Science2 publishers3 min readPublished
Field Museum interns show a misfiled Mazon Creek fossil is a new 309-million-year-old amphibian relative
Two Field Museum interns named Jeanerpeton, a 309-million-year-old amphibian relative the museum had filed under a known species. A co-author says that species name has become a catchall in the collection, so more fossils filed under it may prove to be separate animals.
The Scientist · Science desk
What happened
- The fossil came from an amateur collector who found it at Mazon Creek in Illinois and gave it to the Field Museum, which catalogued it as Amphibamus grandiceps.
- The 2026 paper in the Journal of Vertebrate Paleontology names it Jeanerpeton mazonensis, or 'Jean's crawler from Mazon Creek', after the two students' mothers, who are both named Jean.
- The 2020 paper also reported the first evidence in a temnospondyl body fossil of toe pads comparable to those of living amphibians.
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Why it matters
- precedent Other fossils the Field Museum holds under Amphibamus grandiceps are now candidates for the same one-by-one re-examination, and some may be described as separate species.
- cost Clearing a catchall label takes a long time. A mismatch noted in 2020 needed six more years and a full formal description before the specimen got its own name.
- constraint The pads place a living-amphibian feature more than 300 million years back, but they cannot settle whether Jeanerpeton climbed, so claims about its behaviour stay inferences from living species.
Someone had spotted the mismatch before anyone gave the fossil a name. In a 2020 paper, Arjan Mann and the vertebrate paleontologist Bryan Gee studied this fossil's toe pads. They noted that it had a different number of vertebrae from Amphibamus grandiceps, but did not give it a new species name [7]. The formal description came in 2026, and it makes the fossil a new genus and species of late Carboniferous amphibamiform [3][20]. Six years passed between flagging the difference and publishing the case for a new animal [9]. Payton Kohlberg and Allison Sefcovic, two 2025 interns in Mann's lab, were co-lead authors [1][8].
Cam So, a postdoctoral researcher at the Field Museum and a co-author, describes the specimen as one of several [12]. "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," So said [10]. A waste bin taxon is a name that collects specimens that seem related but have not been well studied [23]. "With this publication, we officially pull Jeanerpeton out of this waste bin taxon," So said [11].
That backs the idea that museum drawers hold misidentified species, but so far only in one direction. The press accounts did not disclose how many fossils the Field holds under A. grandiceps or how many have been re-examined, so there is no misidentification rate to quote. One specimen has left the bin. In Discover's account, So said that examining the others individually could show some belong to separate species [13]. With a lab describing "a bunch of things" under one catchall name, I would expect more names to follow.
The detail that makes this work possible comes from the site. Mazon Creek was a river delta where mud covered dead animals before their finer features disappeared [6]. "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!" Sefcovic said [19].
The feet are the best part of the specimen. Temnospondyls were an extinct group of four-legged animals, and Jeanerpeton was one of them [22]. Mann and Gee reported the first evidence in a temnospondyl body fossil of toe pads comparable to those of living amphibians [14]. The animal's broad toe tips differ from the thinner, tapered toes of its aquatic relatives [15]. Some living amphibians use toe pads to grip surfaces. According to Discover, though, the fossil cannot tell researchers whether Jeanerpeton climbed or how else it used its feet [16].
The two press accounts also differ on how far to take the lineage. Phys.org calls J. mazonensis an early ancestor of today's amphibians and says the findings point to a 309-million-year-old origin of classic frog and salamander traits [18][17]. Discover calls it a relative and limits the claim to a feature of living amphibians already being present more than 300 million years ago [4][24]. In my view, "relative" is the word this anatomy supports. The paper's own title calls the animal an amphibamiform [20].
What to watch
- Whether the Field Museum group publishes counts of how many A. grandiceps specimens it has re-examined, and how many prove anatomically distinct.
- Whether other specimens are referred to Jeanerpeton mazonensis, which would test the new diagnosis against more material.
- Whether other Mazon Creek temnospondyls turn up with broad toe pads, which would show whether the trait was common among land-dwelling forms.