Random tracks

One thing about being in palaeontology is that you tend to cover quite a diverse range of subject. Working with the material available means that even specialists in one group or on one method with often dabble quite widely in other areas of research. There’s not too many theropod guys who haven’t at least looked at ornithischians or sauropods or even crocs, pterosaurs and lizards and have probably published on a few of them. So too it is with things beyond bones and again a lot of people will have had some involvement with work on eggs or trackways.

In my case this means that for all my interest in all living things and my tendency to take in local pigeons and squirrels as much as species in zoos and museums, I now also get interested even by things like random footprints in bits of sand. Even this little can provide a bit of food for thought. Here for example the bird tracks are rather deeper than those of the cat and seem to interact with the substrate in a different way. The sand hasn’t changed as such between the two sets being laid down but of course could have been wetter or drier at different times – even a single base can react very differently according to local conditions and affecting how tracks are made and, by extension, how we interpret them.

Guest Post: Tracking Dinosaurs en-masse

Sadly I wasn’t able to get across to the recent conference on dinosaur tracks that was held in Obernkirchen, Germany. However, occasional Musings contributor Peter Falkingham did make it, and he was kind enough to write up an illustrated review of proceedings.

Variation in footprints

I’ve written a bit about the variation inherent in footprints before, but now I can show quite a nice example I spotted on the beach recently. These two tracks were left by a dog running on the sand. Now I’m not sure if they are two hindfeet, two forefeet or a fore and a hind, but given the normal footfall pattern I suspect one of the former. It doesn’t really matter too much in any case given the fundamental similarity between the feet of a dog – you’d expect them to be pretty much identical.

As you can see through, it’s quite obvious that one foot has left a track rather different to the other with the two lateral footpads effectively missing. This might be down to ow the animal was running, some subtle variation in the substrate or something else (other tracks confirmed that the foot itself was normal as elsewhere there were normal tracks). Quite simply, tracks will vary and you want a decent set of them to make sure that any variations are accounted for, and therefore one must be especially careful with unusual, isolated tracks.

Tracks

While I rarely delve into ichnology (that’s the study of trace fossils for those who’ve not been keeping up), I (fairly) recently came across something that was well worth sharing on the Musings. Many aspects of palaeontology require lots of extrapolation from limited data, or complex reconstructions of various organisms or mechanics etc. to try and work out some details of the biology of an animal known only from a few partial skeletons. Happily however, there’s certainly one aspect of palaeo at least that pretty much anyone can explore experimentally at home – making tracks.

Lots of people have done lots of experiments generating tracks on different substrates using different animals or models or even just recording tracks in the wild. It’s obviously incredibly simple to do and can help generate some real data for comparison to the fossil record. In this case, some nice slick mud following a rainstorm led to some beautifully clear tracks being laid down and early morning light left them wonderfully clear. What’s nice for me is that several features that I have seen described in various archosaur tracks are shown up really quite nicely here and it’s easy to see how similar factors would have led to these being laid down in the same way.

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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.

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.

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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Limulus tracks

Despite the title of this blog, I am a genuine enthusiast of pretty much all animals, though like most I am certainly far more partial to hypercharismatic megafauna* (an apparent piece of horrible corporate-speak but actually a genuinely useful term, c/o Desmond Morris) than others. Despite my bias, this is one of my favourite fossils of all time, and sadly the photo really does not do it justice.

As many palaeontologists will tell you, and as I mentioned recently with the new Chinese theropod tracks, working out which animals left which tracks can be a very tricky thing indeed. What you *really* need therefore is something like this to help you out:

This poor little Limulus (that’s a horseshoe crab) was tottering along on the bottom of a nice lagoon leaving a beautiful trial of tracks for future palaeontologists, right up to the point where he snuffed it and he himself was covered. Yep, this is one of only a handful of fossils worldwide where we have both the tack and trackmaker in direct association such that the identity is pretty much unambiguous. This is of course incredibly useful if you want to know what kind of tracks these animals leave, and also just an amazing fossil.

It comes, perhaps not surprisingly, from the German Solnhofen beds that yield Archaeopteryx (the urvogel), numerous pterosaurs (hooray) and plenty of other fossil life including numerous insects, plants, marine reptiles and rarities like squid and jellyfish (yep, they *do* have a fossil record). Even so it’s an exceptional piece about three metres long (it carries on to the right for soem way, the crab is only about 15cm across) and of course it is a wonder that not only was it found, but recognised for what it was (not may people bother to collect invertebrate trackways, well, not huge slabs of them) but that it was recovered unbroken and in superb condition. You can see it at the Jura Museum, base of the great Helmut Tischlinger and a number of other famous fossils including an Archaeopteryx, the dimunitive theropod Juraventaor, and plenty of important pterosaur specimens.

* If you had not worked it out, it basically means ‘big exciting animals’. It was originally intended to be somewhat tongue in cheek I think, but it has gained a certain popularity.