Post-depositional processes as the main determinants for the alterations caused in cockle shells found in São Julião archaeological site (Mafra, Portugal)

Abstract

The presence of cockle shells in Mesolithic and Neolithic Portuguese archaeological sites located near the coast or estuaries is relatively common. Here we report the study of a collection of complete and fractured Cerastoderma edule (Linnaeus, 1758) shells (with umbo) recovered from a well-preserved midden dated from Late Mesolithic found at the São Julião archaeological site, unit C (Mafra, Portugal).In order to interpret the main morphological alterations observed in the archaeological shells, these were compared with modern specimens from alive animals and dead molluscs present in thanatocoenosis along the Portuguese coast, and also with shells experimentally subjected to overburden in soil.It was observed that shells from dead molluscs collected in thanatocoenosis present wear all over the surface and especially in the umbo region with the formation of a hole, which was not observed in the archaeological shells. Fractured archaeological shells resembled modern shells fractured by overburden in the laboratory. Experimentally, shells were found to be very resistant to fracture, requiring high pressure to be exerted from above the burial soil in order to crack. In usual conditions, this would correspond to a layer of soil over 20 m high, much higher than conditions that existed at the São Julião archaeological site.It was concluded that cockle shells studied in the present work were collected alive and probably consumed as food. Their fracturing was post-depositional, occurred during burial and was probably determined by the pressure exerted by a house of brick and cement that had existed above the archaeological site.