Article by: Lewis Haller
Edited by: J. D. Dixon and Harry T. Jones
A herd of sauropods walks along a lagoon’s shore, sinking into the mud with their weight. A Tyrannosaurus rex stalks along a riverbed, its feet compressing the sand beneath. Trilobites scuttle along the seafloor, searching for food, leaving tiny dimples in the substrate. These are all examples of how trace fossils may arise. The field of ichnology studies trace fossils, which are diverse in both their form and the rocks in which they are found. Trace fossils can be found from the Cambrian, with burrows or feeding traces, and all the way through the Cenozoic, with our own ancestors leaving their footprints around the world.
Trace fossils are an invaluable resource for palaeontologists, as they give us insights into how an organism lived. Whilst footprints aren’t as dramatic or popular as body fossils, they are much more common. This is because during an animal’s lifetime it will leave many footprints, but only one skeleton. Footprints, the focus of the rest of this article, give us amazing details about biomechanics, behaviours, and ecology of prehistoric life. However, due to the low probability that we will discover a skeleton at the end of a set of footprints, we cannot say for certain what animal made the tracks. As such, footprints are given an ichnotaxon; these work in the exact same way as regular scientific names, with a genus and species name. These are distinct from a biological taxon and aren’t necessarily tied to a specific biological species.
Footprints are formed when an animal walks across a soft surface, such as sand or mud. Whether these footprints are preserved is not just a matter of weight, however. The substrate that the animal is walking on plays an important role in preservation. The moisture levels, and the depositional environment, affect whether the footprint enters into the fossil record. If the substrate is compressible and has the right amount of moisture to hold its shape, then a footprint will more likely be preserved. If the substrate is too dry or too wet, then it’s unlikely that the shape will be retained. Likewise, if the environment is low energy, such as a lakeshore, then the footprint is less likely to be disturbed, more likely to be buried, and has a higher chance of preservation. You’ve probably experienced these parameters before when you walk along a beach, and notice only a certain section of the beach allows you to leave your traces behind.
Before we continue the dive into this topic, we need to clarify some terminology. A track is a single footprint fossil, a trackway is two or more tracks of the same type all going in the same direction. If there are multiple footprints at a location that don’t form trackways, we say these are tracks/footprints. Tracks and trackways are preserved in a few main ways: true tracks and undertracks. True tracks are the actual preserved footprints that were originally laid down by the animal, whereas undertracks represent the resulting deformation under the surface of an animal walking on top. Heavier animals result in greater compression and so their undertracks extend deeper into the substrate and deform more layers. Tracks also act as molds for a natural cast; when the footprint is filled in by another layer of sediment, if the sediment is different, a cast will be formed. This cast can be separated from the original track by various geological processes, such as erosion. Now that’s out of the way we can start to analyse what these tracks and trackways tell us.
As mentioned above, by looking at trackways we can learn about how fast an animal was moving, what way it was going, and how big it was. There are a few simple measurements we need to make in order to do this. Firstly, we need to know the stride length (the length between two footprints made with the same leg) and then we also need to know the animal’s hip height. Whilst we can directly measure the stride length, we have to calculate the hip height. Alexander (1976) worked out an approximate way to do this for dinosaurs. For any dinosaur, their hip height is four times the length of their hind footprint (or in bipedal dinosaurs, just their footprint). So, once we have hip height worked out we can then put the numbers into an equation*, and this will give the speed. The stride length and the hip height can also tell us if an animal was walking, trotting or running. By dividing the stride length by the hip height you get a ratio called relative stride length – if this ratio’s result is less than 2, then the animal was walking, if it’s greater than 2.9 then it was running, and if it’s between those values then it was trotting. These measurements give us an insight into how an animal was behaving at that moment in time. They also give us data for use in biomechanical models, improving their accuracy.
Some trackways are isolated trackways – only one animal moving along by itself. However, some trackways show evidence of many individuals travelling together. The localities where many different trackways cross each other are incredibly useful as evidence for dinosaur behaviour. For example, some of these large trackways are nearly entirely sauropod tracks, meaning that it’s likely that sauropods did, in fact, move in herds. One such site is the Davenport Ranch site in Texas, where a herd of 23 sauropods left their traces. Another such site, Cal Orck’o, a 1.6km, 100m tall wall in Bolivia, documents over 12,000 footprints. The three major dinosaur clades (sauropods, theropods and ornithischians) are represented at the site, including specifically ankylosaur tracks. These are of particular importance due to their global rarity within the fossil record. However, not all tracks that may be preserved together may have been laid down together. There could be a gap in time between one animal leaving its traces, and then another one walking through the same area days later.
There are also trackways which give us examples of potential predatory behaviour, as those of theropods have been found in (what appears to be) pursuit of herbivores. Another trackway site in Texas, this one in the Dinosaur State Park near the Paluxy River, shows a herd of 12 sauropods moving along in the same direction, with these gentle giants being followed later on by three theropods. This most likely represents tracking behaviour, with these theropods looking to target one of the herd. However, they may just be following in the same direction towards a shared resource, such as access to water.
Finally, trackways can actually give us a better insight into evolution than body fossils can. Due to the higher likelihood of trace fossils being preserved, you can work out a more accurate date for the first occurrence of a specific group. For example, within the UK, Upper Triassic-aged footprints from Bendrick Rock in South Wales tell us dinosaurs were present in the UK earlier than evidence from the body fossil record. Trace fossils also allow us to document soft bodied organisms, or details that have low chances of preservation, such as skin impressions. Skin impressions from sauropod footprints found in South Korea show that they had a polygonal texture to their skin, much like elephants do today. These textures aid in stability when walking over muddy or wet ground (like the ground the prints were preserved in).
Overall, trace fossils provide invaluable insights into the lives of prehistoric animals, and this article only scratches the surface of what can be done with them. Invertebrate trace fossils are equally valuable and deserve an entire article devoted to themselves. But vertebrate traces are especially useful when combined with the existing body fossil data. By using all the data available to us as palaeontologists, we can construct a more accurate picture of past worlds, therefore increasing our general understanding. So, the next time you’re walking along a beach, leaving your footprints behind, take a moment to consider what someone might think when they dig them up in a few million years time.
*v = 0.25g0.5s1.67h-1.17 where v is the velocity, g is the acceleration due to freefall (~9.81ms-2), s is the stride length (in metres) and h is the hip height (in metres).
Image References
[1] A sauropod manus by Lewis Haller.
[2] Diagrams showing the differences between the different track types and the distance measurements we can use for working out speed. Adapted from Lockley (1991).
[3] The dinosaur trackway surface from Cal Orck’o near Sucre, Bolivia. Image by Giuseppe Leonardi.
Information References and Further Sources
[1] Alexander, R. (1976). ‘Estimates of Speeds of Dinosaurs’, Nature, 261 (5556), pp.129-130.
[2] Lockley, M. (1991). Tracking Dinosaurs: A New Look at the Ancient World. Cambridge: Cambridge University Press. Accessed 13th December 2020. Click Here.
[3] Lockley, M., Schulp, A. S., Meyer, C. A., Leonardi, G., and Mamani, D. K. (2002). ‘Titanosaurid trackways from the Upper Cretaceous of Bolivia: evidence for large manus, wide-gauge locomotion and gregarious behaviour’, Cretaceous Research 23 (3), pp. 383-400. Accessed 13th December 2020. Click Here.
[4] Naturhistorisches Museum Basel. (2016). ‘Cal Orck’o’. Accessed 13th December 2020. Click Here.
[5] Paik, I. S., Kim, H. J., Lee, H., and Kim, S. (2017). ‘A large and distinct skin impression on the cast of a sauropod dinosaur footprint from Early Cretaceous floodplain deposits, Korea’, Scientific Reports, 7 (1), pp. 1-7. Accessed 13th December 2020. Click Here.