Diagnosis. Thallus tenuissimus, continuus; blastidia et soredia destituta. Apothecia adnata, frequentia, plerumque contigua; discus celeriter convexescens; excipulum thallinum celeriter excludens; algae cellulis grandibus, ad 17.5-19.5 µm longae; epihymenium atrobadium. Ascosporae octonae, in modo A evolutae, formae Physconia, (12.5-)15.5-16.5(-19.0) x (6.0-)7.0-7.5(-8.5) µm, similes R. pyrinae sed grandiores.
Description.Thallus very thin, dark grey or grey-brown, continuous; surface plane to minutely rugose, matt; margin indeterminate; prothallus absent; vegetative propagules absent. Apothecia broadly attached, frequent, mostly contiguous, to 0.30-0.40 mm in diam.; disc black, plane, quickly becoming convex; thalline margin concolourous with thallus or brownish, <0.05 mm wide, quickly becoming excluded; excipular ring rarely present, raised. ApothecialAnatomy. Thalline exciple 40-60 µm wide laterally at first; cortex 5-20 µm wide; epinecral layer usually present, 5-10 µm wide; crystals absent from cortex and medulla; cortical cells pigmented, to 4.5-5.5 µm wide; algal cells to 17.5-19.5 µm long; thalline exciple 40-100 µm deep below; cortex absent or 10-15 µm wide, cellular; epinecral layer rarely present, or ca. 10 µm wide; proper exciple hyaline, 5-10 µm wide laterally, expanding to 15-40 µm above; hypothecium hyaline, 30-60 µm deep; hymenium 60-70(-90) µm high, not inspersed; paraphyses 2.0-3.0 µm wide, conglutinate, with apices to 5.0-6.5 µm wide, heavily pigmented, forming a dark brown epihymenium; asci 35-45 x 13-19 µm. Ascospores 8/ascus, Type A development, Physconia-type, (12.5-)15.5-16.5(-19.0) x (6.0-)7.0-7.5(-8.5) µm, average l/b ratio 2.1-2.3, lumina with apical and septal wall thickening (Physcia-like) during development, quickly becoming rounded although sometimes retaining slight septal thickening, frequently curved; torus sometimes present, very thin; wall ornamentation lacking. Pycnidia not seen.
Chemistry. Spot tests all lacking; secondary metabolites not tested.
Substrate and Ecology. On decaying plants, typically Selaginella, in high and dry alpine habitats at elevations of 3,140-4,025 m. Rinodina imshaugii has been collected with Phaeorrhiza sareptana and Rinodina terrestris. According to the MSC database R. mniaraea and R. olivaceobrunnea have also been collected with R. imshaugii, and Amandinea punctata was present at six of the eight sites where Imshaug found this species.
Distribution. A North American endemic, confined to high alpine elevations in the Rocky Mountains of the United States.
Notes. Due to the similarity of spore type, R. imshaugii is most likely to be confused with R. pyrina. Indeed, the majority specimens on which the species is based had originally been labeled as R. pyrina by Imshaug. Although its spore morphology is very similar to R. pyrina, spore size is significantly larger in R. imshaugii. Other similarities with R. pyrina are the large size of the algal cells and the dark brown rather than red-brown epihymenium usually associated with Physconia-type spores. However, the thalline margin of R. pyrina is much more persistent than in R. imshaugii and being corticolous, the species has a very different ecology. The two species are certainly closely related but the larger spore size and often subsumed thalline margin suggest that R. imshaugii is a distinct species. This conclusion is supported by the different substrates and habitats of the two species, resulting in them being allopatric with respect to elevation.
The spores of R. imshaugii may first appear to belong to the Beltraminia-type on casual inspection. Careful searching will reveal Physcia-type lumina (both septal and apical wall thickening) in developmental stages prior to full wall pigmentation and some mature spores also retain slight septal wall thickening. The small apothecia of R. imshaugii with quickly excluded thalline margins may sometimes suggest a small buellioid species such as Amandinea punctata, commonly found in the same habitat. This species has Beltraminia-type spores, a lighter coloured thallus and a dark hypothecium. It is easily distinguished by its habit when growing with R. imshaugii.
Other oro-arctic Rinodina species are widely distributed in the northern hemisphere and in the case of R. olivaceobrunnea its distribution extends into the southern hemisphere (Mayrhofer and Moberg 2002). The discovery of an apparent endemic alpine species is therefore of considerable interest and invites speculation about where else it might be found. Weber (2003) has pointed to similarities between the southern Rocky Mountain and the Russian Altai floras, suggesting that their common origin dates back to the Tertiary and a widespread, intervening oro-boreal flora. He further indicates that the remnant species of this flora are more common in middle Asia than they are in North America. The Altai may therefore be a logical place to search for other populations of R. imshaugii or its relatives. Alternatively, R. imshaugii may have evolved in situ from R. pyrina during uplift and formation of the Rocky Mountains during the Eocene (35-55 Ma).
Specimens examined. U.S.A. COLORADO. Boulder Co., Wild Basin, R.A. Anderson 2134 (COLO); Gunnison-Chaffee Cos., Cottonwood Pass, H.A. Imshaug 11649; summit White Rock Mountain, H.A. Imshaug 11003a; Lake‑Pitkin Cos., Independence Pass, H.A. Imshaug 11604; San Juan-Hinsdale Cos., ridge between Engineer and Gravel Mountains, H.A. Imshaug 17048b. UTAH. Uinta Co., summit Leidy Peak, H.A. Imshaug 17207; summit Marsh Peak, H.A. Imshaug 17263. WYOMING. Park Co., Mount Washington, H.A. Imshaug 9541; Sublette Co., Mount Lester, H.A. Imshaug 19077a (all MSC).