Type: Honduras. Morazán: El Zamorano, Standley, P.C. 236, (F – holotype!, US – isotype).
Description.Thallus saxicolous; upper surface whitish gray to bright white, shiny, epruinose, emaculate, not cracked; lacking vegetative propagules, but densely lobulate; lobes small to moderate-sized, 2–7 mm wide, ± rotund, eciliate, distinctly delimited along the lobe edge by a conspicuous black rim; lowersurface with a ± narrow, deep brown erhizinate, ~ 1–2.5 mm wide margin, blackening and rhizinate towards the thallus center; rhizines short, stout, black, mostly simple, rarely sparsely branched; medulla white. Apothecia rare to moderately abundant, 0.3–8 mm in diam., with a cupulate thalline margin, disc dark brown, deeply concave, imperforate; asci clavate, Lecanora-type, ascospores 8/ascus, simple, broadly ellipsoid, (10.5–)11.6–13.0(–13.5) × (6.4–)6.5–6.8(–7.0) μm (n = 40), thick-walled (~ 1 μm thick). Pycnidia immersed, flask-shaped to globose; ostiole brownish black; conidia sublageniform, (5–)6–8 × ~ 1 μm (n = 30).
Chemistry. Cortex with atranorin [P+ yellow, K+ yellow, KC–, C–, UV–]; medulla with protocetraric and echinocarpic acid [P+ yellow turning orange, K+ dirty yellowish brown, KC± rosé to faintly orange, C–, UV–].
Ecology and distribution. Central and South America (Hale 1965; Brako et al. 1985; Nuñez-Zapata et al. 2015). First reported from the Galapagos by Weber (1986), subsequently by Elix & McCarthy (1998) and online (Bungartz et al. 2016). A rare species in the dry zone and transition zone, typically found on sunny, wind- and rain-exposed boulders, outcrops and cliffs (see also comments for the sorediate P. dilatatum).
Notes.Parmotremaeborinum, P. dilatatum and P. weberi are superficially similar. They share the same white, shiny thallus with conspicuous lobulate lobes that are distinctly blackened along their edge. In the field the three species can be confused, especially if not producing soredia. Parmotremaeborinum and P. dilatatum share the same secondary chemistry, but specimens of P. eborinum are generally fertile, never forming soredia, although we observed one specimen with echinocarpic acid that is not fertile and lacks soredia [Bungartz, F. 6287 (CDS 34499)]. This material can at this point unfortunately not reliably be distinguished as either P. dilatatum or P. eborinum. In the Galapagos, Parmotremadilatatum is typically sorediate; fertile specimens of this species have not been found. Elsewhere, however, where fertile specimens of the two species have been found, the two species can also be distinguished by their spore size. Parmotremaeborinum consistently has spores much smaller (11–14 × 5–7 μm) than those of P. dilatatum (20–28 × 8–10 μm). Morphologically very similar but chemically distinct is P. weberi. With P. eborinum and P. dilatatum the medulla of P. weberi shares protocetraric acid, but instead of echinocarpic acid the latter species contains caperatic acid (KC–). Most specimens of P. weberi are sorediate (including the type). Some specimens are fertile and esorediate, but even though the ascospores of these specimens are similar in size to those of P. eborinum, this material can still reliably be distinguished by its secondary chemistry (caperatic, not echinocarpic acid).