Amphibamus: a small Carboniferous temnospondyl

Amphibamus grandiceps was a small temnospondyl from Pennsylvanian coal-bearing deposits of Illinois. Its name is closely tied to Mazon Creek, where compressed fossils preserve early tetrapod anatomy. A major 2026 revision reassigned the unusual specimen FMNH PR 558, long discussed as an aberrant Amphibamus, to a new genus and species, Jeanerpeton mazonensis. The distinction matters when reading the animal’s anatomy and diversity. See other early tetrapods in the ancient amphibian catalogue.

Quick facts

A name from the coal measures

Edward Drinker Cope named Amphibamus grandiceps in 1865 from fossils associated with the Coal Measures of Illinois. The name reflects its amphibian-like interpretation in the early history of fossil tetrapod research. The species belongs to Temnospondyli, a diverse group of early tetrapods, and is often discussed among amphibamiforms because of its small size and anatomical relationships.

The original name-bearing specimen was later reported destroyed, complicating comparisons with newly collected fossils. Mazon Creek, near Morris and Braidwood in Illinois, has produced additional specimens in siderite concretions. These ironstone nodules can preserve delicate outlines and skeletons, but many fossils are compressed or incomplete. Their preservation is exceptional in some respects and limited in others.

Older classifications grouped several small Mazon Creek temnospondyl fossils under Amphibamus. A 1982 study by Andrew Milner reviewed small temnospondyls from the Middle Pennsylvanian of Illinois and distinguished multiple forms. This history shows why the locality’s fossils should not be treated as one uniform set of juveniles of a single animal.

The 2026 reassignment of FMNH PR 558

In September 2026, Allison Sefcovic and Payton Kohlberg described Jeanerpeton mazonensis from an exceptionally preserved Mazon Creek specimen held by the Field Museum. The holotype includes a nearly complete skeleton and an impression of the body outline in a concretion. The fossil had previously been considered unusual material of Amphibamus grandiceps.

The authors identified a combination of differences, including a narrower snout, pointed teeth and 22 trunk vertebrae, rather than the 20 described for Amphibamus. Their analysis supports a separate genus and species. This does not replace Amphibamus at Mazon Creek: it clarifies that at least two amphibamiforms are represented there.

The reassignment also affects how the fossil record is read. FMNH PR 558 can no longer be used as straightforward evidence for the body proportions or soft-tissue outline of Amphibamus. It is evidence for a close relative living in the same fossil ecosystem. A specimen’s old label is a historical hypothesis, not an immutable part of the fossil.

What the fossils show about Amphibamus

Material assigned to Amphibamus documents a small-bodied temnospondyl with a compact skull and slender skeleton. Its anatomy contributes to discussions of early amphibamiform diversity and the evolutionary history of tetrapods. However, the number of specimens securely attributable to the genus is more limited than the broader Mazon Creek collection might suggest.

Fossils from the Francis Creek Shale are preserved in concretions formed around buried organisms. The surrounding sediment and associated plants and animals help reconstruct a coastal wetland and delta plain with channels, ponds and forested ground. This is a regional environmental model. A fossil’s burial place does not necessarily mark the exact habitat where the animal lived immediately before death.

Some small temnospondyl fossils may represent larvae or juveniles, but growth stage has to be demonstrated through anatomical features and comparison. Size by itself is insufficient. The 2026 study is an example: characters of the skeleton supported recognition of a separate taxon rather than an immature Amphibamus.

Reconstruction and uncertainty

A reconstruction of Amphibamus should be kept distinct from the well-preserved body outline of Jeanerpeton. The latter supplies comparative information, but its proportions cannot simply be copied onto another genus. Skull and vertebral differences show that the two animals were not interchangeable.

Diet, reproductive behaviour and exact habitat use are not demonstrated by the available fossils. A small predator or insect-eater is plausible from the teeth and body size, but no gut contents confirm a menu. Fossils do not establish colour, vocalisation or parental care.

Amphibamus remains important as an early temnospondyl name and as part of the Mazon Creek record. Recent taxonomic work makes that story more precise: the locality preserves more than one amphibamiform, and a famous specimen once attached to Amphibamus belongs elsewhere. The updated classification improves our picture without filling every gap in the animal’s life.