Underprinting and more tricky tracks

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.

7 Responses to “Underprinting and more tricky tracks”

  1. 1 Oliver 09/10/2009 at 6:24 pm

    I was wondering about those 2m feet when reading articles on the find, so thanks for clearing that up.

  2. 2 Roger 09/10/2009 at 7:07 pm

    Good post. I assumed that was the case when I read the news reports, but I didn’t see anyone else mention it…

  3. 3 steveshervais 09/10/2009 at 11:59 pm

    Not quite in the serious vein that prevails on this blog, but monster footprints have been seen before.


    I had hoped to find a simple link to the image, but this was the best I could do.

    Dave can delete this, if he likes.

    • 4 David Hone 10/10/2009 at 3:44 pm

      Well there are lots of ‘monster’ tracks out there on the web, just as there are lots of photos of fairies (insects), dinosaurs (elephants) and pterosaurs (birds). Though at least the footprints do often get distorted and are this easier to misidentify!

  4. 5 Ron 01/06/2014 at 6:19 am

    I think that’s a reasonable explanation. Even the largest known sauropods wouldn’t have left tracks 2 meters wide. Also not discussed is whether sauropods when walking could have placed a hind foot partially in a front foot footprint, leaving the false impression of an extremely large footprint. I see this all the time with mammals. A certain deer track, for example, may appear gigantic but close inspection of a fresh, well-formed track reveals superimposition as the culprit.

    On the other hand some sauropods were true behemoths and I wonder if such beasts could successfully traverse thick mud. As we walk bipedally, we exert maybe 6 lbs/square inch of force on the soil under our foot. We are NOT good marsh or mud walkers. Despite the fact that even when walking, the great sauropods probably always kept three feet against the soil, we are still talking about enormous forces. Using the very rough assumption that the latest giant sauropod found in Argentina had 1,600 square inches of plantar area per foot, we get over 36 lbs/square inch of force on the soil under each foot in contact with the ground. Once such an animal–kind of like a large truck stuck in mud up to its axles– got bogged to its belly, it likely couldn’t escape. Up and down movement of its legs would make its situation worse, not better.

    I realize sauropod footprints are usually found in stone that was wet or muddy soil when the animal traversed it. On the other hand, this is an environment most likely to produce deep tracks. Sauropods, because of their weight, may have been excellent judges of mud and recognized those places safe to walk and those places with especially thick mud that were dangerous.

  1. 1 Guest Post: Tracking the hand prints of sauropods « Dave Hone’s Archosaur Musings Trackback on 30/06/2010 at 8:58 am
  2. 2 Did Wee Little Sauropods Stand Up to Run? | Dinosaur Tracking Trackback on 02/11/2010 at 2:09 pm
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