Pannaria squamulosa

Pannaria squamulosa forms an effuse (spreading) thallus up to 4 cm in diameter. The thallus consists entirely of incised, grey-blue to olivaceous squamules (scale-like structures) up to 3 mm wide and about 150 μm thick. The upper surface has a well-developed paraplectenchymatous cortex (a tissue composed of cells arranged in a jigsaw puzzle-like pattern). These squamules rest upon a well-developed, crustaceous, blackish prothallus (the initial growth of the lichen), which creates a distinctive contrast with the grey-blue squamules.^[2]

Apothecia (fruiting bodies) are common, particularly in the central parts of the thallus, and can reach up to 1.5 mm in diameter. They have a flat, brown disc and a squamulose, grey-brown thalline margin up to 150 μm thick. The hymenium (fertile tissue layer) is about 100 μm high and stains blue with iodine in the vicinity of the asci, which lack internal amyloid structures. The asci contain eight spores each, and the ascospores are colourless, simple, subglobose (almost spherical), measuring 10–15 by 8–10 μm, with a smooth surface. All chemical spot tests (PD, K, C) are negative, and no secondary metabolites (lichen substances) are detected by thin-layer chromatography.^[2]

Pannaria squamulosa is a corticolous species, meaning it grows on tree bark. As of its description in 2003, it was known only from forests in the Cape Province of South Africa, including the type locality at Diepvalle Forest Reserve and the Ysternek Nature Reserve. It was found growing on native trees including Cunonia capensis.^[2] The species appears to prefer humid forest environments in the Cape region. Its restricted distribution may indicate that it is part of the distinctive Cape floristic element, which includes many endemic species.^[2] Its known range was expanded considerably when it was reported from the remote tropical island Réunion in 2011. There, it was found growing on bark and overgrowing mosses growing on rocks.^[3]