Allopyrenis tenuis is a species of squamulose lichen-forming fungus in the family Phylliscaceae. It was first described as a new species in 1963 by Aino Henssen, as Phylliscum tenue. The type of Phylliscum tenue was collected by Henssen in Humboldt County, California, at about 1,000 m (3,280 ft) elevation. The specimen was gathered from seepage-streaked exposed volcanic rock. Matthias Schultz and MarÃÂa Prieto recombined the taxon into the new genus Allopyrenis in 2024, following a molecular phylogenetics-informed reorganization of the class Lichinomycetes.
It has dark reddish-brown, roughly rounded measuring about 0.5âÂÂ0.75 mm across. The thallus surface is because it is covered with clusters of lichenized cyanobacterial cells, about 20âÂÂ50 üm wide; it lacks both lobules and surface . Its fruiting bodies (apothecia) are small, about 200âÂÂ250 üm wide, and , with a prominent but no . The asci are club-shaped with rounded tips. In young apothecia, only a few primary paraphyses are present, and the are or only slightly opened.
Initially described from specimens collected in California, and later found in Alaska, it has also been recorded from Venezuela.