TYPE: U.S.A. NEW MEXICO: environs of Las Vegas, A. Brouard 19950 (holotype, HB. B. DE LESDAIN
[n.v., presumed lost]; isotype, US!).

Description. Hypothallus endosubstratal, IKI–, no algae observed in substrate. Thallus of flat to convex areoles contiguous in patches up to 5 cm wide, or dispersed, as a lichen, or as a facultative lichenicolous lichen, scattered among other lichens, morphing out of usually Aspicilia and Acarospora
species, areoles 0.3–1.5 mm wide, 0.2– 0.5 mm thick, broadly attached, sometimes developing an elevated mycelial base, or becoming subsquamulose with narrow white underside. With or without abscission fissures, mostly sterile, replicating by division. Upper surface dark brown to light brown, shiny or dull, epruinose unless morphing out of Acarospora cf. strigata. Lateral and lower surfaces ecorticate, white. Epicortex variable, thin to 30–50 µm thick. Cortex 30–60 lm thick, cells distinct in water, round to irregular, 3–5 µm, upper layer brownish, 10–30 µm tall, lower layer hyaline. Algal layer
70–100(–150) µm thick, even, continuous under apothecia, algal cells dense 5–12 µm diam., usually uninterrupted but with rare hyphal bundles 5 µm wide especially in large squamules. Medulla 100–200 µm thick, continuous with attaching hyphae, thinwalled, 3–5 µm thick, lower portion often obscured by crystals.
Apothecia immersed or elevated in parathecial crown, black, epruinose, rough, punctiform to 0.4 mm wide. Parathecium 10–40 µm wide, rarely visible as a parathecial ring around the disc the color of the thallus. Hymenium usually 85–100 µm tall but could have wider range, epihymenium brown, 10–20 µm tall, paraphyses mostly 1.0–1.5 µm wide, apices expanding to 2 µm in dark brown gel caps, hymenial gel IKIþred (hemiamyloid). Asci 70–80 x 10–20 µm, hundred or more ascospores per ascus, ascospores usually 3–5 x 1–1.5 µm. Subhymenium 20–30 µm tall, IKI+ blue. Hypothecium 10 µm thick of narrow hyphae.
Pycnidia rare, conidia bacilliform 1.0–2.0 x 1.0–1.5 µm.

Chemistry. Not producing secondary metabolites. Spot tests negative.

Ecology and distribution. Growing as a lichen in full sun on sandstone, rhyolite and andesite,
usually forming large patches or as a facultative lichenicolous lichen, parasitic on usually Aspicilia
and Acarospora, often dispersed among the host thallus. Known only from New Mexico at elevations
of 1590–2000 m, common on west side of Organ Mountains, rare on sandstone at Carrizozo Malpais,
and frequent on yellow sandstone in Las Vegas region (B. de Lesdain 1932; Magnusson 1930). Distribution expected to be wider in southwestern U.S.A. and in Mexico.

Differentiation. Acarospora applanata was treated as synonym of A. veronensis in Knudsen (2007).
Influenced by Weber’s broad concept of A. veronensis in his annotations, Knudsen was uncertain
of the species boundaries and treated A. americana H.Magn., A. applanata, A. cinereoalba (Fink) H.Magn., A. superfusa, and A. tenebrica as synonyms of A. veronensis (Knudsen 2007). Acarospora americana, A. cinereoalba and A. superfusa have already been taken out of synonymy with A. veronensis (Knudsen et al. 2011; Lendemer & Knudsen 2011).
The phylogenetic analyses do not recover Acarospora applanata as closely related to A. veronensis. Acarospora applanata differs from A. veronensis in having a thicker cortex (30–60 vs. 10–15 µm) and usually an expanded parathecium as well as being a lichenicolous lichen (Knudsen et al. 2011; Magnusson 1929a). We do not recognize A. applanata as a synonym of A. veronensis (Esslinger
2019; Knudsen 2007). Acarospora applanata is sympatric with A. tenebrica and can be hard to
determine based on appearance unless A. applanata is cross hatched with fissures and mostly sterile.
Besides usually being covered with fissures, A. applanata overlaps the anatomical measurements of A. tenebrica but has a thicker cortex (30–60 µm vs. 20–30 µm). Acarospora americana occurs at least
in northern New Mexico and differs from A. applanata in having a usually smooth thallus vs. a
usually fissured thallus, a wider parathecial ring if present (ca. up to 90 µm vs. 40 µm wide), and in not
being a lichenicolous lichen (Knudsen et al. 2011; Magnusson 1930).

Discussion. Acarospora applanata is not an obligatory parasite. It is often a solitary lichen pioneer. But it is a facultative, opportunistic parasite. When being lichenicolous, A. applanata enters the microbiome of other lichens as a juvenile fungal parasite, expropriates the algae of the host and destroys the mycobiont, morphing out of the host, forming a new and different lichenized thallus (for example and discussion of morphing see Knudsen et al. 2014). It was not included in the latest checklist of lichenicolous fungi (Diederich et al. 2018). In herbarium collections it is expected to often be found mixed with other crustose lichens more often than solitary and non-parasitic. Acarospora applanata is often sterile and dissected by fissures. For propagation, A. applanata depends on asexual replication and parasitism more than sexual reproduction based on our field observations. Magnusson (1930) saw only two specimens of A. applanata and did not describe fissures, but the large population in the Organ Mountains also had many areoles without fissures. Magnusson never recognized replication by division though it is actually quite common in Acarosporaceae.