Molecular and morphological insights into Phaeoceros himalayensis (Notothyladaceae) and related species: evidence for two new species from Thailand

The hornwort genus Phaeoceros is morphologically diverse, particularly in sporophyte and spore characters. Among its members, P. himalayensis and P. kashyapii, two previously known species from the Himalayan region, are distinct in their stalked tuber thalli, dark brown sporophytes with valves apically adherent at maturity, and vermiculate spores with or without hump-like projections or verrucae on the distal face. In this study, we combine detailed morphological and molecular evidence to investigate species boundaries within the group of species possessing these traits, comprising P. himalayensis, P. kashyapii, and related taxa.

Within Phaeoceros, two fully supported clades were recovered, here recognized as subgenus Phaeoceros and subgenus Himalayanus, subgen. nov. Molecular and morphological data also support the recognition of two new species, P. aequatus and P. stenothallus, both from northern Thailand. The new species are distinguished by unique spore ornamentation together with the production of tubers.

These findings support the ancient evolutionary divergence of the Himalayan Phaeoceros lineage and underscore the taxonomic significance of spore features and tuber formation. This study adds valuable information to our knowledge of hornwort diversity and evolution, providing a foundation for future systematic and evolutionary studies.