The Cladoniaceae are a family of lichenized fungi in the order Lecanorales. It is one of the largest families of lichen-forming fungi, with about 560 species distributed amongst 17 genera. The reindeer moss and cup lichens (Cladonia) belong to this family. The latter genus, which comprises about 500 species, forms a major part of the diet of large mammals in taiga and tundra ecosystems. Many Cladoniaceae lichens grow on soil, but other can use decaying wood, tree trunks, and, in a few instances, rocks as their substrate. They grow in places with high humidity, and cannot tolerate aridity. Many Cladoniaceae species are characterized by a thallus that has two distinct forms: a scaly or crust-like primary thallus that, depending on the species, can be permanent or temporary, and a secondary fruticose thallus called a podetium or pseudopodetium. Cladoniaceae members form symbiotic associations with green algae from the class Trebouxiophyceae, usually the genus Asterochloris. Five Cladoniaceae species are included in the International Union for Conservation of Nature's Red List of Threatened Species. Taxonomy Cladoniaceae was formally introduced to science in 1827 by German naturalist Jonathan Carl Zenker in a publication of Karl Goebel. Zenker's initial concept of the family included genera that are now recognized as separate families, including Baeomycetaceae, Icmadophilaceae, and Stereocaulaceae. William Nylander included 53 Cladonia species worldwide in his 1860 work Synopsis lichenum. When Edvard August Vainio published his three-volume monograph on the Cladoniaceae (Monographia Cladoniarum universalis, 1887, 1894, and 1897), he included 134 species and subspecies. In his circumscription of the family, the genera Pycnothelia, Cladia, and Cladina were included in the genus Cladonia. Cladoniaceae is now one of the largest families of lichen-forming fungi, with about 560 species distributed amongst 18 genera. The type genus, after which the family was named, is Cladonia, circumscribed by Irish physician and botanist Patrick Browne in 1756. He included 8 species in his new genus. Of their occurrence, he wrote: "All these species are found in great abundance in the mountains of Liguanea: they grow mostly on the ground, among other sorts of moss, but a few ... species chiefly are found upon the decaying trunks of trees." Synonymy Several phylogenetic studies have shown that Cladoniaceae is a member of the order Lecanorales, and is closely related to the family Stereocaulaceae. The family Cetradoniaceae, which was created in 2002 to contain the endangered species Cetradonia linearis, was folded into the