TYPE: U.S.A. WESTERN TEXAS: 1879, I. Boll s.n. (holotype, G!).
Description. Hypothallus endosubstratal, no algae observed, IKI–, the endolithic base stipe-like but not distinctly raised above the substrate. Thallus of dull or shiny dark brown epruinose areoles, usually flat, often with black edges, becoming subsquamulose and slightly lobulate, covering several centimeters, contiguous or dispersed, about half sterile, 0.2–0.5 mm wide, less than 0.5 mm thick, epruinose, replicating by division. Upper surface dark brown, epruinose or pruinose, usually without fissures. Lower surface usually narrow, white, or stained by substrate. Epicortex indistinct or less than 5 µm thick. Cortex 20–30 µm thick, cells mostly round 2.0–5.0 µm, upper layer one cell thick, dark brown, swelling in K, lower layer hyaline. Algal layer 50–100 µm thick, uninterrupted, even, continuous below apothecia, algal cells 5–15 µm diam. Medulla 90–120 mm thick, obscure with substrate crystals in water, hyphae 2–4 µm thick. Apothecia immersed, usually one per areole, disc brown, epruinose, rough, 0.2–0.4 µm wide. Parathecium expanding around disc, 20–30 µm wide, of narrow hyphae, the terminal three or four cells noticeable expanded to 2 µm wide. Hymenium 60–90(–110) µm high, epihymenium less than 10 µm tall, dark brown, paraphyses 1.0–2.0 µm wide at midlevel, apices unexpanded or up to 3 µm wide in a dark pigment cap, hymenial gel IKI+ red (hemiamyloid). Asci 50–80 3 10–15 lm, hundred or more ascospores per ascus, ascospores 3.0–6.0 x 1.0–2.0 µm, variable from narrow ellipsoid to ellipsoid. Subhymenium ca. 20 µm thick, IKI+ blue (amyloid). Hypothecium narrow to indistinct, IKI–. No pycnidia observed.
Chemistry. Not producing secondary metabolites. Spot tests negative.
Ecology and distribution. Growing on noncalcareous rock in full sun in New Mexico (B. de Lesdain 1942; Magnusson 1929a, 1937).
Differentiation. In Knudsen (2007) Acarospora tenebrica was treated as a synonm of A. veronensis (for reason see discussion of A. applanata). Phylogenetic analyses found it was not closely related to A. veronensis. It is similar to A. veronensis but differs especially in having an expanded parathecium around the disc (Knudsen et al. 2011; Magnusson 1929a). We do not recognize A. tenebrica as a synonym of A. veronensis (Esslinger 2019; Knudsen 2007). Acarospora tenebrica overlaps A. applanata in anatomical measurements except A. tenebrica has a thinner cortex (20–30 µm vs. 30–60 µm) and usually has many fertile areoles without fissures. Acarospora tenebrica could be confused with A. americana, but in most specimens of A. americana the parathecium is wider around the apothecia (up to 90 µm vs. 20–30 µm) and the upper surface is usually a lighter brown or densely pruinose (Knudsen et al. 2011; Magnusson 1930). Acarospora americana can be much bigger than A. tenebrica (up to 2.5 mm wide) but is often as small as A. tenebrica in desert habitats.
Discussion. Acarospora tenebrica was described from Texas (possibly in area that is part of New Mexico now). Otherwise, it is only known from a few collections from northern New Mexico (B. de Lesdain 1942) and our collections from Chihuahuan Desert. Collections in herbaria are probably identified as A. veronensis especially any specimens annotated by Knudsen or Weber. Magnusson described Acarospora tenebrica from a single small specimen from Texas (Magnusson 1929a). He described the species as having a hymenium height of 68–85 lm, but with more specimens we found a range of 60–110 µm. This is not an unusual range of hymenium height in Acarosporaceae (Knudsen et al. 2019). One of the problems in Magnusson’s taxonomy is he often published very narrow ranges of hymenium height which he used as diagnostic characters and never revised them with new specimens (Knudsen 2004). Nonetheless with a species described from one specimen, a taxonomist can only describe its anatomy. It should always be kept in mind that the type specimen is a nomenclatural type (it establishes the name of a taxon), and only through integrative taxonomy can we understand the circumscription of a species (Dayrat 2005).