Diagnosis. A saxicolous species with thalli composed of dull, yellow-orange to deep orange, moderately to strongly convex or even bullate marginal areoles with downward turned edges and ± flattened, angular, central areoles, their surface often coarsely pruinose (especially if growing on calcareous substrates) or epruinose (on siliceous rock), some specimens delimited by a black prothallus, apothecia immersed to adnate, deeper in color than the thallus, disc epruinose; ascospores (7.9–)8.1–9.9(–10.3) × (4.7–)4.8–5.9(–6.3) μm, with a moderately thickened, (2.6–)2.7–3.2(–3.6) μm wide septum (n = 13). The new species is reliably distinguished from other morphologically extremely similar species in Squamulea by its different ITS sequence.

Type. Ecuador, Galápagos: Santiago, directly below the summit at the NE-exposed slope, 0°12′19″S, 90°47′6″W, 828 m alt., humid zone, formerly with scrub or forest, disturbed by former grazing therefore shrubs and trees missing, artificial pampa with Solanum americanum, Portulaca oleraceum, Senna occidentalis, Borreria laevis, and grasses, basalt boulders and outcrops, on rock, 22-Mar-2006, Bungartz, F. 4711 B (CDS 56235‒holotype; GenBank Accession number nrITS: MT967440).

Chemistry. Thallus and apothecia P–, K+ purple, C–, KC± purplish, UV– (dull); thallus and apothecia with a large proportion of parietin and smaller proportions of teloschistin, fallacinal, parietinic acid and emodin (chemosyndrome A sensu Søchting 1997).

Ecology and distribution. Although most specimens examined are from the Galapagos, at least one specimen included in our phylogenetic analyses comes from the Caribbean island of Nevis (‘West Indies’; Buck 29560, MIN); the species therefore most likely is generally more common throughout the Neotropics. In the Galapagos, it occurs from the dry through the transition into the humid zone. It is a saxicolous species that grows in both shaded and ± sheltered as well as in sunny, wind- and rain-exposed habitats.

Etymology. Named as a belated recognition of Alexander von Humboldt whose 250th birthday was celebrated in 2019. Although Humboldt never visited the Galapagos, he is perhaps the most important explorer of the natural history throughout the South American continent and thus pioneered the ecology of the Neotropics, the region inhabited by S. humboldtiana.

Notes. Squamulea humboldtiana is one of several ‘cryptic’ species of Squamulea in the Galapagos. Phylogenetically, the species is well-defined forming a distinct clade together with S. chelonia and S. oceanica. Morphologically, these three species are all very similar and not well distinguished from other Squamulea species in the archipelago (particularly S. subsoluta s.l.). Specimens of S. humboldtiana are frequently found in nitrophytic, often slightly calcareous habitats; like S. osseophila also on bone. Areoles of the two species can be similar, both convex, almost bullate, but growing side-by-side, thalli of S. osseophila are epruinose, almost ‘waxy’; those of S. humboldtiana, growing on the same piece of bone, are pruinose. The thallus morphology of S. humboldtiana can nevertheless be misleading; some specimens have distinctly more angular areoles, and not all material is pruinose.