A couple of weeks ago I was out walking in North Wales near Conwy, and came across this:
Intriguing! This was on the top of the hill near Conwy, and there were no animals immediately visible. But it did get my mind thinking about apparent swimming dinosaur tracks I’d seen figured before:
I’ve picked out Xing et al 2013 because it was the first paper on swimming dinosaur tracks that came to hand, but there are many others. Sometimes the tracks look more like feet than these, but not always. A mechanism for the formation of such tracks was nicely summarised in a figure by Coombs in 1980:
I’ve been fairly open with colleagues at conferences, in person, and via email, that I’m not a huge proponent of ‘swimming’ dinosaur tracks, for several reasons: Firstly, the scenario requires the water to be pretty much exactly at hip-height with very little tolerance. For individual tracks, ok, but for long trackways where track morphology remains consistent? It sets my spidey-sense tingling. Secondly, there’s an awful lot of reported dinosaur swim tracks in the literature, usually attributed to theropods (Andrew Milner and colleagues gave a nice review in The Track Book). Again, given the specificity of conditions required, it does seem a little odd to me that we would find so many. I’m yet to see any swim tracks exposed at the edge of duck ponds that have since dried up, though of course depositional conditions will be changing and that could mess things up. Thirdly, I’ve seen lots of surfaces in real and simulated deep tracks that look quite similar, and were not formed during swimming (there are better, currently unpublished examples than what I’ve linked to, that will hopefully make an appearance this year).
For clarity, I’ve no problem with theropods swimming (non-adapted chickens can do it fine, though perhaps more due to buoyancy than swimming prowess), I just don’t think we have sufficient evidence in most cases to positively (let alone ‘definitively’) attribute such a behaviour to fossil dinosaur tracks.
But I digress… Back to the welsh tracks, which intrigued me so. I was wracking my brain to think what could have made them. A walker’s stick? The scratches were always in parallel pairs, so probably not. Maybe some sort of yaktrax or other grib on a hiker’s boot? Maybe, but the marks seemed quite long and scrape-y. Could be something hanging down from a mountain bike maybe?
I kept looking around the area and found some particularly long trackways, which of course I wipped out my scale bar and camera for, and produced a 3D model (as is now a standard protocol!):
They seem to alternate fairly regularly left-right. Eventually I found some where the parallel lines met in a rounded shape:
And further still I found some that looked very hoof like:
Sure enough, before long we found some roaming horses. So my interpretation is that the long traces were made by horses breaking as they ran down the hill. I can’t be certain of that, and there’s still room for these to have been made by walkers or cyclists. I’d love to hear from readers as to any other possible interpretations.
But the moral of this story is that even interpreting modern tracks, when the trackmaker is potentially stood just a few meters away is really difficult sometimes. When working with fossils, it can be really tempting to fit the data to an idea, without really testing that idea. Coomb’s 1980 illustration is a really nice ‘just-so’ explanation, and it’s easy to fit data to the hypothesis. But is it really the best explanation in all these cases?
Falkingham, P. L. and S. M. Gatesy (2014). “The birth of a dinosaur footprint: Subsurface 3D motion reconstruction and discrete element simulation reveal track ontogeny.” Proc Natl Acad Sci U S A 111(51): 18279-18284.
Falkingham, P. L., K. T. Bates, M. Avanzini, M. Bennett, E. M. Bordy, B. H. Breithaupt, D. Castanera, P. Citton, I. Díaz-Martínez, J. O. Farlow, A. R. Fiorillo, S. M. Gatesy, P. Getty, K. G. Hatala, J. J. Hornung, J. A. Hyatt, H. Klein, J. N. Lallensack, A. J. Martin, D. Marty, N. A. Matthews, C. A. Meyer, J. Milàn, N. J. Minter, N. L. Razzolini, A. Romilio, S. W. Salisbury, L. Sciscio, I. Tanaka, A. L. A. Wiseman, L. D. Xing, and M. Belvedere (2018). “A standard protocol for documenting modern and fossil ichnological data.” Palaeontology 61: 469-480.
Milner, A. R. C. and M. G. Lockley (2016). Dinosaur Swim Track Assemblages: Characteristics, Contexts, and Ichnofacies Implications. Dinosaur Tracks. P. L. Falkingham, D. Marty and A. Richter. Bloomington, Indiana University Press: 153-181.
Xing, L. D., M. G. Lockley, J. P. Zhang, A. R. C. Milner, H. Klein, D. Q. Li, W. S. Persons and J. F. Ebi (2013). “A new early cretaceous dinosaur track assemblage and the first definite non-avian theropod swim trackway from China.” Chinese Science Bulletin 58(19): 2370-2378.
Additional models of individual scrapes: