Rosselia socialis from the Ordovician of Asturias (Northern Spain) and the early evolution of equilibrium behavior in polychaetes

Stacked specimens of Rosselia socialis from the Middle to Upper Ordovician Castro Formation of Asturias, northern Spain represent an example of an infaunal community able to move upwards simultaneously to avoid burial due to sedimentation from unidirectional currents. This ichnofabric is similar to those described from younger deposits and attributed to equilibrium structures produced by polychaetes belonging to the Order Terebellida. Although high density of specimens has been documented in Cambrian Rosselia suites, the Ordovician occurrence from Asturias is so far the oldest example of a Rosselia ichnofabric recording synchronous equilibrium behaviour of a whole community. Despite uncertainties derived from their patchy fossil record and potential for behavioral convergence, trace-fossil evidence seems to indicate that some groups of polychaetes were efficient burrowers since the early Cambrian. Rosselia ichnofabrics reflect an increased complexity of the trophic web and the ability of infaunal polychaete communities to colonize sandy, relatively high-energy settings by the early Paleozoic.