Description.Thallus of dispersed areoles 0.3–1.0 mm wide, 0.2–0.5 mm thick, rarely contiguous, not becoming lobulate, usually with one apothecium, though in European specimens sometimes having several apothecia. Upper surface epruinose, dull brown (black in some European specimens), sometimes with a black edge around the areole. Epicortex thin or lacking. Cortex usually 15 μm thick, rarely thicker up to 30 μm, narrow upper layer brown, lower layer hyaline, cortical cells 3–6 μm wide. Algal layer uninterrupted, 50–130 μm thick, algal cells variable in size. Medulla 140–300 μm thick, usually not obscure, of intricate hyphae 2–4 μm wide. Apothecia immersed, disc 0.2–0.5 mm wide, brown, epruinose. Parathecium indistinct or up to 15 um wide, not expanding around disc. Hymenium 65–90 μm , rarely 100 μm high, epihymenium brown 15 um thick, paraphyses 1.5–2.0 μm thick, hymenial gel IKI- red, hemiamyloid. Asci 60–70 × 14–20 μm, ascospores 3.5–5(–6) × 1.5–2.0 μm, ellipsoid. Subhymenium usually 15–20 μm tall. Pycnidia rare, conidia 1–1.5 × 0.5 μm.
Chemistry. Not producing secondary metabolites. All spot tests and UV-reaction negative.
Distribution and ecology. Northern Asia, Europe, North America on granite and volcanic rock, very rarely on calcareous rock. It is probably rare in North America. Magnusson and Knudsen have identified a few specimens from Maine (UPS), Missouri (NY), New Mexico (ASU) and from California [Santa Barbara (W), San Gabriel Mountains (FH), Santa Cruz Mountains (NY), and San Bernardino Mountains (SBBG).]
Discussion. Many specimens of Acarospora veronensis in North American herbaria are misidentified. Weber annotated all small brown C- species of Acarospora as A. veronensis. Knudsen in the Acarospora treatment in the Sonoran flora, unsure how to circumscribe the species as morphological species concept, annotated specimens of A. americana, A. applanata, A. cinereoalba, A. superfusa, and A. tenebrica as A. veronensis. In later studies including using phylogentic analysis these species have been taken out of synonymy with A. veronensis. The picture of A. veronensis in Vol. 3 of the Sonoran flora is an untypical specimen of A. americana.
Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2007. Lichen Flora of the Greater Sonoran Desert Region. Vol 3.
Thallus: areolate to rimose-areolate, squamulose or verruculose, overall up to 4 cm wide areoles: round to angular, sometimes lobed, 0.2-0.5(-1.5) mm in diam., less than 0.6 mm thick, dispersed or contiguous; rim: down-turned, often a darker brown to black upper surface: usually medium to dark brown, seldom light brown, sometimes black, dull, plane to convex, smooth or rarely fissured, epruinose to heavily pruinose upper cortex: paraplectenchymatous, thin, 10-20(-55) µm thick; cells: distinct in K, round to rectangular, (1-)2-3(-5.2) µm wide or 4-9 x 4 µm; syncortex: lacking or up to 20 µm thick; eucortex: upper layer dark brown and c. 10 µm thick, lower layer hyaline algal layer: even medulla: white, subparaplectenchymatous to prosoplectenchymatous, continuous with attaching plectenchyma, distinct in K lower surface: narrow, corticated, cells one to several layers thick, usually white, sometimes dark or light brown, or light green from algal layer; sometimes raised above substrate layer at least 10 µm by stipe, allowing soil particles to collect under squamule, sometimes giving edges of scattered squamules a fuzzy appearance at 10 X attachment: broad to rarely forming a stipe Apothecia: deeply immersed, rounded to angular, one to several per areole, 0.1-0.4(-1) mm in diam.; cortex between apothecia usually without fissures; sometimes apothecia merging leaving sterile remnants as an umbo or peninsula disc: dull reddish brown to blackish brown, plane to convex, smooth to rough parathecium: 10-20µm wide, sometimes merging cortex overlapping disc epihymenium: light yellow or reddish brown to dark brown; sometimes with dark pigment caps, 10-20 µm thick hymenium: hyaline, ±110 µm tall; paraphyses: 1.3-2.1 µm wide at base, spreading (in K); apices 3.3-4.1 µm wide; rarely bifurcating in upper third subhymenium: hyaline or pale yellow, indistinct, 10-15 µm thick; hypothecium: 10-50 µm thick asci: cylindrical, 75-90 x 15-23 µm, 100+-spored ascospores: hyaline, simple, narrowly to broadly ellipsoid, (3-)3.8-5 x 1-2.1 µm Spot tests: UV-, all spot tests negative Secondary metabolites: none detected. Substrate and ecology: on acidic rocks and basalt, occasionally on wood World distribution: Northern Hemisphere Sonoran distribution: common in Arizona, southern California, Baja California, Baja California Sur, Sonora, and Durango (just outside of the Sonoran region). Notes: Acarospora veronensis is interpreted as an extremely variable species, and it is one of the most common species in the Sonoran region. Acarospora americana was applied by Magnusson to specimens usually forming a rimose-areolate crust. Acarospora cinereoalba (Fink) H. Magn. and A. superfusa H. Magn. were applied to specimens that are heavily pruinose, sometimes forming verrucae with a single disc. Such specimens are common in arid areas of southern California, and in the deserts of Arizona and Mexico, but similar specimens have been seen collected from near waterfalls in Minnesota (Fink, MIN) and from New York (Wetmore, MIN). Some of these heavily pruinose specimens can only be distinguished from similar, non-faveolate specimens of A. obpallens by the C test or TLC. Sometimes they look like A. strigata, which always has wider spores. As with several other highly polymorphic Acarospora taxa, combined molecular, morphological and ecological studies may eventually segregate new species from this species concept. As with yellow Acarospora, the narrow taxonomic concepts of Magnusson are avoided (Knudsen 2004).