TYPE: MEXICO. MEXICO CITY: Tlalpan, 1926, Frère Amable s.n. (holotype, hb. B. de Lesdain [n.v., presumed lost]; isotype, UPS[n.v.]).
Description. Hypothallus endosubstratal, IKI–, no algae observed. Thallus indeterminate, rimose or areolate, dispersed or contiguous and covering up to several square centimeters, areoles irregular to angular, mostly 0.5–1.0 mm wide, 0.3–0.5 mm high, flat to convex, areoles along edges of thalli same size and shape as inner areoles or sometimes either elongate up to 1.0 x 0.5 mm or rounded up to 2 mm wide. Upper surface a pale-yellow color, epruinose, smooth or rough. Epicortex lacking. Cortex mostly 60–70 µm tall, all yellow or upper layer dark yellow and 20–30 µm thick, obscuring hyphae, lower layer white, 30–40 µm, thick with abundant crystals, sometimes with IKI+ reddish reaction, cortical cells mostly globose to oval, 2–5 µm wide, scattered algal cells sometimes in lowest depths of cortex. Algal layer scattered to dense, upper and lower edges even to uneven, uninterrupted, 30–100 µm thick, algal cells mostly 8–12 µm wide. Medulla clear or obscured with crystals, 50–100 µm thick, of hyphae intricate to vertical, 2.0–4.5 µm thick, variable. Apothecia 0.1–0.2(–0.5) mm wide, immersed, punctiform, disc black or red brown, sometimes yellow from crystals of rhizocarpic acid and epanorin, usually one per areole. Parathecium expanding up to 100 µm sometimes forming a yellow parathecial ring around the disc, but not apparent in early ontogeny of apothecia. Hymenium (120–)150–170 µm tall, epihymenium yellow to yellowish brown with crystals, variable in thickness, paraphyses mostly 1.0–1.5 µm wide, apices barely expanded or not, some branching in upper part, hymenial gel IKI+ blue becoming a dirty brownish red (hemiamyloid). Asci usually cylindrical 90–110 x 10–20 µm, 100–200 ascospores per ascus, (3.0–)4.0–5.0(–7.0)31.0– 1.5(–2.5) µm, variable but mostly narrowly ellipsoid. Subhymenium IKI+ dark blue, 30–35 µm tall. Hypothecium ca. 10 µm thick of narrow hyphae. Pycnidia ca. 100 x 60 µm, ostiole not visible on upper surface, conidiogenous cells mostly 15 x 0.5 µm, conidia mostly globose to subglobose 1.0–2.0 µm wide.
Chemistry. Rhizocarpic acid (major) and epanorin (minor) in cortex and epihymenium. Spot tests negative. UV+ orange.
Ecology and distribution. Growing on granite, sandstone, and rhyolite in full sun to shade.
Acarospora amabilis was described from the mountains near Mexico City and Puebla (Magnusson 1929a,b). Magnusson reported two specimens from the Las Vegas area (Magnusson 1930). When mostly in shade on rhyolite in Soledad Canyon in the Organ Mountains, the thalli were flat and rimose with narrow cracks, the thallus mostly 0.3 mm thick with or without rounded outer lobes up to 2 mm wide. These thalli eventually become areolate, dividing into a cluster of angular areoles divided by distinct cracks. Normally in open situations in full sun, A. amabilis has from the beginning an indeterminate thallus of angular areoles 0.3–0.5 lm thick, contiguous or dispersed, with outer areoles sometimes elongate 1.0 3 0.5 mm to sublobate without being effigurate like most specimens of the similar looking A. erythrophora H.Magn. (Knudsen 2007).
Differentiation. Two species with a high hymenium, Acarospora brouardii and the new species A. organensis differ from the rimose-areolate A. amabilis especially in having squamulose thalli (Knudsen 2007). The areolate A. erythrophora is similar looking to the rimose-areolate A. amabilis (Knudsen 2007). Both species have a high hymenium and have a similar beautiful yellow hue. Acarospora erythrophora produces epanorin (major) and rhizocarpic acid (minor) in the cortex and gyrophoric and lecanoric acid in the cortex, apothecia and medulla (Knudsen 2007). Acarospora amabilis differs in producing rhizocarpic acid (major) and epanorin (minor) in the cortex and producing no other secondary metabolites. Areolate specimens of A, amabilis could be mistaken for A. contigua but that species has a lower hymenium (85–125 µm vs. 120–170 µm). In Knudsen (2007) Acarospora amabilis was treated as a synonym of the squamulose A. socialis. Squamulose yellow Acarospora begin as broadly attached areoles. When depauperate in poor microhabitats they do not develop into squamules and are reduced in size. Acarospora amabilis was interpreted as depauperate A. socialis. It differs from A. socialis especially in having in having rimose-areolate thallus vs. a squamulose thallus. We do not recognize A. amabilis as a synonym of A. socialis (Esslinger 2019; Knudsen 2007).
Discussion. The holotype description was based on a specimen from Mexico with punctiform apothecia in an early stage of ontogeny in which it is stated that the parathecium is indistinct (Magnusson 1929b). In his study of several specimens from New Mexico, Magnusson revised the description of A. amabilis stating that the parathecium is indistinct or distinct and expanding to form a parathecial ring around the apothecial disc (Magnusson 1930). The specimen collected at Soldier’s Camp by A. Brouard (FH) Magnusson called ‘‘typical.’’ It is growing in full sun and has an areolate thallus. The species is a good example of morphological differences if growing in full sun or in partial or full shade.