Clypeosphaeria mamillana is a saprotrophic fungus in the family Clypeosphaeriaceae classified within the class Sordariomycetes. This microscopic ascomycete decomposes dead woody plant material, particularly dead stems and branches of various herbaceous and shrubby plants, playing a role in nutrient cycling in temperate European ecosystems. The species is distributed across northwestern and northern Europe, with confirmed records from Belgium, Denmark, Norway, and Sweden, occurring in deciduous woodland, hedgerow, and scrubland habitats where suitable dead plant substrate is available. Clypeosphaeria mamillana produces small, black, flask-shaped ascocarps, known as perithecia, embedded within the surface of decaying plant material and visible as minute raised bumps, a morphology reflected in the species epithet mamillana, meaning nipple-like or breast-shaped. The asci within the perithecia contain elongated, multi-septate ascospores adapted for wind dispersal. As a lignicolous fungus, it contributes to the breakdown of complex plant polymers including cellulose, hemicellulose, and lignin in the forest litter layer. The species is classified as Not Evaluated by the IUCN, as most saprotrophic fungi of this type have not been formally assessed due to the challenges of accurately determining population size and geographic extent for microscopic organisms.