Two student interns at Chicago’s Field Museum have described a newly identified 309-million-year-old amphibian relative from Illinois, naming the genus Jeanerpeton in honor of their mothers—who both happen to be named Jean.
Long before dinosaurs roamed the earth, the area surrounding modern-day Chicago was a swampy river delta swarmed with giant insects, early land-dwellers, and strange swimming creatures. That ancient environment laid down a wealth of Carboniferous period fossils at Mazon Creek, where sediment rapidly buried dead plants and animals to create an exceptional preservation site. Within that museum collection, a fossil collected years ago by an amateur donor has finally been recognized as a distinct animal rather than just another specimen of a known group.
Revisiting the Waste Bin Taxon at Mazon Creek
When the fossil first arrived at the institution, experts classified it as Amphibamus grandiceps, a well-established amphibamiform from the region. In paleontology, researchers often rely on a catch-all classification—metaphorically described as a waste bin taxon—to hold poorly understood specimens that appear related but lack immediate close study. Study co-author Calvin So explained how museum archives often conceal hidden diversity behind these broad historical groupings.
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. Jeanerpeton is one of those species. And now, we’ve found enough evidence to support it being an anatomically distinct animal.
Calvin So, postdoctoral researcher at the Field Museum
That detailed anatomical re-examination was carried out by Allison Sefcovic and Payton Kohlberg. Working as 2025 Women in Science interns, the two students analyzed the fossil’s dorsal and lateral skull elements and its axial skeleton under the guidance of Arjan Mann, the museum’s assistant curator of fossil fishes and early tetrapods.
Anatomy and Exceptional Preservation of Jeanerpeton mazonensis
Published in the Journal of Vertebrate Paleontology, the description establishes Jeanerpeton mazonensis as a creature that looked somewhat like a salamander but appeared squat and chubby like a frog. The specific name mazonensis refers to Mazon Creek.
The rapid sedimentation at the ancient Mazon Creek river delta captured delicate soft tissues that usually vanish entirely from the fossil record. Sefcovic noted that the specimen preserves remarkable physical features that offer a direct window into ancient animal biology.
What’s really cool about Mazon Creek is that it preserves all of these unusual details—things like toe pads and body outlines. 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.
Allison Sefcovic, first author of the study and a Women in Science intern from the Field Museum
Honoring Their Mothers and Overcoming Scientific Uncertainty
The genus designation Jeanerpeton carries a personal tribute.
Before working in the Mann Lab, I didn’t know much about paleontology. 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.
Payton Kohlberg