Terpenoids are hydrocarbons, comprising isoprene as their fundamental building blocks, many of which serve defensive functions in plants and protect them from potential enemies in the environment. Preservation of volatile terpenoids in amber is unusual and rarely detected in fossilized remains. In the present study, a remarkable preservation of monoterpenoid constituents are detected in Pliocene–Pleistocene ambers collected from the Siwalik strata of eastern Himalaya. Amongst the monoterpenoids, eucalyptol, borneol, α-pinene and p-cymene are detected in significant abundance. The sesquiterpenoid fraction comprises both biomolecules and geomolecules. Biological compounds include copaene, selinene, OH-bearing compounds such as spathulenol, globulol and α-cadinol as well as isomers of elemene and muurolene. Calamenene and dihydro-ar-curcumene are the diagenetically altered sesquiterpenoids recorded in the samples. High abundances of biotriterpenoids like β-amyrin and α-amyrin as well as moderate concentrations of altered products such as double-bonded ketones and alcohols generated from the parent triterpenoids are detected in the ambers. The results demonstrate that the monoterpenoids, which are particularly susceptible to degradation, could survive exhaustive diagenesis and alteration over a long span of time on rare occasions. The biosynthetic pathways of the secondary metabolite terpenoids, particularly the monoterpenoids, evolved in plants long back in time and are present in the angiosperm clades that evolved much later in Earth history. This indicates the unique efficiency of these volatile terpenoid compounds as defense tools in more evolved and complex biota.

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