A three-dimensional geometric morphometric study of the effects of erosion on the morphologies of modern and prehistoric footprints

Introduction: Fossilised footprints have been discovered all over the world and can provide information regarding the foot size and subsequent body size estimates of the track makers or an insight into the kinematics of the foot/lower limb. After exposure, these fossils rapidly erode. It is predicted that footprint morphology is compromised after creation, prior to fossilisation and that erosion after exposure will affect the morphology of a footprint after exposure. To date, no studies have assessed if degradation prior to fossilisation and/or after fossilisation, and subsequent exposure, affects the morphology of the print, thereby affecting any measurements taken. This study aims to quantify these pre-and post-erosional processes. Materials and methods: A set of experimentally generated footprints were created to test the effects of degradation of footprint morphology prior to fossilisation. In addition, Holocene footprints were recorded at Formby Point, Sefton, UK. In just over a week tidal action had completely eroded the Holocene beds. Photogrammetry was applied to the experimental human footprints and a selection of Holocene human and animal footprints. Three-dimensional Geometric Morphometric methods were utilised to estimate differences in shape and size. Results: Results from the experimental footprints indicate that weather action affects the size and shape of a footprint prior to fossilisation. When the weather was dry, footprint shape and size showed little difference for two weeks, but rainfall caused significant changes. The Holocene footprints show that after fossilisation and exposure to coastal erosion, footprint rigidity is highly compromised. The human footprint borders progressively recede, increasing length and width each day. Footprint depth, often used to inform upon speed and kinematics, varied considerably in one week. Some regions becoming shallower, others increasing in depth. Similar results were found for the animal footprints, but with less significant changes in shape and size determined. Conclusion: Observed significant differences in measurements result in problems for predicting stature, mass, sex, and kinematic analyses. This warrants caution when making interpretations from fossilised footprints. Rapid recording of fossilised prints from first exposure and assessing pre-fossilisation processed are necessities when recording footprint surfaces