squamulose

Squamulose describes a lichen growth form in which the thallus is composed of small, scale-like units called squamules. These squamules are typically discrete and only loosely attached to the substrate, producing a mosaic of overlapping scales rather than a continuous sheet.

Squamulose thalli are usually flattened and may range from a few tenths of a millimeter to several millimeters across. The margins are distinct and the surface may be smooth, cracked, or wrinkled, often displaying a range of colors from gray and green to brown, depending on species and the photobiont. Reproductive structures, such as apothecia or pycnidia, may occur on the squamules or at their margins.

Squamulose lichens are distinguished from crustose lichens, which form a tight, contiguous crust, and from foliose lichens, which have larger, leaf-like lobes that are more widely attached. In some contexts squamulose forms are viewed as an intermediate growth form between crustose and foliose types, and many species can show squamulose thalli under certain environmental conditions.

Distribution is global; squamulose lichens inhabit a variety of habitats, including rocks, soil, and bark, from arctic to tropical regions. They are well adapted to exposed, dry environments where small scales help reduce water loss while maintaining photosynthetic tissue contact with substrates.

Taxonomically, squamules describe a phenotype rather than a taxonomy: many families contain squamulose species. Identification relies on thallus morphology in combination with microscopic features and chemical tests.