Bioturbation is the disturbance of sediments and substrates by organisms or plants. In some cases, organisms disturb the sediment in distinct shapes or patterns; termed trace fossils.
Most of the fossils recognized by collectors and scientists are body fossils. Body fossils represent a part of a distinct organism or plant, and usually the hard part remains of a body or plant. In contrast, trace fossils are the tracks, trails, burrows, and other markings left behind by organisms, rather than an actual part of an organism’s body. An example would be footprints. The footprint is an indentation left by a body that may remain in the sediment, even after the maker of the footprint has walked away.
Trace fossils are extremely varied in shape and size. They can be made by soft-bodied organisms or organisms with hard parts. In fact, trace fossils provide evidence for some soft-bodied organisms that lack hard parts, because soft-bodied organisms without hard parts do not usually leave body fossils. Not only do trace fossils provide evidence of the existence of ancient organisms in a particular sediment or rock bed, but in many cases, they provide insight into behaviors of organisms that could not be interpreted from their hard-part fossils alone. For example, footprints may provide information on the direction an organism is moving, if it was moving by itself or with others, if it was walking or running, etc. Trace fossil abundance and diversity can also provide information on environmental factors (salinity, sedimentation, oxygen levels, etc.) of the beds in which they are found.
It is important to understand that, unlike hard-part remains (body) fossils, (1) the same trace fossil can be made by different organisms with the same size and behavior, (2) the same organism can have different behaviors, which make different traces (e.g., a burrow, a trail, a feeding structure, and excrement all from the same individual or type of individual), (3) the same organism can leave different traces in different types of sediments (e.g., soft mud versus firm mud versus sand). The same organism can leave different traces in different parts of a bed (on top, within, or at the base).
Trace fossils are also called ichnofossils. The study of trace fossils is called ichnology.
Some Kentucky trace fossils