A Bichordites-dominated ichnofabrics from Spanish Pliocene calcarenites: traces of marine life in migrating dunes

Abstract

A series of Pliocene ichnofabrics is described from the Cuevas Formation, Almería-Níjar Basin of south-eastern Spain, where a thick, cross-stratified, mixed bioclastic-siliciclastic succession is exposed along a laterally continuous section. It records the dynamic conditions of ancient subaqueous dunes during their deposition and the activities of organisms colonizing them. The ichnofabrics are dominated by Bichordites, traces likely made by burrowing sea urchins adapted to live in shifting sand. Ichnofabrics range from those showing weak bioturbation with little else but Bichordites (representing high-energy, continuously migrating dunes) to ichnofabrics featuring a high degree of bioturbation containing a low to moderately diverse ichnofauna (representing more physically stable environments where organisms could gather food in less agitated waters). Strong burrowers like the Bichordites producers could have acted as ecosystem engineers. Piscichnus producers may have preyed on benthic sand-dune organisms. The non-uniform distribution of ichnotaxa in these Pliocene sand dunes, alongside previous studies, suggests that these ancient bioturbating communities may have been similar to those in modern seas.