Morphometry of Middle Bronze Age palstaves. Part II – spatial distribution of shapes in two typological groups, implications for production and exportation

Abstract

For archaeologists, metallic artifacts are key materials to assess Middle Bronze Age production areas and cultural exchanges. Here, a set of 629 bronze palstaves excavated in northern France, belonging to Breton and Norman typological groups, was treated by (open) outline-based morphometrics with orthogonal polynomial regression. Using robust statistics developed for outlier detection, these Norman and Breton palstave outlines can be divided into two groups: those for which the shape fluctuates close to the standard shape, called "congruent" axes, and those which are far enough from this standard to be considered as "non-congruent", although they possess most of the features of the typological group. The highest density of discovery (whether congruent and non-congruent in shape) is in the extreme east of Brittany for the Breton axes, while the Norman axes are concentrated in northern Normandy, hence the choice of names. However, the distribution of congruent and non-congruent artifacts appears to be spatially dependent for the Norman group, and to a lesser extent for the Breton group, as there are proportionally more congruent specimens inside the supposed production areas than outside. This contradicts the generally accepted archaeological scheme which hypothesizes that all axes in a group originate from the same production center, and that some items were exported from there to supply neighboring regions. Other minor production centers probably existed, copying the original model with greater shape variation.

Unsupervised model-based clustering for typological classification of Middle Bronze Age flanged axes

“…In fact, the presence of two distinct areas: (i) zone A, with flanged axes belonging to G1 and G2, and (ii) zone D, with axes from G5 and G6, tends to confirm a finer structuration of geographical space, already evoked by Butler (1963), and supported by the study of all metallic Middle Bronze Age artefacts discovered in north-western France (Gabillot, 2003). Note that zone D also corresponds to an area presenting a high density of contemporary Norman palstaves (Forel et al, 2009;Monna et al, 2013), suggesting the existence of a specific cultural area (Gabillot, 2006;Monna et al, 2013). Comparison with existing typologies reveals that most of the artefacts from G5 and G6 are members of concave-blade flanged axes ( Table 2).…”

“…Data acquisition, drawings, and outlines Acquisition follows the procedure described in Forel et al (2009), Monna et al (2013) and Wilczek et al (2014). Briefly, all published images obtained from the available documentation (Supplementary materials S1, Fig.…”

“…Metalworkers produced axes following set technological rules of fabrication, adapted to the social and cultural standards of Bronze Age communities (Monna et al, 2013;Gabillot et al, in press). It is tempting to think that the zones with the greatest number of axes represent the areas of indigenous production.…”

“…They provide a continuous morphospace allowing more complex statistical analyses, including the reconstruction of the mean shape and shape diversity within the group of interest (Adams et al, 2004;Navarro, 2003;Zelditch et al, 2004;Slice, 2005;Wilczek et al, 2014). Two recent studies undertaken on Bronze Age palstaves (Forel et al, 2009;Monna et al, 2013) have already demonstrated that combining geometric morphometrics with spatial analyses can be very effective for the better understanding of artefact production and use.…”

The Provenance, Use, and Circulation of Metals in the European Bronze Age: The State of Debate

“…Likewise, by combining chemical data with detailed studies of style and manufacturing traits, it is possible to assign artifacts to individual workshops or artisans, or specific metal batches, under the broader level of "provenance regions," even when the geological source of raw materials may remain unknown (e.g., Leusch et al 2015;Martinón-Torres and Uribe-Villegas 2015). Microtypological analyses, aided by geometric morphometrics, can help identify objects that were cast in the same molds (e.g., Forel et al 2009;Li et al 2014;Monna et al 2013). This information is useful for a more detailed characterization, not only of production organization or the make up of a hoard but also of mechanisms for trade and interaction across longer distances.…”