This paper reports preliminary results of investigations into the use of bivalves as seasonal indicators in archaeology. Specifically, it addresses three proposed methods used for deriving seasonality for the southern California coastal region (Drover 1974; Lyons 1978; Macko 1983) based on external shell features. Basic to each of these seasonality methods is the premise that each year, usually during winter, Chione species bivalves will form an externally visible growth-cessation band (often termed the "winter" or "annular" break ring or band) along the outermost margins of their shells. As the bivalve resumes growth during the remainder of the year, this "annular" band is incorporated as a feature of the external shell structure of the bivalve. It is assumed that these "annular" bands therefore are permanent marks recording each year in the life of a bivalve across the surface of its shell from the hinge to the margin, and that the growth between each band occurred in one year. Based on this premise, three methods (each simply variations on the same theme) have been proposed to obtain seasonal data from shell remains recovered from archaeological sites on the southern California coast. A fourth method (Weide 1969) employing Pismo clams is not considered here because that species is somewhat sparse in most southern California shell middens.

The study results presented in this paper are concerned only with external shell growth. In particular, this research centers on highly defined ridges and grooves on the external shell surface. These pronounced external features, or external periodic landmarks, are presumed to represent fortnightly and annual periodicities in shell growth. The presumed fortnightly ridges of Chione clams are evident as raised concentric sculptures on the shell surface, and are each supposed to represent a two-week period of shell growth. The presumed annual bands or grooves, which more often appear as checks in the shell surface, are visible on most molluscan skeletons, and are believed to record the winter cessation of growth. Biologists counting these checks achieve the chronological age of the organism. From the archaeological perspective, it has been proposed that the season of death can be inferred from the number of fortnightly ridges per year, and from the position of the last annual band in relation to preceding annuli or to the shell margin itself (Fig. 1). This paper reports the final results of a test for three seasonality methods (Cerreto 1988).