British distribution: Throughout Britain.
Orthotrichum anomalum is a locally frequent moss on walls and boulders and is an associate of species such as Schistidium apocarpum and Grimmia pulvinata on mortared walls. It usually demands moderately high levels of calcium and is no doubt sensitive to the acidification effects of sulphur dioxide pollution as well as to the sulphite toxicity that affects most mosses. It seems to have increased here in Paisley in recent years, presumably as levels of sulphur dioxide pollution have fallen.
It is, however, known to occur also on base-poor rocks, though not commonly, and such colonies are reported as showing slight morphological differences from those on base-rich rocks. Perhaps there is some ecotypic variation within the species.
It is a cushion moss, adapted to avoid water-loss on the exposed boulders that constitute its most typical habitat. However, unlike related species, the capsules project just a little beyond the leaves. The narrow, recurved-margined leaves close together on drying but spread widely when moist, as shown in the photographs. The cells of the distal part of the leaf are small and thick-walled, no doubt aiding resistance to water loss.
This species is autoecious (male and female gametes produced on the same plant) and typically produces abundant capsules (sporophytes). Reliable spore-production no doubt aids it as a rapid, urban colonist of new or short-term sites. As shown in the photographs, the capsule is protected when young by a thin sheath, the calyptra, which is noticably fibrous-hairy.
Photographs: Paisley, Renfrewshire, March 2002 (same colony but on different days).