Novel oxygenated fossil nor-diterpenoids from Cretaceous amber (South-Western France) as potential markers from Cupressaceae and/or Cheirolepidiaceae

Abstract

In the frame of palaeobotanical studies aimed at identifying reliable molecular markers for the determination of the botanical origins of Cretaceous ambers, four diterpenoids from the amber from Archingeay (Charente-Maritime, South western France) were identified, after chromatographic isolation and detailed nuclear magnetic resonance studies, as 13-hydroxy- and 8,13-epoxy-nor-labdanoids. These compounds are structurally related to cupressic acid and torulosol, which are C-13 oxygenated labdanoids sometimes occurring as predominant constituents in resins from Cupressaceae s.l. and Araucariaceae. A diagenetic pathway leading from cupressic acid and torulosol to the fossil terpenoids is proposed. This involves incorporation of the parent diterpenoids into macromolecular constituents of amber during burial, followed by the progressive release of the identified nor-labdanoids during maturation. The newly identified nor-labdanoids might thus be considered as potential biomarkers for amber originating from Cupressaceae s.l and/or Araucariaceae. Nevertheless, detailed investigation of the other terpenoid assemblages from the Archingeay amber points to an origin from Cupressaceae s.l. However, since biomarkers exclusive to this botanical family have not been identified, the possibility that the amber originates from Cheirolepidiaceae is also considered.