Anyone who has visited a fossil track site will know that there’s something wonderful about being able to follow the steps of a creature that was alive hundreds of millions of years ago. It’s remarkable to consider that we can find not only the remains of these organisms, but also some of the various traces they left on the world as they went about their lives.
Edward Hitchcock (1793-1864) captures this excitement vividly in his Ichnology of New England – he was an early worker on fossil footprints that are now associated with dinosaurs:
‘What a wonderful menagerie! Who would believe that such a register lay buried in the strata?… At first men supposed that the strange and gigantic races which I had described, were mere creatures of imagination, like the Gorgons and Chimeras of the ancient poets. But now that hundreds of their footprints, as fresh and distinct as if yesterday impressed upon the mud, arrest the attention of the sceptic on the ample slabs of our cabinets, he might as reasonably doubt his own corporeal existence as that of these enormous and peculiar races.’
Tracks are only one kind of ‘trace fossil’ that can be found – trace fossils encompass all the fossils that preserve the activity/behaviour of organisms – e.g. trackways, burrows, fossilised faeces etc. All these traces offer unique insights into the lives of ancient organisms and the environments they lived in.
Why study trace fossils?
The study of traces (termed ichnology) is split into two main fields: neoichnology (the study of traces made by modern organisms in the world today) and palaeoichnology (the study of trace fossils). But why study trace fossils?
Body fossils (the remains of the organism itself) can tell us a lot about what that
organism looked like and how it lived. Yet, as these are the remains of a dead animal/plant, there is a limit to what it can tell us about the behaviours of those organisms. How did they feed; how did they move? Trace fossils, being created while the organism is still alive, can offer extra information to help answer these questions.
e.g. measurements of trackways can tell us something about how that organism moved and how fast. Also, looking at different burrowing structures can tell us something about how that creature was likely to have fed – e.g. was it a suspension feeder?
Where are the body fossils?
Trace fossils can also help fill gaps in our knowledge where body fossils are scarce. E.g. the types of body fossils that preserve are very biased to groups that contain hard parts (like bones). Soft-bodied animals are much more rarely preserved, but their traces can be quite abundant. E.g. burrows made by soft bodied organisms can often be found.
Also, as trace fossils and body fossils are quite different in terms of their structure / composition – the environments that tend to fossilise them can be quite different. So, for example, you may end up with a rock sequence representing a land environment where the only evidence of animals are the various tracks and traces they created.
Trace fossils are also useful indicators of the sort of environment the rocks containing them represent. E.g. if you find bird tracks then the rocks were clearly not deposited in the deep sea!
Different combinations of trace fossils can indicate whether the environment was terrestrial or marine, how deep the water was, how oxygenated the water was etc… Groups of trace fossils that are often found together and indicate something about the environment are known as ‘ichnofacies’.
For example, the Skolithos ichnofacies is characterised by a variety of different trace fossils (including a number of vertical burrows) and is usually indicative of beach foreshore and shoreface environments (although it can occur in a variety of others – see the refs at the end for more details)
To summarise, trace fossils offer us unique insights into the behaviours of organisms and also their environments. They come in a wide range of forms so I’ll just dip briefly into tracks, burrows and coprolites to give some indication of the variety that can be found.
Tracks and Trails
Fossil tracks and trails are some of the most evocative trace fossils – seeing marks made by movement many millions of years ago is always rather exciting. Everyone is likely to have seen images of dinosaur trackways, but in fact traces of the movement of many different organisms can be found. One of my favourite examples comes from the Isle of Arran, where long, train-track like traces of many small indentations can be found (see the picture below). Looking at the size of these tracks, it is rather difficult to imagine what could have made a trace like this.
In fact it has been attributed to ‘Arthropleura’, a large millipede like creature that was around 1m long! These lived in the Carboniferous period around 300 million years ago.
Analysis of trackways such as this can tell us about the size of the track-maker, how its legs moved in relation to each other and how fast it may have moved.
A key consideration with these tracks (along with all the other traces I’ll discuss) is the issue of how do you match the trace to the trace-maker?
It’s very rare to find the trace and trace-maker preserved together, although in some cases this can occur. Some fossils from the Solnhofen limestone exhibit trackways that end, conveniently, with dead horeshoe crabs. In this case you can definitively say that the track was made by a horeshoe crab and in fact represents its final steps.
For most other cases though, this needs a bit more detective work. Comparison of fossil traces to those of modern animals can be extremely useful to narrow down the sort of organism that could have made it. Also, comparing the known fossil organisms to the size / morphology of the trace can help narrow this down.
Often though, we can only have a broad idea of what type of organism made a trace. This is not surprising considering, for one, that many of the trace-makers are soft-bodied organisms that are not well-preserved in the fossil record. Also, this is complicated by the fact that one type of trace can be made by many different kinds of organism. E.g. the simple vertical burrow Skolithos that I mentioned earlier can be made by annelids, crustaceans, phoronids etc…
Also, consider that each type of organism will make many different kinds of trace. E.g. a crab may make a burrow, but will also produce a different trace when it walks and will produce faeces (coprolites) too.
Finally, the same organism doing the same thing can look quite different depending on the material they are on. E.g. consider the shape of footprints made when a person walks on quite dry sand vs very deep, gloopy mud. These will look quite different even though the same foot created them.
All these factors mean that it is often not possible to assign a trace to one particular trace-maker. Still, even with only a broad idea of what made a trace, these fossils give valuable information about the environment the rocks were deposited in and the types of behaviours that the animals/plants were exhibiting.
Another common type of trace fossil is a burrow. Many different kinds of animal create burrows from worms to crabs etc.. Again, their different shapes and sizes are useful indicators of the type of environment the rocks were deposited in – as mentioned above Skolithos (a simple vertical burrow) is (unsurprisingly) part of the Skolithos ichnofacies.
The shapes of burrows can also tell us about how the animal inside was feeding. For example, suspension feeders (that pick small suspended particles out of the water column) tend to live in straight or U-shaped vertical burrows. Conversely, deposit feeders (that eat organic material within sediments) may produce burrows which show active infilling (as they’ve passed the sediment through their gut) that will contrast in grain size / organic content with the rock around it (e.g. Planolites).
As a final example of trace fossils – let’s take a look at coprolites. No less amazing, but perhaps less glamorous, than the fact that you can walk in the footsteps of now extinct creatures is the fact that you can also find piles of their fossilised faeces.
These ‘coprolites’ provide direct evidence of what different creatures were eating – e.g. a coprolite from a herbivore may contain fragments of plant matter, while those of a carnivore may contain bones.
Coprolites are very difficult to assign to a particular organism. It can be done in some cases though – E.g. spiral coprolites can be assigned to sharks that have spiral intestinal valves.
Trace fossils, in all their different forms, represent the behaviour of an organism. Be that walking, burrowing or defecating, these traces mark the actions of a creature that was alive millions of years ago. As a result, they give unique insights into the organisms and environments of the past.
References / suggestions for further reading
Introduction to Paleobiology and the Fossil Record by Michael J. Benton and David A. T. Harper – chapter 19 is a great introduction to trace fossils
Ichnology: Organism-Substrate Interactions in Space and Time by Luis A. Buatois and M. Gabriella Mangano – chapter 1 is another good introduction to the field of ichnology
KU Ichnology – http://ichnology.ku.edu/ – fantastic website by a group at the University of Kansas that covers the basics of ichnology and has lots of great videos/pictures of trace fossils
Palaeocast – podcast series available on iTunes – they have various episodes that address trace fossils e.g. episodes 14 and 44
Dr Peter L. Falkingham’s blog – https://pfalkingham.wordpress.com/ – blog of a fossil trackway researcher, has some great 3D models of tracks under the ‘3D data’ tab
The Complete Dinosaur by Brett-Surman, Holtz and Farlow (chapters 28 and 34) – has nice introductory chapters on dinosaur trackways and coprolites
Pond, S., Lockley, M. G., Lockwood, J. A. F., Breithaupt, B. H. & Matthews, N. A. Tracking Dinosaurs on the Isle of Wight: A review of tracks, sites, and current research. Biol. J. Linn. Soc. 113, 737–757 (2014) DOI: https://doi.org/10.1111/bij.12340 – paper on tracks present on the Isle of Wight