Flattened fossil footprints: Implications for paleobiology

• Large theropod track casts are flattened in some sand-dominated sequences.
• Flattening occurs if casts are deformed by vise-like pressures between massive beds.
• Flattened casts have deceptively wider digit traces and reduced hypex dimensions.
• Flattened theropod track casts distort original track morphology.
• Flattened theropod track casts may not be suitable for ichnotaxonomic studies.

Studies of natural casts of dinosaur footprints associated with very thin mudstone and siltstone intervals in thick sand-dominated sequences often reveal casts that are significantly flattened due to the differential effects of overburden pressures on different lithologies. They are in effect squeezed, vise-like, between two thick, non-compactable sand layers. Thus, the sand filled tracks (casts) are flattened or widened as the ductile layers are compressed. Such flattening, here described from five localities, is a previously unreported phenomenon with implications for vertebrate ichnology. Present evidence suggest that significant flattening is not evident in most sequences in which mudstone and siltstone intervals are thicker, even though overburden pressures may have been comparable. Examples from the Jurassic of North America and the Cretaceous of China show that the flattening (widening) of tridactyl theropod tracks leads to predictable changes in track cast morphology, which may influence interpretations of track maker identity, and ichnotaxonomy. In the theropod dominated samples described here, such extramorphological changes differentially affect the shape of the whole cast and individual digit trace casts making them appear more “fleshy” and sometimes deceptively convergent with ornithopod tracks.