Footprints and footpads – trusting those tricky tracks

Just a quick point this time out on an obscure (as far as the literature goes at least) and unusual little fact about footprints. If you look at the palm of you hand it is pretty obvious the each joint of the fingers and even the base of each finger on the palm has a fleshy pad on top of it, such that if you were to place you hand on some nice soft mud, you would get both a good representation of your hand *and* this would also give you a pretty clear picture of where the joints are (the gaps) and the bones are (the depressions in the substrate). You might therefore think that this pattern is pretty much the same for other animals and that hand (or foot) prints give a clear picture of the actual bones involved.

Milan 2006

A rhea foot bones superimposed on the pads and gaps of the foot. Modified from Milan, 2006.

Not so in fact (as you have probably already guessed) and for two big reasons. First off while obviously humans do at least have a nice ratio of pads-to-bones and gaps-to-joints this is not consistent. First of all many animals do not have this ratio and foot-pads can cover several bones, or several pads can cover one bone, and gaps can occur in the middle of bones as opposed to at joints. There is also inevitably an issue of natural variation here and not all individuals have the same pad structure on their feet as other members of the species and some are highly variable and can even be different on the left and right feet of an individual. As such the number and position of pads and gaps can be very different to the actual bones and joints and not much of an indicator of the anatomy of the foot.

Secondly, footprints themselves are enormously variable. Obviously it can make a huge difference whether you are making tracks on mud or sand or hard soil or whatever, and if you are walking or running you can end up leaving rather different prints. However it is perhaps not obvious just how variable these can be. You might think that if you maintained a steady pace and gait over a fairly uniform surface then the prints would be consistent. Not so – even here pad and gaps can appear and disappear from track to track and between left and right.

All this variation I should point out has been recorded in living animals and trackways from live animals including controlled experiments. As such we can be pretty confident that these effects are real and a result of variation from the animals themselves and the tracks being laid down in addition to of course the inevitable variation as a result of preservation and erosion of trackways before their discovery. The practical upshot of this is that tracks become even harder to identify and analyse since for some tetrapods at least (and much of this work has been done on ratites and thus is particularly relevant to theropods) the actual pattern of the pads and gaps in the footprint can have little to do with the foot bones that they enclosed. In short don’t trust those tricky tracks.

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14 Responses to “Footprints and footpads – trusting those tricky tracks”

  1. 1 Andy Farke 15/09/2009 at 12:20 pm

    Very interesting post! So all of those nice reconstructions that Richard Lull made of dinosaur foot skeletons are pretty speculative then. . .another childhood myth smashed to bits (which reflects the nerdiness of my childhood as well as the types of myths I have in my head)!

    • 2 David Hone 15/09/2009 at 12:49 pm

      Sorry to say, but yes, it does look like it. I should stress of course that often the prints do match the pads well and of course in some animals the pads do match the bones (like humans) so can be useful or correct. The trick of course is knowing *which* ones are correct and there we are rather stuck with extinct taxa. Certainly I’d not count some pads in a theropod and as a result go, OK, it has 4 pads and 4 phalanges – there’s jsut no basis for it.

  2. 4 Tor Bertin 15/09/2009 at 1:44 pm

    I recently read Philip Manning’s study on the 3 dimensional properties of footprints–thought it very interesting, especially the range of variation found within fresh prints from the top layer to the bottom.

    • 5 David Hone 15/09/2009 at 9:29 pm

      Well often you can at least tell undertracks from the originals which helps aleviate this problem, but certainly it is another issue that fits in here.

  3. 6 Nathan Myers 15/09/2009 at 2:39 pm

    It’s posts like this that keep me coming back. Things I never could have guessed are the root and trunk of science.

  4. 7 Peter Falkingham 15/09/2009 at 4:21 pm

    One of the problems I’ve had is convincing people that vertebrate tracks are so massively variable. At the risk of pimping my own work, it’s not just pads which can be problematic, there can be artefacts due to sediment failure, such as interdigital webbing, that have nothing to do with the track maker’s foot (Falkingham et al 2009, Palaeo 3). This variation, and that you mention, makes vertebrate ichnotaxonomoy really problematic.

    However, I don’t think we should underestimate the usefulness of tracks… Difficult as the individual fine details are to interpret, we can draw some pretty nice general conclusions from track assemblages and trackways.

  5. 8 Mike Taylor 15/09/2009 at 4:40 pm

    This kind of thing (and Phil Mannion’s observations about angulation changing in undertracks and suchlike) does rather undermine my confidence that there is ANY useful information to be got out of tracks — interpreting feels more like reading chicken entrails than science. Is there really any point in studying tracks at all? Can someone restore my faith?

    • 9 David Hone 15/09/2009 at 9:25 pm

      Well I’ll stay away form the mechanics as it’s really not my field, but certainly there is some very useful behaviour stuff that can be got from trackways (admittedly not much if any from most) and some can be useful becuase they are diagnostic (e.g. the two-toed paravian tracks – you could defintively state that there were for example dromaeosaurs in the Triassic if the right tracks turned up, even more general ones like ‘sauropod track indet.’ is handy if it tells you they are presetn in a certain time / place / fauna). So there is some stuff out there to be got for sure, however I would agree that there is less than many might think.

    • 10 Mike Keesey (@tmkeesey) 20/02/2013 at 4:44 pm

      Why are the pad patterns less interesting than the phalanges? What are you, an osteocentrist?

  6. 11 Dan Varner 16/09/2009 at 3:57 am

    Heilmann in _The Origin of Birds_ dicusses the toe pads and Lull’s ideas about them on pp.179-182.

  1. 1 Underprinting and more tricky tracks « Dave Hone’s Archosaur Musings Trackback on 09/10/2009 at 5:12 pm
  2. 2 Variation in footprints « Dave Hone's Archosaur Musings Trackback on 16/10/2010 at 12:40 pm
  3. 3 Dinosaur Duty Trackback on 22/11/2014 at 5:23 am
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