Human trace fossils

Fossils are any trace of ancient life. They can be body fossils – mineralised remains of the physical organism, such as bones, shells, teeth – or trace fossils – preserved traces of the behaiour of organisms, such as footprints, burrows, scratch marks.

Trace fossils are also known as ichnofossils, a fancier term. Ichnofossils are radically different from body fossils, not only in how they look, but how they preserve, how they are studied, and what they can tell us about past life.

I am not 100% sure of how trace fossils are preserved, since I have never really found out, but I can imagine that they are typically formed when a footprint, trackway, burrow – whatever – is overlain by a different type of sediment than it was made in. That way, the shape is preserved in the interface, or boundary, between these different sediment types. It is important that they are different types, because otherwise the actual trace would disappear in the middle. The trace fossils are exposed when one of the different layers is eroded away, so again it is important that the two sediment layers are different: one harder than the other. If the substrate where the trace was made is the one that is eroded, the trace fossil appears inverted, and the examiner needs to think backwards, or just make an inverted cast of it.

Burrows, footprints and the like are rarely well preserved, which makes them difficult to study. But, the biggest challenge of study of ichnofossils is figuring out what type of organism it belonged to. Take a footprint for example. First of all, the footprint is a reflection of the fleshy pads of the foot, so matching it with the foot bones of an animal requires quite a bit of thinking. Second, you are lucky if the footprint you have found belongs to any species whose fossils are known! Or maybe some bones are known, but not the feet? What do you do if the trace belongs to something unknown? Therefore, trace fossils are safer to match with larger groups, rather than trying to pin it down to species level. It is a shame, but cannot be helped.

Regardless, trace fossils are a valuable asset to the record of body fossils, as they are direct evidence of behaviour rather than anatomy, physiology, etc. Burrows indicate burrowing behaviour, implying an infaunal (living in the sediment) or subterranean (underground) ecology. Tracks can tell us about how the animal moved, including estimates of how fast. However, what we can learn from these traces is not very meaningful unless we can link them to any known organism, and the conclusions we draw from them must be taken with a pinch of salt.

Just as we study body fossils by comparing them with living analogues, ichnologists examine trace fossils in the light of relationships between living animals and the traces they leave. What aspects of the animals affect the traces? The answer to that question reflects what we could infer from tracks alone. Below is an example of how an ichnologist might go about studying traces of life.