The genus Pteridium is a member of the homosporous fern family Dennstaedtiaceae. Generic relationships within the family still are not fully understood, but the genus is morphologically quite distinct. The taxonomy and classification within Pteridium continues to be controversial. Traditionally and as monographed by Tryon (1941), the genus was thought to comprise a single, highly variable and nearly cosmopolitan species, P. aquilinum, which could be divided into two subspecies, with a complex series of eight and four varieties respectively. However, over time, some of the morphological variants became elevated to separate species status, with the result that more than a dozen species epithets have been published within the genus. Long-term research on the systematics and phylogeny of Pteridium, mainly by John Thomson (National Herbarium of New South Wales) and his collaborators (Thomson, 2004) has resulted in the recognition of four or five species: a New and Old World northern hemisphere diploid (P. aquilinum sensu stricto), a pair of closely related New and Old World mostly southern hemisphere diploids (P. arachnoideum, P. esculentum), an uncommon neotropical tetraploid (P. caudatum), and a Malaysian/Australian tetraploid (P. semihastatum, aka. P. yarrabense). Within P. aquilinum sensu stricto, infraspecific classification also continues to be controversial, with ten or more geographically and morphologically defined subspecies possibly recognized.