Bioerosion versus colonisation on Bivalvia: A case study from the Upper Miocene of Cacela (southeast Portugal)

A study of the bioerosion structures and the skeletobionts associated with the most common bivalves (infauna and epifauna) from the classic Upper Tortonian site of Cacela, Algarve region, SE Portugal, revealed 24 different ichnotaxa and five systematic groups of encrusters (Foraminifera, Annelida, Bryozoa, Balanomorpha and Bivalvia). Despite a relatively high ichnodiversity, the percentage of bioerosion in the specimens analysed is quite low (10–12%). This is explained by rapid sedimentation with only short periods of exposure on the sea-floor. The dominant bioerosion structures were linked to the boring activity of nonpredatory organisms. Algal microborings are the most common, followed by annelid borings (Caulostrepsis-Maeandropolydora), sponge borings (Entobia) and ctenostome bryozoans (Pinaceocladichnus). Spatial distribution of bioerosion structures and encrusters allow the reconstruction of three successive stages. The first was restricted to the biosubstrate lifetime, with structures showing a preferred orientation and situated exclusively on the outside of the shells. The second comprises the period immediately after death, with structures that extend outwards and start with the colonization of the interior of the valves, losing their initial orientation. The third stage relates to later postmortem colonisation, with structures on both sides of the valves and without a preferential orientation