Classification of Zygomycetes has evolved with the description of new taxa and the rediscovery and redescription of known species. Taxonomy of orders in Zygomycetes is based on the morphology of the spore forming structures, sporangia and (or) sporangiola, or merosporangia. Some organisms produce only zygospores, azygospores, chlamydospores, or a combination of these structures. The occurrence and morphology of any of the aforementioned structures and others (e.g., stolons and rhizoids, apophyses, branching pattern, fertile vesicles) are used to ascertain the phylogenetic relationships in Mucorales. Our understanding of the morphology, development, and phylogeny of Zygomycetes has been enhanced by using microscopic observations and cladistic analysis of data sets derived both from small subunit rDNA and morphology. Many morphological characters (e.g., trophocyst, yeast cell formation) still appear to be reliable phylogenetic indicators while others (e.g., spore morphology) are too variable. The value of zygospore morphology is reduced because the sexual spore has never been reported for many taxa. Many characters used to circumscribe mucoralean families probably do not indicate relationships but still are useful in identification. Sporangiola should be considered indistinct from sporangia.
