Symbiotic association of a pyrgomatid barnacle with a coral from a volcanic middle Miocene shoreline (Porto Santo, Madeira Archipelago, Portugal)



Abstract:  The bioclaustration of the pyrgomatid barnacle Ceratoconcha aff. costata within the carbonate skeleton of a colonial hermatypical coral (Tarbellastrea reussiana) is described from a middle Miocene basaltic rocky palaeoshore on a small north-eastern Atlantic islet of Porto Santo (Madeira Archipelago, Portugal). The resulting structure is named as a new trace fossil Imbutichnus igen. nov., characterized by a small, funnel-like cavity, a circular to oval cross-section, with a nearly cylindrical shape in the upper part and a conical shape towards the base. Imbutichnus costatum isp. nov. is defined as a bioclaustration structure produced by the overgrowth of a pyrgomatid barnacle by a coral. From an ethologic point of view, Imbutichnus is attributed to the Impedichnia category. In terms of palaeoecology, it is interpreted as the result of a parasitic relationship. This is also the oldest record of pyrgomatid barnacles in the eastern Atlantic and clearly demonstrates that the Miocene palaeogeographic range of Ceratoconcha was much wider than previously assumed.