|University of Bristol|
Once a track has been filled and buried, it must withstand the expanse of time to become a fossil. Over geological time scales, sediments may be transformed by diagenesis. Rock morphology can change and with it the fossils therein, including any tracks. This may result in the malformation or even destruction of preserved tracks.
Many sediments are reworked by bioturbation and invading sea levels (transgression). In such circumstances the preserved print may be churned and washed away. Equally, the track may be lifted away, but leave a trace in the underlying sediments.
Traces of tracks produced underneath the footprint are known as undertracks. Undertracks are often preferentially preserved because they are already buried when they are made. Most undertracks represent the footfalls of larger dinosaurs since it is unlikely that the smallest dinosaurs could produce enough pressure to create undertracks.
When we discover tracks, particularly those of limited quality, we must decide "whether the tracks were of poor quality from the time that they were made or, whether they began as well-preserved footprints but underwent deterioration prior to burial and preservation in a modified state" (Lockley, 1986).
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Lockley, M.G. (1986) The Paleobiological and Paleoenvironmental Importance of Dinosaur Footprints. Palaios. 1, pp.37-47.