Issues in the identification of umbones in California mussel shell assemblages

Archaeologists investigating California coastal sites have used fragments of California mussel (Mytilus californianus) shells with umbones for two purposes: estimating the minimum number of individuals (MNI) represented in a shell assemblage and obtaining measurements that serve as proxies of mussel valve length in studies of variation in mussel sizes. Identification of umbones depends fundamentally on the ability of laboratory personnel to identify umbones, particularly if fragments are small. However, two factors affect identifiability: degree of fragmentation and tactics used by prehistoric mussel collectors. Both factors affect the proportion of small and difficult-to-identify umbones in an assemblage. Difficulty in identifying mussel umbones can be mitigated by laboratory protocols that ensure identification of umbo fragments and assessment of the degree of success in implementation of the protocols.