Identifying design and reduction effects on lithic projectile point shapes

Since lithic tools are intended to accomplish certain functions as a response to environmental demands, their original design changes considerably during use. Thus, exploring variability on the original designs can be informative of cultural adaptive processes on past populations. However, the complex life-cycle of a stone tool includes loops of damage due to use followed by breakage and resharpening that dramatically blur the size and shape attributes defining the original design. Here we use the Factor Model, a statistical approach recently modified to be used in landmark data, to evaluate original design attributes versus changes attributed to maintenance activities on a sample of Southern Patagonia lithic stemmed points, including arrows and spears. The model enables the separation of shape aspects that tend to covary because of common factors affecting simultaneously the two fundamental modules of a classical stemmed weapon (blade/stem), from those shape features explained only by local factors affecting modules independently. Our results show that original design differences explain most of the total shape variation, and also indicate that maintenance patterns differ among point types considered as different weapon systems (arrows and spears). Whereas arrow reduction is focused on tip modifications, spears present a broader array of shape changes including the tip and the shoulders. These results demonstrate that disentangling the sophisticated interaction among original design and maintenance activities of lithic projectile points enables a proper and independent exploration of adaptation to functional demands and cognitive models of past populations.