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.
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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