Mid-Holocene serpulid build-ups in an estuarine environment (Buenos Aires Province, Argentina)

A bed with several serpulid build-ups is described from a Holocene outcrop at Quequén Grande River, Argentina. The tubes were dated at 6360 ± 80 years BP. The morphology and size parameters of the serpulids were compared to different species of serpulids from the literature and tubes of Ficopomatus enigmaticus, Hydroides dianthus and Hydroides plateni from Argentinean waters. In spite of the fact that today only F. enigmaticus constructs build-ups in Argentine brackish environments, the fossil tubes could not be assigned to this species, but instead, they show close similarities to those of the two species of Hydroides. The sequence is interpreted as having originated from a brackish environment. At present, neither the endemic H. plateni nor any other species of Hydroides construct build-ups in brackish waters of southern South America. This suggests, that suitable environments to construct aggregates were more widespread than today or that the invasion of F. enigmaticus replaced Hydroides; alternatively various causes may have acted jointly.