Diagnosis. Differs from P. subisidiosum by its smaller, ± angular, flattened and closely appressed lobes with simple, cylindrical, mostly unbranched isidia; the thalli growing on rock, not bark.
Type: Ecuador. Galápagos: Isla Floreana, trail going to Post Office Bay off the dirt road between highlands and Puerto Velasco Ibarra, cliff at NE side of trail (Mirador), 1°17′4.298″S, 90°26′36.60″W, 365 m alt., transition zone, ~ 10 m high lava cliff within dense scrub of Clerodendrummolle, Crotonscouleri and Macraealaricifolia, and one single Acaciarorudiana tree, on top of the basalt cliff, sunny, wind- and rain-exposed, 25-Jan-2011, Bungartz, F. 10207 (CDS 47626 – holotype!).
Description.Thallus saxicolous; upper surface white to whitish gray, dull to shiny, epruinose, densely reticulate-maculate, rarely becoming cracked; abundantly isidiate; isidia marginal and laminal, cylindrical, mostly simple to sparsely branched, occasionally clustered, typically longer along the lobe margin, commonly with a brown to blackened tip, eciliate; lobes small to moderate-sized, 0.3–4(–5) mm wide, ± angular, axils rounded, but not forming conspicuous loops, margins sparsely to abundantly ciliate; cilia short and slender, 0.2–0.5(–0.7) mm long, black, mostly simple, very rarely branched; lower surface black, gradually becoming dark brown towards the margin, lacking a distinctly erhizinate zone, the marginal rhizines not much shorter than the central ones; rhizines long, slender, black, mostly simple, rarely sparsely branched; medulla white. Apothecia and pycnidia not observed among the Galapagos specimens.
Chemistry. Cortex with atranorin [P+ yellow, K+ yellow, KC–, C–, UV–]; medulla with salazinic acid [P+ deep yellow, K+ yellow, soon turning blood-red, KC–, C–, UV–].
Etymology. Named for its saxicolous substrate and its isidia.
Ecology and distribution. Currently known only from the type and presumably endemic, collected on top of a basalt cliff in the transition zone of Floreana Island.
Notes. In Galapagos, P. saxoisidiatum is not easily confused with any other reticulate-maculate species that contains salazinic acid in its medulla. In Galapagos the only other isidiate, reticulate-maculate species containing salazinic acid is Parmotrema subisidiosum. It is much more common and more abundant, and also grows on bark, not on rock. The species can be distinguished from P. saxoisidiatum by larger, rotund, undulating lobes that are abundantly ciliate, and isidia that are soon conspicuously coralloid. In South America, two other species that resemble P. saxoisidiatum have recently been described: Parmotrema anchietanum Marcelli, Benatti & Elix and P. asperum Benatti, Marcelli & Elix (Benatti et al. 2008). Both species also have reticular maculae, contain salazinic acid within their medulla, and are also saxicolous. However, unlike P. saxoisidiatum, these two species are sorediate species, not isidiate.