Life habit: saprobic Thallus: immersed Apothecia: 0.4-0.6 mm high, black, epruinose stalk: 0.03-0.06 mm in diam., central part pale, consisting mainly of parallel, periclinally arranged hyphae; lowermost part: with hyphae more irregularly interwoven; outer part: dark brown to greenish brown, the hyphae largely anticlinal and densely interwoven capitulum: broadly obovate, 0.15-0.23 mm in diam. exciple: very poorly developed, brown, formed as a continuation of the outer part of the stalk; hypothecium: pale aeruginose to brown asci: when mature 26-33 x 3-4 µm, cylindrical, with a distinct apical thickening penetrated by a fine canal, with irregularly arranged to biseriate spores ascospores: fusiform, the lowermost spore in the asci often longer and narrower than the others, non-septate, medium to dark brown, 5.5-7 x 3-3.5 µm, under the light microscope with a distinctive, minute areolate ornamentation Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: Saprobic on the bark or more rarely wood of conifers (or rarely on that of deciduous trees), in rather humid situations, often 1-3 m above the ground, on trunks, or in rather open and dry localities in cool-temperate and temperate forests World distribution: Eurasia, Australasia, and North and South America Sonoran distribution: California. Notes: Chaenothecopsis nana is easily mistaken for Mycocalicium subtile, and the ascus tip should always be checked. See under that species for comparisons.