True tracks, undertracks and eroded tracks, experimental work with tetrapod tracks in laboratory and field

To be able to distinguish between true tracks, undertracks and tracks altered by erosion is of great importance in tetrapod track ichnotaxonomy. This paper reports three experimental approaches to the study of undertracks using the footprints of an emu (Dromaius novaehollandiae). Experiment 1 describes successive horizontal sections down through a plaster cast of an emu footprint emplaced in soft mud, revealing a steady downward decrease in the area of the track, particularly the length. This is the result of the movements of the trackmaking limb during impact, forward swing of the body and final kick-off. Experiments 2 and 3 describe vertical sections cut through footprints emplaced in packages of layered, coloured cement, admixed with water to produce different consistencies, firm and semi-fluid. After hardening, the cement block was serial-sectioned vertically, and removal of a lightly cemented layer gave access to two undertrack bedding-plane views. Successive undertracks downward show an increase in horizontal dimensions and decrease in vertical topography of the structure, representing gradual degradation of the track anatomy with depth. In experiment 3, the true track at the tracking surface was deformed by collapse and flow of the substrate after withdrawal of the foot. In this case the undertracks that were formed in layers subjacent to the foot reproduced the morphology of the foot more faithfully than did the true track.