Today Peter Falkingham, fresh from finishing his PhD (congratualtions by the way), tells us about his work on dinosaur footprints. Some prints are rather enigmatic and Peter has been working on how they might have formed:
When I was young, and just starting my long journey towards palaeontology, many of the older dinosaur books I had still displayed sauropods submerged in water, their necks sticking out like snorkels, and the text explaining that this was why dinosaurs like Brachiosaurus had nostrils on the top of their heads. Of course, that view was eventually replaced, and sauropods were brought onto the land. But that left us with a little anomaly – manus only trackways. How these trackways were formed has been discussed for some time.
Initially, it was thought that the track makers were buoyed up by water, and were punting off the bottom with their front legs. A few later studies explored the possibility of preferential preservation of the manus impression due to sediment properties and undertrack phenomena.
In a paper published online today (30/6/10), myself and colleagues have used computer simulation to experimentally test (virtually) how manus only trackways can be formed. By using finite element analysis (FEA), an engineering technique usually seen in palaeontology applied to bones, we can simulate a virtual substrate, and indent it with a virtual foot (see Falkingham et al. 2009, 2010 for earlier studies). There are pros and cons to using virtual methods over physically pushing a foot into a box of mud, but in this case it allowed us to rapidly generate both the substrate and the feet, and easily alter variables. It also allowed us to look at the tracks made by a multi-tonne animal, something rather more difficult with physical modelling.
Before going on and discussing the paper, it’s probably worth giving you a 101 on soil mechanics. For the cohesive substrates we were simulating (think muds and clays), deformation has two parts, an elastic phase and a plastic phase. When the applied load is low, we get elastic deformation, which is recoverable (i.e., if you remove the load, the substrate returns to its original shape). However, if a given load is exceeded (the bearing capacity), the substrate fails, and deforms in a non-recoverable manner.
With this in mind, let’s look at the study itself. My colleagues and I simulated track formation by two sauropods; Diplodocus and Brachiosaurus. In order to simulate track formation, we needed the foot outline, the mass, and the CM position in order to apply the correct force to each foot. For the foot shape, we used reconstructions by Wright (2005), and for the mass and CM we used values published by Henderson (2006).
FEA Brachiosaurus trackways generated in virtual substrates of 3 different shear strengths (Cu). Darker areas represent deeper impressions. As the substrate becomes firmer, there is a range of substrates (in this case, around 80 kN/m2) where only the manus produces tracks.
Depending on the centre of mass (CM) in a quadrupedal animal, force (from its weight) is applied accordingly to the fore- and hind limbs. A CM position at the hip, or more correctly directly above the hind feet, will result in the hind feet supporting 100% of the load. As the CM moves towards the shoulder, a greater and greater proportion of the mass is supported by the front limbs, which subsequently apply a greater force to the substrate. On top of this, the shapes and sizes of the feet differ between the front and back, altering the pressures exerted on the substrate.
By altering the properties of the substrate, we found the values of shear strength (which defines the bearing capacity) where tracks would form under the forefeet and hind feet. In simpler terms, we looked at how soft the substrate had to be to deform under the manus or pes, and leave a track. We showed that for the values used, there was a range of soft substrates for both dinosaurs where both the manus and pes would leave tracks, resulting in a full trackway. However, above this shear strength, there was another range where only the front or hind feet could produce tracks. In Brachiosaurus, which had a rather anterior CM, there were a range of substrates where only the manus indented to a noticeable depth, leaving a manus-only trackway. The reverse was true for Diplodocus – there was a range of substrates where only the pes was able to indent due to the more posterior CM.
Our paper presents a relatively straightforward mechanism for manus-only track formation that doesn’t rely on complex sediment mechanics or submerged sauropods. This in itself has implications not only for the palaeobiology of these animals, but also palaeoenvironmental interpretations based on the substrates their tracks are found in. The study also illustrates the danger of inferring locomotion from trackways, and not only for sauropods. There are some animals for which the jury is still out on whether they were bipedal or quadrupedal – some pro-sauropods for instance. To base interpretations on isolated trackways may be to do so where only half of the information has been recorded.
The paper in question is released online today:
Falkingham, P. L., Bates, K. T., Margetts, L. and Manning, P. L., in press. Simulating sauropod manus only trackways. Biology Letters.
References:
Falkingham, P. L., Margetts, L., Smith, I. M. and Manning, P. L., 2009. Reinterpretation of palmate and semi-palmate (webbed) fossil tracks; insights from finite element modelling. Palaeogeography, Palaeoclimatology, Palaeoecology, 271(1-2): 69-76.
Falkingham, P. L., Margetts, L. and Manning, P. L., 2010. Fossil vertebrate tracks as palaeopenetrometers: Confounding effects of foot morphology. PALAIOS, 25(6): 356-360.
Henderson, D. M., 2006. Burly Gaits: Centers of mass, stability, and the trackways of sauropod dinosaurs. Journal of Vertebrate Paleontology, 26(4): 907-921.
Wright, J., 2005. Sauropod tracks and their importance in the study of the functional morphology and paleoecology of sauropods. In: Curry Rogers, K.A. and Wilson, J.A. (Editors), The Sauropods: Evolution and Paleobiology. University of California Press, Ltd., London, pp. 252-284.
Dave Hone's Archosaur Musings 
From an engineering perspective, one might guess substrate preservation, bedding place complexity (homogeneity and inclination) and animal stance, gait and mass centroid would be central to this issue. Day (Science, 2002) suggests that manus negative rotation that increased surface shear. The large, flat hindfeet impressions are described as shallow and not well preserved, particularly if the substrate is prone to water erosion effects, in an aquatic or wetland setting.
The concave shape of the front foot, placing enormous weight onto a relatively small surface area with rotation, for animals with head and shoulder weight placed well forward over the shoulders would be key.
Interesting reading, thanks!
Peter, that’s awesome work! I was always quite suspicious of all the swimming sauropod stories, especially after attending Adolf Seilacher’s trace fossil course in Tübingen (although I must admit that I slept about 20% of the time, when other students were AGAIN too dense to get the concept of an undertrack for the umpteenth time).