Beachcombing for bioarchaeology
On Saturday I took a walk along the beach from Preston to Weymouth in Dorset, looking for shells and other invertebrate remains to identify and add to my reference collection. Building a collection of modern material is important, as it is much better to have physical examples for comparison than pictures online or in books. Looking at my collection, I had last visited these beaches in 2008, when I found numerous slipper limpets (Crepidula fornicata). These aren’t particularly useful for British archaeology, as they are an accidental introduction from North America which arrived during the nineteenth century when a new species of oyster was imported from the United States to be grown in British waters. They are significant in the ecosystem however. They eat plankton either in suspension in the water or in deposits, and can out-compete many native molluscs including our native oyster Ostrea edulis. They are often found stacked on top of one another – the oldest animals at the base of the stack are females, the youngest at the top males. When females die, the males nearest the bottom become female.
Despite the dominance of slipper limpets, a few other species were to be found, especially on Weymouth beach. Mollusc shells included the queen scallop Aequipecten opercularis, the dog cockle Glycymeris glycymeris, the great top shell Gibbula magus, the common otter shell Lutraria lutraria, and the common whelk Buccinum undatum.
As well as mollusc shells, I was looking for bryozoans. These are colony-forming invertebrates that are quite often found on marine shells from archaeological sites, and occasionally in sediments on coastal sites too. Identifying the species on shells may help identify the region or environment from which the shell was harvested. Back in 2013, I published a (now happily open access) paper reviewing bryozoans in archaeology, and I was keen to build up my reference collection.
On a frond of Fucus seaweed, I found a colony of Electra pilosa, a relatively common lower shore species found on all British coasts.
The zooids (individual animals) of this particular colony had especially pronounced medial proximal spines looking rather like bristles:
The inner surface of the otter shell I found was also home to two species of bryozoa – Plagioecia patina, which forms disc-shaped colonies and is found in shallow water on all British coasts, and Tubulipora plumosa, which is also found on all British coasts. Along with these were long tubes secreted by the worm Pomatoceros triqueter. In his 1978 report on the shells from the Graveney boat, Ken Thomas noted that the presence of organisms on the inner surface of a shell from an archaeological context suggests that it was already an empty shell at the time it was collected from the shore, meaning it is not food waste, so may have arrived at a site as a curio or for some other reason.