Gastrochaenolites as Palaeoenvironmental Information Traps: The Case of the Tholos of La Pastora (Valencina de la Concepción, SW Spain)

In the Mio-Pliocene calcareous sandstone slabs used as capstones at La Pastora tholos in the Copper Age megasite of Valencina de la Concepción-Castilleja de Guzmán (SW Spain), marine bioerosion structures produced in the 3rd millennium BCE were recognised. In this work, we focus on the perforations made by the lithophage bivalve Petricola lithophaga, called Gastrochaenolites, specifically on its functionality as a ‘trap’ of sediment of the environment where they were produced and of which there are no observable outcrops. The microfaunal content consisting of planktonic and benthic foraminifera as well as ostracods was studied. It was possible to infer the palaeoenvironmental conditions of the place where the slabs were quarried, c. 4700–4600 years ago, which correspond to a wide marine bay. The presence of benthic foraminifera like Astrononium stelligerum, Elphidum crispum, Ammonia beccarii and ostracods like Loxoconcha elliptica and Cytherois fischeri implies that the sector of the bay had a water temperature of no more than 20°C, a bathymetry between 0 and 25 m of depth. Also, episodes of fluvial influence caused a variation in water salinity between 30 and 35‰, attested by the presence of euryhaline species of ostracods from low energy environments and clay substrates, thus confirming estuarine conditions.