Rhizoplaca parilis

Thalli of Rhizoplaca parilis usually form tightly attached, placodioid rosettes that can be mistaken for R. melanophthalma: they have radiating lobes with slightly up-turned tips, an upper surface suffused with yellow-green usnic acid, and a dark, often glossy disc when apothecia (fruiting bodies) are present. Minute features such as lobe width, marginal crenulation (scalloping) or pruina provide no reliable separation from lookalike taxa. Instead, identification rests on a combined assessment of chemistry and DNA sequence data. Thin-layer chromatography typically reveals usnic and psoromic acids as major constituents, accompanied by constipatic, dehydroconstipatic and dehydroprotocetraric acids. Many thalli also contain lecanoric and orsellinic acids in significant quantities, while gyrophoric acid and several O-demethylated derivatives appear sporadically. No single metabolite profile is diagnostic, but the suite involving orsellinic-series compounds is otherwise unknown in the complex.^[2]

Superficially the lichen presents as a compact crust; however, rare collections possess slightly looser margins that suggest an incipient tendency toward a vagrant life-form. The photobiont is a green alga of the genus Trebouxia. Microscopic examination shows one-celled ascospores typical of the genus, but these fall within the range of dimensions reported for allied species and therefore offer little taxonomic utility.^[2]

Rhizoplaca parilis colonises exposed, nutritionally poor siliceous rocks—basalt, granite and schist are frequent substrates—yet it also tolerates calcium-rich sandstones and limestones. In North America the species occupies ecological zones from pinyon–juniper woodland through montane conifer forest to the lower fringes of alpine tundra, generally at elevations between about 2,000 and 3,500 m (6,600 and 11,500 ft). Overseas material has been gathered in analogous settings across Central Asia, western China, parts of Europe and the Andean cordillera, indicating a broad circumpolar to temperate distribution.^[2] In 2021, R. parilis was recorded on James Ross Island in the Antarctic Peninsula, where it was found growing near nutrient-enriched bird nesting sites.^[3] In 2023 it was reported from northern Pakistan.^[4]

The lichen favours fully insolated rock faces where melt-water or summer thunderstorms supply intermittent moisture. Its scattered records suggest that it may be common in suitable microhabitats yet overlooked because of its close resemblance to members of the R. melanophthalma aggregate.^[2]