Abundant tetracyclic terpenoids in a Middle Devonian foliated cuticular liptobiolite coal from northwestern China

Cuticular liptobiolite coal, defined by its dominant cutinite, is rare and only known in a few regions of the world. An occurrence of this type of coal, found in the Devonian of the Hefeng Basin, Xinjiang (northwestern China), is described for the first time by organic petrography and biomarker analysis. The cuticular liptobiolite has distinct sheet-like layers and thus splits easily into leafy laminae, which are segregated during weathering, forming a so-called “paper coal”. It is comprised dominantly of cutinites (> 75 vol% on a mineral-free basis), with minor vitrinites (< 20%). Lycopsids are suggested to be the dominant coal-forming plants. The aliphatic fractions of these samples contain only small amounts of n-alkanes and common biomarkers, such as steranes and hopanes. Tetracyclic diterpanes (CnH2n-6) with mainly beyerane and atisane skeletons are the dominant constituents. The abundant tetracyclic diterpanes present in this and other Devonian cuticle-rich coals indicate that their parent natural product precursors occurred abundantly in the early vascular land plants of the Devonian, i.e. lycopsid and psilopsid species. It seems that these early land plants secreted massive amounts of resins from their vascular tissues coating their cuticle surfaces to protect against water evaporation. Massive cuticle accumulation in the Middle Devonian likely resulted from a simple assemblage of unique early land plants.