This is Mazon Monday post #342. What’s your favorite Mazon Creek fossil? Tell us at esconi.info@gmail.com.

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The Mazon Creek fossil biota has a new member… a vertebrate, which are rare in Mazon Creek. Jeanerpeton mazonensis, which means “Jean’s crawler from Mazon Creek”, was named by two student-scientists from the Field Museum. The name honors their mothers, who are both named Jean. See the press release on the Field Museum website.

The study’s co-lead authors, Payton Kohlberg and Allison Sefcovic were part of the Women in Science Interns program at the Field Museum in 2025. They teamed up with Arjan Mann, Assistant Curator of Fossil Fishes and Early Tetrapods, and Cal So, lead scientific advisor and research and postdoctoral researcher, at the Field Museum.

Jeanerpeton mazonensis is an early ancestor of modern day amphibians. It look something like a salamander, but more squat and chubby like a frog.

The fossil specimen was collected from Pit 11 in 1965 by Ken Kietzke, an amateur collector. Ken donated the fossil to the Field Museum. There, it was classified as an Amphibamus grandiceps, a species described by Edward Drinker Cope in 1865.

Although its true identity remained unrecognized, the specimen was already familiar to researchers studying early tetrapods. In 2020, Arjan Mann and paleontologist Bryan Gee included it in a study for its unusually broad toe pads. They concluded that the specimen preserved the earliest fossil evidence of lissamphibian-like toe pads in a temnospondyl, an extinct group of tetrapods that includes the amphibamiforms.

Over time, however, Amphibamus grandiceps became what paleontologists sometimes call a “wastebasket taxon”, which is a convenient place to group fossils that were poorly understood or difficult to classify more precisely. As additional specimens have been studied, researchers have begun to recognize that some of these fossils actually represent distinct animals.

According to Cal So, a co-author of the study and postdoctoral researcher at the Field Museum, “What we’re finding from the Field Museum’s collection are a bunch of things that historically were grouped together. 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.”

So, what does this new species reveal about Mazon Creek? The researchers focused on the dorsal and lateral anatomy of the skull, along with the axial skeleton. Their analysis identified a unique combination of anatomical features that supported recognizing Jeanerpeton as a distinct genus and species.

Jeanerpeton is also a great example of how museum collections can continue to yield discoveries decades after fossils were first collected and identified. In many ways, we are experiencing a new golden age of Mazon Creek research, as scientists take fresh looks at familiar specimens and use detailed anatomical comparisons to recognize differences that were overlooked when the fossils were first studied.

The paper “Jeanerpeton mazonensis, gen. et sp. nov.: a new, exceptionally preserved, late Carboniferous amphibamiform from the Mazon Creek Lagerstätte, Illinois” was published in the Journal of Vertebrate Paleontology.

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 & Arjan Mann

ABSTRACT—The Mazon Creek fossil assemblage has provided the earliest fossil records of several important early tetrapod lineages, including the oldest dissorophoid temnospondyl, Amphibamus grandiceps. Recent systematic efforts have unveiled exceptional diversity of recumbirostran tetrapods at Mazon Creek; however, similar attention has not been paid to the numerous temnospondyl fossils from the site. Here, we provide an anatomical and systematic revision to a uniquely preserved amphibamiform from Mazon Creek, FMNH PR 558, that was previously reported to have the earliest body fossil evidence of lissamphibian-like toe pads in temnospondyls. Restudy of this fossil, with particular attention devoted to the dorsal and lateral skull and the axial skeleton has revealed distinct differences from coeval and other PermoCarboniferous amphibamiforms, allowing for the designation of the new taxon Jeanerpeton mazonensis, gen. et sp. nov. Based on preliminary observations of varying presacral vertebral counts among Mazon Creek amphibamiform fossils there is more diversity of this group from the Moscovian amongst known material than previously recognized.

SYSTEMATIC PALEONTOLOGY
Superclass TETRAPODA Jaekel, 1909
Order TEMNOSPONDYLI Zittel, 1888
Superfamily DISSOROPHOIDEA Bolt, 1969
Unranked clade AMPHIBAMIFORMES Schoch, 2019
Genus JEANERPETON, gen. nov.

Type [and Only] Species—Jeanerpeton mazonensis
Zoobank LSID—urn:lsid:zoobank.org:act:A10337DF-49A1-
41D2-86B5-8883528B1833
Etymology—The generic epithet, ‘Jeanerpeton’ is derived from the combination of ‘Jean’ in honor of Payton Kohlberg and Allison Sefcovic’s respective mothers, and the combination of ‘erpeton’ derived from the Greek meaning ‘crawler.’
Diagnosis—As for the type species.
Zoobank LSID—urn:lsid:zoobank.org:act:E6CBB12A-0360-
447C-9894-751838BC9A84
Etymology—The specific epithet ‘mazonensis’ is derived from
the locality Mazon Creek.
Holotype—FMNH PR 558, a part and counterpart concretion, bearing a virtually complete (with the exception of the tail), albeit uniquely preserved skeleton and soft tissue impression; counterpart preserves dorsal skull fragments, and parts of the axial skeleton.
Locality, Horizon, and Age—West side of Pit 11, 2 miles south of Braidwood, Mazon Creek, Will County, Illinois, Francis Creek Shale above the Morris (No. 2) Coal, Carbondale Formation, Pennsylvanian (Moscovian). Collected by Ken Kietzke in 1965.
Diagnosis—Apomorphies of Jeanerpeton: a long prefrontal orbital process that extends posteriorly past the midpoint of the orbit. Differing from Gerobatrachus, Rubeostratilia, Plemmyradytes, Doleserpeton, Amphibamus, Eoscopus (Daly, 1994), and Platyrhinops but at least sharing with Milnererpeton and Apateon pedestris: presence of 22 presacral vertebrae. Differing from Amphibamus, Gerobatrachus, Georgenthalia, Eoscopus, and all branchiosaurids but sharing with Doleserpeton, and other ‘micropholids’: narrow preorbital region and longer preorbital skull table relative to the postorbital skull table. Differing from Nanobamus in: absence of keyhole-shaped naris. Differing from Amphibamus and Doleserepton but sharing with branchiosaurs, Nanobamus, Gerobatrachus, Georgenthalia, Eoscopus, and ‘micropholids’ in: monocuspid, homodont teeth. Differing from Milnererpeton in: shorter postfrontal, shallower otic notch, and wider postorbital bar. Differing from Milnererpeton and branchiosaurids in: absence of ossified basibranchial. Differing from Pasawioops, Tersomius, and Kermitops in: absence of strong anastomosing ornament. Differing from Amphibamus in: absence of punctate ornament on dorsal skull.