Terpenoid composition of fossil resins from western India: New insights into the occurrence of resin-producing trees in Early Paleogene equatorial rainforest of Asia

Abstract

The terpenoid composition of well-preserved fresh and oxidized fossil resins (ambers) from Early Paleogene lignite-bearing sequences of India have been examined using gas chromatography-mass spectrometry for determining their botanical origin. The samples were collected from two sedimentary basins viz. Barmer (Akli Formation) and Bikaner-Nagaur (Palana Formation), western India. The total extracts of the studied ambers are represented by hydronaphthalenes; sesquiterpenoids and significant occurrence of diterpenoids. Such terpenoid distribution is typical for gymnosperm, especially conifer derived sources. The diterpenoids are comprised of majorly abietane, phyllocladane, kaurane, pimarane/isopimarane and podocarpane class compounds. Tertracyclic diterpenoids such as 18-norkauranes, ent-16β(H)-kaurane and 16α(H)-phyllocladane and a rearranged tricyclic diterpenoid, rosane are detected in the fossil resins from Barmer Basin. The occurrence of tetracyclic diterpenoids and rosane together with 8,13-dimethyl-16-norpodocarpa-6-ene suggests that Podocarpaceae family is the possible source of the ambers. However, chemotaxonomic assignment at family level for the fossil resin from Bikaner-Nagaur Basin is difficult due to the absence of source specific diterpenoids. Previous reports had documented the occurrence of angiosperm-derived dammar resins from Early Paleogene sediments of India (Dutta et al., 2009; Rust et al., 2010). The present study suggests that the Early Paleogene resins of the equatorial rainforests were not always represented by the dammar resins derived from Dipterocarpaceae, a family of angiosperm, some of them were also contributed by the conifers.