Enhanced preservation of delicate arthropod trackways by microbial mat biostabilisation in a Cretaceous marginal-marine setting from Patagonia: A preliminary study of preservational implicancies in trace fossils

Abstract

Observations in several localities of the Lower Cretaceous Agrio Formation (Neuquén basin, Patagonia, Argentina) during the last years confirmed that marginal-marine environments where widespread. Particularly, the top of the unit has recently been documented as a tidally-dominated depositional setting. In the interval studied here, intertidal conditions were set on a tidal flat. The analysis of sandstone beds in different areas permitted to identify exquisite trackways preserved on ripples prior to exposure. Some are assignable to xiphosurids (Kouphichnium), while others are well preserved but lacking in features diagnostic of the producers, which are nevertheless most likely arthropods. Microbially induced sedimentary structures (MISS), and therefore, the presence of microbial mats, are known from tidal flats. The presence of wrinkle marks (a type of MISS) on the trackway-bearing levels pointed towards the involvement of microbial mats. Samples of the surface of these levels were taken and photographed under a scanning electron microscope (SEM). Between the sand particles of the uppermost millimetres of the surface, filament-like microstructures were observed, mostly oriented parallel to the surface and laterally continuous, but also some perpendicular to the bedding plane. They resemble ensheathed forms of cyanobacteria, a common morphotype involved on mat and biofilm formation commonly described in tidal flats. After this preliminary analysis, it was found that the existence of delicate arthropod trackways coincides with these filamentous mats, which are considered a necessary condition for the exceptional preservation of the traces, through the binding and biostabilisation of the layer, preventing erosion. Future studies will go into greater detail on the specific role microbial mats play in trace fossil preservation in these Cretaceous marginal-marine settings.