How Does a Footprint Turn Into Rock? A Narrow Set of Circumstances

Preserving a footprint requires a sequence of unlikely things. The sediment must be soft enough to take an impression and firm enough to hold it, which is a narrow range that anybody who has walked on a beach has encountered, since dry sand collapses and wet mud flows. The surface must then dry or firm up sufficiently to resist the next thing that happens to it. It must be buried by a different sediment, which requires an event depositing material without washing the surface away, most often a gentle flood or windblown sand. And the whole sequence must then be preserved through burial, lithification and eventual exposure without being destroyed. Each step is common individually and the combination is not.

Several distinct things can survive and confusing them causes errors:

• •The true track, the actual impression made by the foot on the surface it walked on
• •The natural cast, the filling of that impression by the overlying sediment, which appears as a raised shape
• •Undertracks, deformation transmitted into layers beneath the surface, which are less detailed and frequently mistaken for true tracks
• •Overtracks, where a layer draped over the impression reproduces it faintly
• •The surface as exposed may be any of these depending on which layer splits
• •A single trackway can therefore appear at several levels in a quarry face

The shape of a preserved print is not the shape of the foot, and the differences are systematic rather than random. Sediment collapses inward as the foot lifts, narrowing the impression. Mud sticks to the foot and is pulled up, deforming the rim. The animal's weight is distributed unevenly and changes through the step, so parts of the print are deeper than others and the outline reflects how the foot moved rather than its static shape. Sediment properties change the result enormously, so the same animal walking on different surfaces leaves different-looking prints. Experimental work with modern animals walking across prepared surfaces of known consistency has quantified these effects and is how track interpretation has been put on a firmer footing.

Identifying what made a print is harder than popular accounts suggest and is handled by a separate naming system. Because a track rarely occurs with a skeleton, track types are given their own names independent of any animal, which prevents an unjustified identification being built into the literature and allows the tracks to be classified on their own terms. Assigning a maker relies on matching foot anatomy, size and the geological age, which usually narrows it to a group rather than a species. Several distinct animals can produce indistinguishable prints, and one animal produces different prints on different substrates and at different speeds. Claims that a specific named dinosaur made a specific track should generally be read as a claim about the group it belonged to.

Despite the difficulties, tracks supply information no skeleton contains. They record a living animal in motion at a specific moment, including its speed, its direction and whether it was turning. They record behaviour, including group movement and interactions. They record the environment directly, since the sediment the animal walked on is preserved with the print. They occur in settings where bones do not survive, so they document the presence of animals in places and periods with no body fossil record at all. And they vastly outnumber skeletons in many formations, since one animal leaves thousands of prints in a lifetime and only one body, which means tracks give a better estimate of what was actually walking around.

Sediment must be soft enough to take an impression and firm enough to hold it, then be buried gently before the surface is destroyed, which is a narrow combination. True tracks, natural casts and undertracks look different and are frequently confused. The preserved shape distorts systematically as sediment collapses and the foot lifts, which experiments with modern animals have quantified.