Podocarpaceae and Cupressaceae: A tale of two conifers and ancient adhesives production in South Africa

Abstract

Research on ancient adhesives from the South African Stone Age is expanding, driven by excellent preservation conditions of adhesives and the potential to address diverse archaeological questions. These adhesives are primarily characterized through microscopic and chemical analysis. Despite geographic variability, a consistently identified component is Podocarpus resin or tar. We challenge these identifications, considering another Podocarpaceae genus, Afrocarpus, and the Cupressaceae genus Widdringtonia. Gas Chromatography-Mass Spectrometry was employed to analyze molecular signatures of modern wood, tar, resin, and seed cones from these genera. The results form an extensive reference database and reveal challenges in distinguishing these genera based on the diterpenoid signature. While Podocarpus is frequently cited, we advocate for a broader classification as Podocarpaceae when phenolic diterpenoids are found in high abundances and pimaranes and abietanes in lower abundances, and Widdringtonia when the opposite is true. The study differentiates materials used in adhesive production, including leaves and wood, highlighting the significance of α,ω-dicarboxylic acids, hydroxy acids, n-alkanes, and alcohols. Tars produced from leaves are characterized by odd-numbered n-alkanes, while tars produced from twigs and branches are characterized by long-chain α,ω-dicarboxylic acids, hydroxy acids, and alcohols. Because the differences between these adhesives in terms of raw material procurement and production are great, a more nuanced and cautious approach that acknowledges the challenges in differentiating tree species on a molecular level and considers archaeological and environmental context is required.