 The holotypes of Archaeomarasmius consist of mushrooms and associated basidiospores. Specimen AMNH NJ-90Y is a nearly complete mushroom, broken off near the base of the stipe (stem). The pileus (cap) is up to 6 mm (0.24 in) in diameter and has a convex shape sporting an umbo (a broad raised central region). The mushroom is a medium-dark brown color with thin, minutely textured flesh and an incurved margin. The lamellae or gills are distantly spaced, with 12 gills extending fully from the cap edge to the stipe, and lack lamellulae (short gills which do not reach the stipe from the edge of the pileus). The pileus is centered on the stipe, which is 0.5 by 2.2 mm (0.020 by 0.087 in) long and is broken off above the base. The stipe lacks a veil and is smooth and cylindrical. The top of the pileus is exposed on a fracture plain, and to prevent oxidation, the area was coated in a fine layer of synthetic resin, which also resulted in slightly improved visibility of the mushroom.
