[2] It is distinguished by its pale to dark grey thallus, which can appear very irregular and uneven, often with a thick, coarse, wart-like texture.

This species complex includes multiple cryptic clades that are distinguished by their fatty acid profiles rather than morphological differences.

These findings indicate that chemical differentiation, particularly in fatty acid composition, often precedes morphological changes and may drive speciation within the complex.

[6] The taxonomy of M. sanguinarius has been further complicated by historical recognition of unpigmented forms, which were sometimes classified as separate species, such as M. affinis and M. alpinus.

Molecular data have now clarified these relationships, supporting the distinction of M. sanguinarius from other similar taxa and highlighting the cryptic diversity within this species.

Soralia (propagules for asexual reproduction) are rare but, when they do appear, they are rounded, scattered, often few in number, and can be the same colour as or paler than the thallus.

Underneath the apothecia, the tissue and hypothecium are carmine to blood-red, reacting K+ (bright red), with the pigment diffusing into the solution.

This lichen is particularly abundant in boreal and montane forests, where it often grows on the bark of coniferous trees such as spruce and fir.