Geometric morphometric analysis of grain shape and the identification of two-rowed barley (Hordeum vulgare subsp. distichum L.) in southern France

Hulled barley is one of the most frequently recovered cereals in European archaeological sites from Roman and medieval periods. In southern France this cereal is common in carbonized contexts such as cultural layers, ditches, pits, hearths, etc. The distinction between the two subspecies, two-rowed (Hordeum vulgare subsp. distichum L.) and six-rowed barley (H. vulgare subsp. vulgare L.) is usually based on morphological characters. The following criteria can be used to discriminate both subspecies from archaeological remains: the number of fertile spikelets per rachis segments, the linear or horseshoe shape depression of the lemma base, the maximum width of the caryopses and the proportion of twisted grains. The recovery of thousands of caryopses, some clearly twisted, and of rachis segments with sterile spikelets from the site of Petit Clos (Perpignan, Pyrénées-Orientales, France) dating to the Roman period suggests that both subspecies were cultivated during this time in southern Gaul. However evidence for two-rowed barley is usually scarce in archaeobotanical reports from Roman and medieval sites. To confirm the presence of two-rowed barley in the carbonized assemblage from Petit Clos and its cultivation, we developed a new method for analysing caryopses shape using geometric morphometrics with landmarks and sliding semi-landmarks. We compared modern reference specimens to the archaeological grains from several excavations from southern France dating from the 1st to the 11th century AD. Several varieties of both subspecies were correctly identified in the modern reference sample using GMM, both before and after carbonization. Archaeological specimens could then be accurately identified. The results confirm that both subspecies of barley were cultivated in southern France during the Roman period.