Formation and preservation of vertebrate tracks in semi-liquid sediments: Insights from tidal flats and laboratory experiments

Abstract

Despite the valuable palaeoecological and palaeoenvironmental information provided by vertebrate tracks, those made in semi-liquid sediments have been largely overlooked because they are assumed to be preserved as a mass of disrupted sediment and to have a low preservation potential. Nevertheless, understanding their mechanisms of formation, infilling and preservation is crucial since they could be more abundant in the fossil record than expected or be misinterpreted as other soft-sediment deformation structures. To solve these aspects, this study analyses consecutive cross-sections performed along a human track made by a shod foot in semi-liquid sediments in the upper intertidal flats of the Bay of Mont-Saint-Michel (north-west France) and monitored until its complete burial. These were compared with cross-sections of tracks made in a flume tank. Cross-sections reveal that the sediment structures associated with these tracks reflect the mechanism of their formation and infilling, and even the footstep dynamics. These structures comprise: (i) marginal rims that developed at both sides of tracks during foot penetration; (ii) upward deformation structures produced during foot withdrawal; (iii) a syn-track infilling, which almost entirely fills the tracks during the withdrawal, formed by sediment collapsed from the track walls or by liquefied sediment; (iv) a post-track infilling that fills the tracks completely during their subsequent flooding. This work demonstrates that these tracks have a high preservation potential in tidal settings, especially if they are made after the peak of a spring tide period, and undergo desiccation and consolidation during neap tides, which prevents their erosion and favours their burial by sediment. The identification of the above-mentioned structures in fossil counterparts provides useful palaeoenvironmental information, because they allow discriminating these tracks from those made in sediments with less water content and from other soft-sediment deformation structures (i.e. convolute bedding and balls-and pillows) with which they share strong resemblances.