Making footprints without feet

Last week saw the publication of a new paper by me and my colleague Angela Horner concerning the production of traces by terrestrially locomoting lungfish. Lungfish, as their name suggests, are perfectly capable of breathing air.  When the body of water they are living in dries up, they are able to move over land to find a new pond/lake.  Our paper documents the traces left behind by lungfish on substrate.

This has been a long time coming.  Angela and I were post-docs together at Brown University between 2012 and 2014 (I was there as part of my Marie Curie), and during discussions at a local watering hole, Angela was explaining how she had worked on lungfish locomotion, and conversation moved to what kind of weird traces they would leave.  You see, when moving over land, the lungfish tends to push the head into the substrate, and use this as a pivot point to ‘flick’ the body forward.

So, we set to and got a 1m square tray, filled it with mud, and let the lungfish move freely over it, leaving traces as it did so.  What was observed, was that the head plants left obvious deep impressions in the mud, but the body only slightly disturbed the sediment surface. The result was a series of impressions, often alternating left and right as the lungfish switched sides to plant the head.

We documented traces from the animal moving over mud and sand, as well as a resting trace in sand (the lungfish was rarely particularly cooperative).

All of the trackways are available in a Sketchfab Collection, but here’s one of the better trackways:

And here’s the resting trace, or the ‘lungfish sand angel’:

Our paper documents what might be expected if we saw lungfish trackways in the fossil record. Lungfish as a clade have been around since the Devonian, and there are plenty of body fossils and some records of lungfish burrows.  However, no tracks have ever been attributed to lungfish; could this be because no one has though to look?

We conclude our paper with a more contentious and tentative observation that there are similarities between our lungfish tracks and some early tetrapod traces, and that if discovered out of context, lungfish traces (or traces made by other head-planting terrestrially locomoting fish) might be misidentified as originating from tetrapods.

I shall leave you to read the paper and decide how much you agree with that assertion. In the mean time, here’s our subject moving gracefully across the mud: