Arthoniaceae is a family of fungi in the order Arthoniales, within the class Arthoniomycetes. The family consists primarily of lichenized fungi, though it also includes several lichenicolous (fungi that grow on other lichens) and saprobic species. It is one of the largest families within its order and is distributed globally across various terrestrial habitats.
The thallus of Arthoniaceae species is typically crustose, meaning it grows as a thin crust closely adhered to its substrate, such as tree bark, rocks, or occasionally leaves. In many species, the thallus is endophloedic (growing within the bark) or endolithic (growing within the rock), making it barely visible to the naked eye.
The reproductive structures, or ascomata, are a defining feature of the family. They are generally apothecial but often lack a distinct excipulum (a specialized margin). These ascomata are frequently irregular in shape, appearing as dots, elongated streaks, or star-shaped (stellate) patterns. Microscopically, the family is characterized by bitunicate asci—thick-walled sacs that contain the spores—which are typically globose to pyriform (pear-shaped). The ascospores themselves vary significantly in size, shape, and the number of internal divisions (septa) depending on the genus and species.
Taxonomically, Arthoniaceae has undergone significant revisions due to advancements in molecular phylogenetics. Historically, classification relied heavily on the morphology of the ascomata and spores, but genetic sequencing has revealed that several traditional genera were polyphyletic. The most prominent genus within the family is Arthonia, which contains hundreds of species. Other notable genera include Arthothelium, Coniarthonia, and Bryostigma.
Ecologically, members of Arthoniaceae play roles in various ecosystems as pioneers on bark and rock surfaces. They are found in both temperate and tropical regions, with a high level of diversity observed in humid forest environments. While most species are considered specialists regarding their substrate, the family as a whole occupies a wide range of ecological niches.