Stories that old oyster shells tell – part 4

The following is the fourth instalment of an 8 part summary of the work I have been undertaking on British Native or European Flat Oyster shell specimens from archaeological and present day contexts. You can see previous posts about the shells of  Ostrea edulis Linnaeus by clicking here for the Oyster Variations category.

Two thousand years of eating oysters in the UK:

an archaeological perspective

Part 4

A FEW RESULTS

Some results of the analyses of the two main contributors to variability in the shells of Ostrea edulis L. are presented here. These relate first to size and, in the following Part 5, evidence of infestation and encrustation by epibionts.

Size differences

The size of oyster shells recovered from ancient sites results from a combination of factors: natural environment, genetics, and human influence. Examining size can potentially help to distinguish, for example, between oysters originating from different localities or subject to varying fishing practices. Measurements of more than 30,000 oyster shells were used initially and comparisons between the samples made by parametric and non-parametric statistical tests for various categories of sample (Winder 1993 Chapter 9 Intersite variation in size of oyster shells). These categories included samples from different geographical regions, inland and coastal sites, urban and rural sites, and various historical periods.

 FIGURE 3

Comparisons of size for broadly defined historical periods reveal interesting variations in mean size between the Roman, Saxon, Medieval, Post-medieval and Modern oyster shells. This appears to indicate statistically significant temporal differences in the average size of oyster shells. Roman shells are largest but size decreases progressively through successive periods until a recovery to almost Roman dimensions in the Modern period. The data will be reworked using more sophisticated computer software and the much larger database that has been acquired since this analysis was first completed.

Figure 3 is just a simple bar chart representation of the differences in size of oyster shells through time based on the original analysis. There are two bars for each period and these indicate the overall size for left  (blue bar) and right (red bar) valves. The sizes of the two types of shell valves are always different even for the same individual oyster. The right flat valve is smaller than the left and sits somewhat within the shallow saucer shape formed by the left valve. The perimeter of the right valve in life frequently has a flexible new growth of shell that extends to meet the edge of the left shell. In right oyster shells recovered from archaeological excavations this fragile outer margin is absent.

The final bar of the above chart, which represents results for Modern oysters, lacks figures for right valves. The reason for this is that the majority of the measurements in this category were taken from fresh living specimens of oyster in which it was not possible to determine with accuracy the dimensions of the in situ, almost embedded, right shells. The maximum diameter dimensions of the entire oyster were recorded in living specimens. Right valve measurements of living oysters would not be directly comparable in every instance with right valves of archaeological specimens.

In the next instalment, Part 5 will discuss some of the results from analysis of infestation characteristics in archaeological oyster shells.

N.B. Please leave a comment or e-mail me directly on winderjssc@aol.com if you would like to ask any questions or to have a free copy of the complete article sent to you as a pdf file. This article is just a very brief summary of my archaeological oyster research. A small selection of references to publications and reports will be provided with the article.

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