Tiny Carboniferous Steps

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The tiny Batrachichnus salamandroides trackway found in Nova Scotia. Image courtesy Justin Spielmann.

Fossil footprints are lovely vestiges of prehistoric life. Even though skeletons are wonderful things, and immense mounts of defleshed dinosaurs were what inspired my love of paleontology in the first place, the variety of fossil tracks paleontologists have collected and cataloged are intricate snapshots of life in motion. Imprints and trackways record movement and behavior – petrified outlines of prehistoric moments that we can’t observe directly. And it’s this variety of fossil that allowed paleontologists to follow a tiny 315 million year old tetrapod on a little jaunt.

Discovered by amateur paleontologist Gloria Melanson along Nova Scotia’s Joggins Cliffs, the tiny trackway contains what Melanson, Matt Stimson, and Spencer Lucas deem to be the smallest fossil vertebrate footprints yet found. The whole trackway, containing about 30 footprints, is just under two inches long, and the imprints themselves range from 0.09 to 0.06 of an inch long each. This was a tiny, tiny tetrapod that strutted along a floodplain during a time when insects were giants and our own ancestors were small, lizard-like creatures. Based on the details of the tracks, it seems that the vertebrate was walking at a normal pace before slightly speeding up toward the end of the slab. But just what sort of animal made the tracks?

Students of trace fossils – technically known as ichnologists – constantly face what paleontologist Martin Lockley has called the “Cinderella Syndrome.” Unless an organism literally dies in its tracks, paleontologists face the problem of matching the trace to the trace-maker. Indeed, the situations that preserved body fossils weren’t always amenable to setting trace fossils in stone, and vice versa. And even in places like the Joggins Cliffs, where body and trace fossils are both found, many of the body fossils are fragments preserved in different settings than the footprints. In this case, what we know about the skeletons of the Joggins Cliffs animals comes from bones found inside the fossilized trunks of tree-like lycopsids – vascular plants most closely related to today’s club mosses and quillworts.

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A restoration of the kind of amphibian that may have made the Batrachichnus tracks, courtesy Justin Spielmann.

Stimson and co-authors attribute the tracks to the ichnospecies Batrachichnus salamandroides. This is the name for the particular track type, but not the animal itself. (Just like body fossils, trace fossils are organized according to a binomial system which refers to specific trace fossil forms.) But the paleontologists also narrow down the list of possible creatures that could have made the Batrachichnus salamandroides footprints. Their prime candidate is a juvenile Dendrerpeton– a roughly salamanderish amphibian known from an articulated skeleton and other fossils found in the ancient lycopsid stumps. The crawler’s proportions, when shrunk down to scale, seem to match the proportions of the trackmaker.

Then again, Dendrerpeton isn’t the only suitable candidate. A different variety of amphibian, called a microsaur, could have made similar tracks. As Stimson and colleagues conclude, “multiple biotaxa of varying sizes could produce Batrachichnus salamandroides tracks.” We may never know for sure who left the tracks. The trail of itty bitty footprints can only take us so far into the past.