Thallus immersed, usually effuse, indistinct or slightly ‘bleaching’ the bark. Photobiont absent.
Anamorph: not known.
Teleomorph: ascomata apothecia, 0.2-1.4 × 0.1-0.4 μm, rounded, elliptic to linear, rarely branched or substellate, black, not pruinose, usually immersed then erumpent and often with a thin ‘margin’ of bark tissue; in section 40-60 μm tall. Epithecium brown or olive-brown, K+ pale green. Hymenium 26-50 μm tall, hyaline. Hypothecium indistinct, to 10 μm thick, hyaline. Interascal tissue of paraphysoids 1-1.5 μm diam., numerous, the apices thickened to 3.5 μm diam. with dark brown walls. Asci 32-43 x 18-25 µm, saccate, almost sessile, the apex very broad and rounded, distinctly thickened with a very broad shallow subapical chamber, 8-spored. Ascospores 13-23 × (4-) 5-7 μm, cylindric-ovoid to cylindrical, the apical cell not enlarged, 3- (to 5-) septate, hyaline, without a gelatinous sheath or perispore.
Chemistry: lichen products not detected by TLC.
Assessed by Woods & Coppins (2012) as of Least Concern.
Distinguished from Arthonia radiata by the less conspicuous, apparently non-lichenized thallus, and rarely branched, shallower apothecia. Arthothelium lirellans looks similar in the field, but has muriform ascospores.
Common throughout the British Isles and Ireland, except in heavily air-polluted areas. BLS map here.
On smooth bark of a wide variety of trees and shrubs; often a primary colonizer of twigs along with Arthopyrenia punctiformis.