Insights into the palaeo-trophic food web of the Brazilian Romualdo Formation (Cretaceous, Aptian–Albian) based on fossil mercury bioaccumulation

Abstract

In aquatic environments, mercury (Hg) bioaccumulation rates tend to reflect the organism’s position in the food web – the larger the rate, the higher their position. Here we present the first attempt to reconstruct such web for the Romualdo Formation of the Araripe Basin (Northeastern Brazil), based on Hg bioaccumulation of its fossil record; the aim is to understand possible trophic relationships between vertebrate/invertebrate species inhabiting its paleoenvironments. Observed ratios between Hg concentrations [Hg] in fossils and their surrounding concretions ([Hg]sample) suggest bioaccumulation increases with the change in feeding habits and size of the fish taxa analysed, being lowest in Rhacolepis to a maximum peak in the large predator genera Cladocyclus and Calamopleurus. Feeding habits of Vinctifer were also reviewed, and the genus was reinterpreted from filter feeder to mesopredator; durophage bottom-feeding taxa recorded values compatible with their predicted feeding habits. Low [Hg]sample ratios were observed in ornithocheiriform pterosaurs, suggesting it a mesopredators specialized in the smaller fish species, while Thalassodrominae presented intermediate to high [Hg]sample, pointing out to a unique trophic role as a terrestrial opportunistic generalist, ranging from predator to scavenger.