Short Paper: Fossil vertebrate tracks and indenter mechanics

A reconnaissance study is outlined showing how an improved understanding of the formation and character of fossil vertebrate tracks in soft sediments is provided by the application of the mechanical engineer’s indenter theory, supported by complementary scaled laboratory experiments. Only the footprint at the bottom of the shaft cut by the animal's limbs in general offers the best preservation of the shape of the underside of the foot. The character of the foot is less well preserved in the deformed bedding beneath the footprint and among laminae deposited in the shaft after the passage of the animal. This understanding will help to reduce errors in the use of tracks in taxonomic, biostratigraphic, palaeoecological, behavioural and environmental studies. When a limbed vertebrate traverses yielding sediment, the limbs deform the substrate into tracks which vary with the nature, size and gait of the animal, the forcefulness of its limb movements, and the sediment properties. Most previous geological work on tracks and trackways has emphasized their taxonomic, biostratigraphical, palaeocological and behavioural significance (Lockley 1986; Leonardi 1987), but without an adequate understanding of their environmental implications (Lockley 1986) and, in particular, their genesis and preservation as trace fossils. The response of an elastic-plastic material to an indenter or punch, both in terms of theory (Calladine 1969; Hill 1971; Johnson et al. 1982) and in the light of qualitative laboratory experiments, provides a general mechanical model for track formation and a basis for the assessment of track preservation in sediments. The indenter represents the animal's