An Integrated Approach to the Taxonomic Identification of Prehistoric Shell Ornaments

Shell beads appear to have been one of the earliest examples of personal adornments. Marine shells identified far from the shore evidence long-distance transport and imply networks of exchange and negotiation. However, worked beads lose taxonomic clues to identification, and this may be compounded by taphonomic alteration. Consequently, the significance of this key early artefact may be underestimated. We report the use of bulk amino acid composition of the stable intra-crystalline proteins preserved in shell biominerals and the application of pattern recognition methods to a large dataset (777 samples) to demonstrate that taxonomic identification can be achieved at genus level. Amino acid analyses are fast (<2 hours per sample) and micro-destructive (sample size <2 mg). Their integration with non-destructive techniques provides a valuable and affordable tool, which can be used by archaeologists and museum curators to gain insight into early exploitation of natural resources by humans. Here we combine amino acid analyses, macro- and microstructural observations (by light microscopy and scanning electron microscopy) and Raman spectroscopy to try to identify the raw material used for beads discovered at the Early Bronze Age site of Great Cornard (UK). Our results show that at least two shell taxa were used and we hypothesise that these were sourced locally

Shell middens - Unlocking hidden archaeological information using Raman spectroscopy

The study of shell middens and archaeological marine shells is important to investigate palaeoenvironments and human development. Analytical methods, such as isotope analyses, used for these reconstructions can only be trustworthy if the shells have not been drastically changed by the effect of diagenesis or by human intervention, eg., heating of the shells. These changes are difficult to quantify by the current methods, which are either limited or destructive. In this thesis, a quantitative method for the assessment of diagenesis and the evidence of heating in sea shells has been proposed with Raman spectroscopy (RS), which is based on the inelastic scattering of light, whereby a small difference in the incoming and outgoing light energy provides highly sensitive spectroscopic information. The quantitative analysis of the Raman spectrum involved peak fitting procedures to obtain the peak position, amplitude and full width half maximum (FWHM) parameters, which were then compared between modern and ancient Conomurex fasciatus and Nucella sp. shells to obtain markers of diagenesis. Modern Conomurex fasciatus shells were heated at different temperatures and time conditions with the quantitative Raman parameters compared across different heat-treatments that were later compared to burnt archaeological Conomurex fasciatus fragments obtained from the same shell midden. The results have proven that quantitative RS is responsive to diagenetic alterations and heating of sea shells indicating an increase in the calcium carbonate crystallinity caused by the degradation of the organic macromolecules that supported the strained mineral lattice, demonstrating that RS is sensitive to indicate diagenesis in sea shells

PCA scores plots showing examples closest to the beads.

<p>Scores plots from principal components analysis showing only examples from genera with scores closest to the beads. Bead 3870 can be seen, clustered with <i>Unio</i> and <i>Antalis</i> examples on the left of the plots. Although <i>Pecten</i> examples overlap with <i>Nucella</i> and the other beads in the scores plot for the first two principal components (a), separation can be seen along the third component (b).</p

Classification results.

<p>Results of the classification performed using Learning Vector Quantization. Leave-one-out (L-O-O) classification was used for validation and the results show how the test samples were classified. Each row represents one of the 26 genera included in the analysis, as indicated on the left of the table together with the number of examples tested. The four-letter codes uniquely identify each genus (full names in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0099839#pone-0099839-t001" target="_blank">Table 1</a>). The columns show the predicted genera. Numerical values are percentages so that the element in column <i>i</i> of row <i>j</i> shows the percentage of genus <i>j</i> that were classified as genus <i>i</i> and the main diagonal shows the percentage of each genus correctly classified. The dotted lines separate Bivalvia from Gastropoda and brackets above the predicted class names group genera within the same order. The grey-scale intensities are related to the numerical values with white corresponding to 100% and the darkest grey to 0% (for which no numerical value is given). Note that rounding may result in rows not summing to 100%.</p

Great Cornard bead 3682.

<p>(a) Photograph and (b) photomicrograph of the surface. The bulk of the bead is granular but this surface has the remnants of a thin layer of a whiter, more opaque material with a finely striated appearance.</p

Dendogram for the genera most similar to the beads.

<p>Dendogram showing the results of hierarchical cluster analysis on the amino acid signatures for the genera most similar to the Great Cornard beads.</p

Great Cornard bead 3870 and <i>Antalis</i> sp.

<p>Photograph (a, b) and scanning electron microscopy images (c, d) of bead 3870 (a, c) and <i>Antalis</i> sp. (b, d).</p

Coefficient of similarity distributions.

<p>The distributions of CS values for correct (solid line) and incorrect (dashed line) classifications.</p

Details of the molluscan taxa.

<p>The number of biological replicates available for each taxonomic level (genus, family, order if available, and class) is given in parentheses.</p

PCA scores plot for <i>Patella</i>.

<p>Scores plot for the first two principal components obtained from data for shells of the <i>Patella</i> genus. The plot shows no consistent pattern with either age or country of origin. Age bins are given in thousand years.</p