A large class of complex branched, cyclic biomarkers composed of isoprene (C5) subunits, including mono- (two isoprene units), sesqui- (three), di- (four), sester- (five), tri- (six), tetra- (eight), and higher terpenoids. Terpenoids are precursors for important classes of hydrocarbons such as isoprenoids, steroids, hopanoids and carotenoids. They include the saturated terpanes (hydrocarbons) and compounds that may contain double bonds or other elements (in addition to carbon and hydrogen), such as oxygen. Certain structural units may be rearranged or missing compared with terpenes and terpanes. Terpenoids occur naturally in essential oils from tree resins, cellulose, fish-liver oils, and plant pigments.