This acrocarpous moss has a narrow distribution between northern Canada and Alaska, and grows on rocks with basic chemistry, such as rocks rich in calcium.
This species is small, acrocarpous and relatively stiff. When young, the plant is green but becomes dark reddish-brown to jet black when it matures.
When moist, the leaves are falcate-secund and they do not change much when dry. Furthermore, the leaves near the base of the plant are generally smaller, gradually become larger near the apex.
The stems of A. macrosporum are red brown in colour and lack a central strand. At the base is where you can find some uniseriate rhizoids.
During the early stages of maturation the sporangium is red brown in colour and becomes black with age. Initially the sporangium is smooth at maturity and then develops 4-8 ridges with alternating depressions. These depressions eventually develop into the slits from which the spores are dispersed.Spore dispersal is hygroscopically achieved by the opening and closing of the slits, in which the tissues of the valves swell when moist and contracts when dry.
The calyptra, when present, is large, long, light-yellowish brown and covering most of the capsule. At times, the calyptra can remain on the capsule until it reaches full maturity.
The seta of this species is also red-brown in colour and is straight.