Geometric Morphometrics

The study of form may be descriptive merely, or it may become analytical. We begin by describing the shape of an object in the simple words of common speech: we end by defining it in the precise language of mathematics; and the one method tends to follow the other in strict scientific order and historical continuity.  — D’Arcy Wentworth Thompson (1915)

Our emended approach to the analysis of geometric morphometrics employs the geomorph package in R. Focusing on morphological variation among several vessel forms, our recent efforts have been aimed at the analysis of intact and/or reconstructed Caddo NAGPRA vessels from the Washington Square Mound (41NA49) site; however, assemblages from several other sites are also being analyzed.

The vessels from the Washington Square Mound site were used in our first attempts to discern whether it was possible to demarcate between the various vessel shapes using geometric morphometrics. Having progressed substantially since that initial nisus, and since we plan to include all of our 3D scans for Caddo burial vessels in the geometric morphometric analysis of the Turner Collection, each previously analyzed collection is being reanalyzed using this more thorough and rigorous approach.

We have been working with a number of colleagues from around the world to further refine our method, and have defined a series of landmark configurations that can be used for the various analyses (see one of these above). This iteration has since been modified to include additional landmarks at the junction of the base/body and the body/lip. The additional landmarks were added for several reasons; chief among those is that it grants us the capacity to break out specific components of the vessels (i.e., base, body, lip, etc.), and allows us to analyze shape changes in specific surfaces through time. Once populated in Design X, the point data are exported to a .csv file. The .csv files from all of the vessels are then imported to R, where the geomorph package is used to create sliding vectors for each of the semi-landmarks prior to a Procrustes superimposition (GPA) (below) an analysis of principal components, MANOVA, shape covariation, allometry, integration and modularity, group shape differences and disparities,  and asymmetry.

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Results of Procrustes superimposition (GPA) in R.

A variety of approaches were explored with regard to sliding vectors, and we found that vectors along the splines were more true to vessel shape than surface vectors. Preliminary results of the PCA (below) highlight the variation for all intact/reconstructed vessels from the Washington Square Mound site, and further hint at the potential for a study of ceramic morphology to discriminate between discrete vessel shapes. While we are using several additional methods of analysis, we are only illustrating the PCA here until the article is published.

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While the goal of the combined analysis is to eventually posit a hierarchically-nested method of classifying Caddo vessel shape that can easily evolve based on the addition and incorporation of new data, it is not meant as a replacement for the current taxonomic definitions associated with decorative elements. In fact, we have been incorporating relative dates (see those here) associated with decorative elements to assist in better clarifying the temporal dynamics.

Taking this one step further, these 3D data are being modeled (at or below a 0.01mm tolerance with the 3D scan data), where each element (i.e., base, body, neck, rim, etc.) can be segregated and analyzed independently of the whole. This allows us to look at additional manufacturing and design trends concerned with specific elements in aggregate (i.e., how did bottle neck [or base, or body, or rim, etc] shape and form evolve through time?). The shape and form of ceramic vessels are assumed to be products of the same social parameters and craft traditions that influenced decorative elements. To what extent the variation in ceramic shape and form may or may not correlate with elements of decoration and design is a question that previously laid out of reach, but might now be considered using this systematic, rigorous and replicable method.

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Additionally, should it prove possible to identify something akin to a transitional species, that could point to several interesting social possibilities including, but not limited to, (1) identifying the locus of a specific innovation, (2) the spatial and temporal dynamics of morphological variation for specific elements (neck, body, base, etc.) of ceramic design, (3) identifying or refining social networks used by specific Caddo polities/groups during temporal periods previously defined—primarily—through design-based seriations, (4) intra/inter-polity/group variation of shape and form for ceramic design, (5) potential trade relationships based upon the presence of a specific shape/form of vessel outside of known (assumed) social boundaries, and (6) the power or influence that shifted among and between polities through time. These considerations could theoretically be couched in discussions of communities of practice, craft specialization, ceramic technological organization, politics, religion, and—possibly—inter/intra-polity disputes/warfare. Furthermore, this research design has the capacity to inform greatly upon the evolution of ceramic design as it relates to the shape and form of ceramic vessels, and by adding related qualitative measures to our results we might just have the potential to bolster evidence for human behaviors associated with ceramic production and use within the ancestral Caddo territory.

Initially a development in the biological sciences, the study of geometric morphometrics in archaeology will no doubt include some interesting discussions regarding the various analytical and theoretical underpinnings that are most appropriate for a cultural system versus a biological system. There remains plenty of thinking left to do on this subject, but based on the preliminary results, the capacity for geometric morphometrics to inform upon issues related to material culture and cultural systems could be enormous.

To view the beginnings of a similar program of analysis for projectile points, click here.

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