For decades, a fossilized impression that was thought to belong to an already known species lay in the collections of the Field Museum in Chicago. A reexamination of the anatomy of the skull and axial skeleton showed that what lay before the researchers was a new genus and species of proto-amphibian that lived 309 million years ago in the swampy delta of what would become Illinois.
The animal was given the name Jeanerpeton mazonensis. The first part honors the mothers of the two lead authors of the study, both of whom are named Jean. The second refers to the Mazon Creek locality, where sediments accumulated rapidly in the Carboniferous and the finest details were preserved.
The exceptional preservation made it possible to discern scales on the back, abdominal "ribs" — gastralia — and even impressions of the eyes. The scientists note that this may be the oldest evidence of structures resembling the toe pads of modern tailless amphibians. The creature resembled a squat salamander with features that bring it closer to early frogs.
Previously, the fossil was assigned to the species Amphibamus grandiceps. Such "wastebasket taxa" often conceal diversity. The new data indicate that more representatives of amphibamiforms lived in the region than had been supposed. Mazon Creek has already yielded some of the earliest traces of key lineages of early tetrapods.
The study was conducted by Allison Sefkowicz and Peyton Kolberg, interns in the Field Museum's Women in Science program, together with colleagues. Their work shows how a careful second look at old collections opens new pages in the evolution of vertebrates.
Such finds remind us that even in well-studied places, unknown inhabitants of ancient ecosystems may be hiding, whose features shed light on the transition of life from water to land.
