Batrachichnus: footprints of a small Paleozoic tetrapod

Batrachichnus is an ichnogenus: a name for a recurring form of fossil footprint trackway, not for the skeleton of an animal. Its best-known tracks come from Late Carboniferous and Early Permian rocks in Europe and North America. A sequence of prints can preserve how an animal placed its limbs and interacted with soft ground, even when no bones were buried nearby. The possible trackmakers belong among the early tetrapods discussed in the ancient amphibian catalogue.

Quick facts

A trace fossil records a path, not a skeleton

The name Batrachichnus was established for a characteristic form of small tetrapod trackway. The commonly discussed ichnospecies B. salamandroides was named from Carboniferous prints. The resemblance to a salamander-like gait helped describe the pattern, but it does not demonstrate that a modern salamander or a specific salamander relative made it. Ichnotaxonomy classifies traces by their form and recurring features, while zoological taxonomy classifies the animals themselves.

This distinction is useful because footprints can preserve behaviour that bones do not. A trackway records the order of footfalls, the spacing between them, the width of the path and sometimes a tail drag. A skeleton can show how a limb was built; a sequence of prints shows how the feet contacted a particular surface during one passage.

The same evidence also has limits. A foot sinking into wet mud may leave a broader or deeper print than it would on firmer sediment. Water flow, drying, erosion and later compression can remove digits or change their apparent length. Researchers therefore compare whole trackways and repeated impressions, rather than naming a species from a single smudged mark.

Reading the prints and the substrate

Typical Batrachichnus tracks are small and set on either side of a central direction of travel. Four digits are commonly visible on the forefoot and five on the hind foot. The rear prints may be longer or more widely spaced, depending on the animal’s proportions and how it moved. Some slabs preserve a narrow groove between the prints, interpreted as contact by the tail.

Stride length and trackway width can be measured, but they are not direct tape measurements of the animal. A body-length estimate needs a model linking limb length and gait to foot spacing. Because the trackmaker is not known and prints deform with substrate consistency, a precise height or speed would claim more than the trace provides.

Fossil footprints also include true tracks and undertracks. A true track forms at the original surface; pressure can deform lower sediment layers and create undertracks with a softened version of the same pattern. Erosion may expose a track from below rather than the exact surface on which the animal walked. These processes explain why apparently different prints can come from the same passage and why similar-looking prints need not represent different animals.

Who could have made a Batrachichnus trackway?

Small temnospondyls are plausible trackmakers because their feet and body plans can fit the prints. Some early tetrapods historically grouped as “microsaurs” also have postcranial anatomy compatible with parts of the pattern. A 2022 study of a trackway from the Minto Formation in New Brunswick argued that candidate trackmakers should not be restricted to temnospondyls. It compared the hand and foot anatomy of several late Paleozoic groups and emphasized that different taxa could produce traces within the same ichnogenus concept.

That conclusion is not a claim that every proposed group made every Batrachichnus track. It means the shape of a footprint does not uniquely identify a skeletal genus when multiple animals had similar limbs. A track found beside a skeleton would be valuable, but proximity alone could still be coincidental unless the association is unusually direct and alternatives are excluded.

Earlier research on trackways from the Southern Alps described a sequence interpreted as a transition from walking on a submerged surface to swimming. Full impressions became more incomplete as buoyancy reduced pressure on the bottom, leaving scratches or only occasional contact. Such a series captures movement through changing water depth. It is more informative than a lone groove, which might be made by current, a body part or a different organism.

Where and when the traces occur

Batrachichnus is reported from Late Carboniferous and Early Permian deposits across parts of Europe and North America. Individual trackways come from particular beds and environments; a broad distribution of similar traces should not be treated as a continuous range of one biological species. A stable footprint form could have been produced by related animals at different times or by multiple lineages with comparable feet.

Wet mud near channels, lakes and floodplains provided surfaces that could register prints and then be buried quickly. Preservation required a balance: sediment had to be soft enough to take an impression, but not so fluid that it collapsed, and burial had to protect it from later disturbance. The trackway thus records both the animal’s movement and the properties of the ground beneath it.

The illustration emphasizes the evidence itself: a small tetrapod silhouette beside a trackway with distinct front and hind prints and a faint tail trace. The animal is intentionally generalized. No fossil supports assigning this particular artistic body to a named genus, and its colour, skin and exact gait remain unknown.