In 2005-2006 John Speth and Matt LeDuc, at the time an undergraduate Honors student in anthropology at the University of Michigan, began a project for Matt’s Honors thesis to investigate the rims of El Paso Polychrome jars recovered from four late prehistoric village sites in the Roswell area of southeastern New Mexico. It has long been known that the ratio of maximum rim thickness to minimum vessel wall thickness, a ratio known as the “rim sherd index” or RSI, increased steadily over time, but no one had ever determined whether the change reflected increasing rim thickness, or declining vessel wall thickness, or some combination of the two. The assumption has always been that the RSI primarily monitors stylistic characteristics of the rims, although changes in average jar size, a functional property of the vessels, could conceivably also lead to shifts in the RSI. Surprisingly, nothing was known about the relationship between the properties of the rim and the size of the vessel. To investigate these relationships, Speth and LeDuc undertook a study of El Paso Polychrome jars curated by museums and research institutions in New Mexico and West Texas. In a sample of over 40 complete or nearly complete jars, we measured orifice diameter, maximum vessel diameter, vessel height, maximum rim thickness, and vessel wall thickness. We then explored the relationship between attributes of the rim and vessel size. Our first finding, one that we had not anticipated, was that El Paso jars subdivided neatly into just two distinct size classes—small and large—with virtually no overlap between them. And, as we had suspected, the average RSI values of the small and large vessels were significantly different. We used these insights to develop a chronological ordering of four late prehistoric (post-AD 1250) Roswell-area villages. The results of the seriation revealed that these villages underwent a rapid “pithouse-to-pueblo transition” during the 14th and early 15th century, a shift that may have been driven more by socioeconomic changes than by subsistence intensification. Our results will be published in 2007 by the El Paso Museum of Archaeology as part of the Proceedings of the 14th Biennial Jornada Mogollon Conference.
