Thallus endo- or episubstratic, occasionally with thallus surrounding conglomerated apothecia, irregular in outline, to 1 cm in diam., thin, thallus very variable; margin indistinct; when thallus is endosubstratic it is only seen as a greyish tinge on the wood, when it is episubstratic the thallus is yellowish orange with a matt surface.  Anatomy: cortex 20 μm high. Apothecia abundant, scattered or gregarious, 0.5-0.9 mm in diam., small, sessile, occasionally forming raised conglomerates, not to slightly constricted, zeorine; disc plane to weakly convex, yellowish orange to orange, never with an olive tinge, surface structure smooth to fine granulose; margin prominent, thick, persistent, not flexuose or occasionally angular due to suppression, thalline margin yellow, proper margin yellowish orange, slightly brighter than disc, smooth. Excipulum to 100 μm wide; thalline exciple with many algal cells; proper exciple broad, prosoplectenchymatous, strongly glutinized, hyaline. Hypothecium 90 μm high, of unoriented hyphae, with lumina broader and more rounded in lower part, hyaline, with oil droplets; subhymenium very dense and thin, hyaline. Hymenium 70-80 μm high, hyaline; paraphyses not gelatinized, ramifications present; terminal cells are swollen, to 5 μm wide, hyaline; subterminal cells to 4 μm wide; epihymenium distinct, yellow to yellowish orange.  Asci 60 × 25 μm.  Ascospores c. 8 per ascus, broadly ellipsoid to ellipsoid, 16-18 × 7-9(-10) μm; spore septum 3-6 μm.

SECONDARY METABOLITES: emodin, fallacinal, parietin, parietinic acid, and teloschistin; chemosyndrome A.  All yellow parts K+ red, C-.

SUBSTRATE AND ECOLOGY: on bryophytes, wood and detritus in horizontal, eutrophicated habitats. Habitat exposed to sun, dry to moist.

NOTES:  C. sibirica belongs to the large group of Caloplaca with small, yellowish orange apothecia that are very difficult to differentiate on morphology. It is characterised by never getting an olive tinge and by the large spores, 16-18 × 7-10 μm, with a broad septum, 3-6 μm. It is earlier recorded on wood and moss from Svalbard (Søchting 1989), but is also found on rock and old bone. It has been found in North America, Svalbard, Siberia and Europe (Søchting 1989, Santesson et al. 2004, Søchting & Olech 1995).