Triplinerved cinnamon from the Siwalik (middle Miocene) of eastern Himalaya: Systematics, epifoliar fossil fungi, palaeoecology and biogeography

Cinnamomum Schaeffer (avocado, family Lauraceae), commonly known as cinnamon tree, is a highly diverse, economically important evergreen element of tropical and subtropical regions of the world. In the present work, several compressed leaf remains similar to modern leaves of Cinnamomum are recovered from the lower part of the Siwalik strata (middle Miocene) of Darjeeling foothills, eastern Himalaya. The fossil specimens are characterized by an elliptic to ovate lamina, acute to short acuminate apex, round to acute base, basal as well as suprabasal acrodromous type of primary venation, and anomocytic type of stomata. Based on leaf architecture (macro- and micromorphological features) and using leaf epidermal anatomy, these Siwalik fossil leaves are confidently assigned to the genus Cinnamomum and are recognized as a new species: C. miocenicum Mahato, Hazra et Khan. The evidence of current Siwalik specimens and earlier-reported fossil species similar to thermophilic Cinnamomum in appreciable numbers from other Siwalik localities (Darjeeling, Uttarakhand, and Nepal) suggests that Cinnamomum was a common Siwalik element, pointing toward the existence of tropical, warm and humid climate conditions during the Siwalik sedimentation time. This conclusion is also supported by epifoliar fungal remains recovered from cuticular fragments of this lauraceous taxon. This finding also represents an essential source of data for understanding Cinnamomum’s evolution and deep time diversification. We review in detail the biogeographic history and suggest possible migratory routes of the genus from an Asian perspective.