A recent discovery of ‘giant’ sauropod tracks in France has got the media all of a flutter and it seemed a pertinent opportunity to return to the concept of ‘tricky tracks‘ and the misinterpretation of fossil footprints. The media are especially impressed that some of these impressions are nearly 2 m across and while I have not seen anyone *directly* claim that these match the feet that left them (and nor have I looked that hard), I rather imagine most people will jump (not at all surprisingly) to that conclusion. But is this really the case? Are there sauropods out there with a pes six feet across?
Well once again that rhetorical question at the end of the first paragraph has a pretty obvious answer – no, not really. While I have not seen any researchers quoted on the French tracks or indeed seen any decent close-ups, I find it hard to credit that there were sauropods with feet this big, since frankly they would have trouble getting their feet past each other when the walked, and scaling up from the bones of sauropod feet an animal leaving tracks that big would be getting on for a size that is hard to comprehend. Hundreds, even thousands of tons I imagine – in other words, beyond credible. So what’s going on here?
One explanation is that the tracks as preserved are showing the effects of the substrate they were made in. In short, a heavy animal walking across very soft mud will gunge and slop the stuff everywhere and will leave a wide area affected by its passing at each point that a foot hits the substrate. In other words, big feet and soft mud can make for even bigger tracks.
However I suspect the answer is another related but somewhat different artefact – underprinting. Imagine a nice heavy sauropod putting its foot down on some relatively soft, but still firm, sediment that lies in multiple layers (like lots of mud layers that have built up on a tidal flat over a few weeks for example). Now the actual print the animal leaves on the surface of the mud will likely be quite clear and deep, and will indeed match the foot that left it. But as we move down through the layers the force will dissipate and spread out. On the second layer the impression will be less deep, less clear and, crucially, rather bigger. Go down a few more layers experiencing the same effect and what you are left with might well be recognisable as a sauropod footprint, but this undertrack might also be several times bigger and not very distinct. You might well have a 2 m wide sauropod track, but not a 2 m sauropod foot. An incredibly important, and hardly subtle distinction, but one rarely, if ever, discussed in the media or even some palaeontology textbooks.