The discovery of a new foliicolous microthyriaceous fungus associated with Quercus L. from the Siwalik (Miocene) of the Western Himalaya

Microthyriaceous fossil fungi (especially thyrothecia) are well-reported in the Cenozoic sediments worldwide. However, very few microthyriaceous fossils on leaves have been reported to date. Here, we describe the in-situ discovery of a unique microthyriaceous fossil fungus, Palaeomicrothyrium miocenicum gen. et sp. nov., which exhibits a combination of morphological characteristics not observed in any extant fungus. This new fossil fungus was discovered on cuticular fragments of a compressed, serrated margined dicotyledon leaf resembling Quercus L. (Fagaceae), recovered from the middle Siwalik sediments (Late Miocene; 12–18 Ma) of the Himachal Himalaya, western India. Palaeomicrothyrium miocenicum, similar to the modern microthyriaceous genus Microthyrium Desm., is characterized by superficial, flattened thyrothecia with cells of the upper wall radiating in a parallel arrangement from the distinct darkened, circular, central ostiole. It also preserves some unique characteristics such as dark brown, long, septate ascomatal setae (common in some modern members of Microthyriaceae, Trichothyriaceae and Chaetothyriaceae), meandering interwoven hyphae (common in modern members of Micropeltadiaceae), and cylindrical, erect, unbranched, septate, macroconidiophore (found in some reported modern asexual genera namely, Antidactylaria Z.F. Yu, M. Qiao & R.F. Castañeda, and Isthmomyces Z. F. Yu, M. Qiao & R. F. Castañeda of Microthyriaceae). These morphological characteristics are not reported in known extant fungal taxa, suggesting that P. miocenicum likely represents a new epifoliar fungus. The preservation and unique characteristic combination of this fossil fungus provide critical insights into the ancient morphological states during the diversification of modern microthyriaceous fungi. The in-situ presence of sexual and asexual morphs, as well as microthyriaceous germlings of P. miocenicum in considerable numbers on the same host leaf cuticles, suggests a possible biotrophic relationship in the ancient warm and humid tropical climate of the Himachal sub-Himalaya during the depositional period.