Microbial influence on the formation and subsequent changes of vertebrate tracks: field experiments on present‐day coastal sediments

Abstract

The presence of microbial mats is often invoked to explain the good preservation of vertebrate tracks, because they can cover and biostabilize such structures. However, microbial influence on the sediment properties when the track is made and on the track characteristics has not been so thoroughly analysed. This study presents the results of field experiments consisting in the production of human tracks in different inter- to supratidal coastal environments of Argentina that show different degrees of biostabilization. The biostabilization degree, together with the characteristics of the sediment below, strongly control depth, appearance and anatomical fidelity of the tracks, and also influence their subsequent evolution. The development of an epibenthic mat in wet porous sand confers the surface a great resistance to deformation, resulting in shallow (millimetre to 2 cm) tracks with high anatomical fidelity that deform the mat plastically and are resistant to subsequent currents. This contrasts with the tracks made in poorly colonizedsand, which are 2 cm-deep, show vertical walls and are completely eroded by subsequent currents. The development of a biofilm over frequently flooded, wet, soft, muddy or muddy to sandy sediment does not change significantly its rheological properties, resulting in several centimetres-deeptracks with vertical walls, which are preserved through time, although progressively reshaped by currents and burrowing. The development of a biofilm or a microbial mat on supratidal, hard, muddy or mixed muddy to sandy sediments retains moisture and increases adhesiveness, leading to theformation of poorly defined shallow (less than a millimetre to 1 cm) tracks by surficial sediments sticking to the sole and tearing during foot withdrawal. Rapid hardening by desiccation may help to preserve these tracks in supratidal flats.