Field Museum Reveals New Early Amphibian Genus

A 309-million-year-old Illinois fossil, long misfiled under another species name, has been described as a new genus and species.

Updated on Nov. 13, 2026 in Geology

Grey limestone fossil slab with a faint amphibian skeleton impression on a wooden laboratory table beside a brass hand lens. — Researchers at the Field Museum in Chicago have described a 309-million-year-old Illinois amphibian fossil as a new genus and species after it was reassessed. AI Illustration. Upload story photo >

Researchers at the Field Museum in Chicago described Jeanerpeton mazonensis, an early amphibian relative from Illinois, in a paper published in the Journal of Vertebrate Paleontology on September 24, 2026. The specimen had been filed under a poorly studied waste bin taxon, Amphibamus grandiceps, before being reassessed.

Why it matters

The reassessment shows that specimens can sit unrecognized in museum collections for decades when grouped under poorly understood categories. Because other fossils at the Field Museum were grouped the same way, more new species names could follow.

The specimen preserves scales on its back, gastralia and eye impressions, and is dated to 309 million years, roughly the same age range the Illinois State Museum assigns to Mazon Creek fossils. The full number of misfiled specimens awaiting review has not been reported.

The players

Field Museum

The Field Museum is a natural history museum in Chicago that holds large paleontological collections, including Mazon Creek fossils.

Journal of Vertebrate Paleontology

The Journal of Vertebrate Paleontology is a peer-reviewed scientific journal that publishes research on fossil vertebrates.

The details

Allison Sefcovic and Payton Kohlberg, who were Women in Science interns at the Field Museum in 2025, co-led the description. The genus name Jeanerpeton honors two women named Jean, and the specimen was donated to the museum by an amateur collector many years ago.

Carboniferous: a swampy river delta covered the area around present-day Chicago.
September 24, 2026: the paper describing Jeanerpeton mazonensis was published.

Big Picture

Like a skull fragment that London's Natural History Museum acquired in 1924 and later found to be a new dinosaur, the Illinois specimen sat for decades under a mistaken name. The case suggests that misfiled collections may hide further undescribed species.

For Illinois residents, the find adds to the record of fossils from Mazon Creek, a site spanning Grundy, Will, Kankakee and Livingston counties. Visitors interested in the specimens can check the Field Museum's collections in Chicago.

The takeaway

Museum collections can hold overlooked scientific discoveries for decades, so reexamining older specimens can reveal new species. Researchers and the public alike benefit when collections are periodically reviewed.