describe chucking specimens who seem related, but have not been well studied yet. 

“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,” added 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.”

The study’s co-lead authors, Sefcovic and University of Illinois undergraduate student Payton Kohlberg, were part of the museum’s 2025 cohort of Women in Science Interns. The program champions young women to pursue careers in science and museums. Along with So, Kohlberg and Sefcovic are all part of a lab group run by Arjan Mann, the Field’s assistant curator of fossil fishes and early tetrapods in the Negaunee Integrative Research Center.

“Before working in the Mann Lab, I didn’t know much about paleontology,” said 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.” 

With so many physical traits that would be passed on to the thousands of amphibian species that evolved over the next hundreds of millions of years ago and could be considered the mother of future amphibians, the team named it J. mazonensis, after Kohlberg and Sefcovic’s moms. Its discovery adds to our understanding of one of the world’s premier fossil deposits and what life in Illinois looked like even before dinosaurs. 

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