Infraclasses

Infraclasses are taxonomic ranks in zoological classification that denote major subdivisions of a subclass. They sit between the ranks of subclass and superorder, so the typical sequence is class, subclass, infraclass, superorder, order. The rank is optional and not used consistently across all animal groups; some taxonomic treatments employ it to reflect historical groupings, while others prefer to describe lineages as unranked clades within a phylogenetic framework.

Within the class Mammalia, the subclass Theria has often been divided into infraclasses such as Metatheria (marsupials) and Eutheria (placental mammals). These infraclasses group together major, well-supported lineages, but in many modern classifications, placental and marsupial relations are instead treated as clades without formal infraclass status.

Because infraclass usage varies by author and by phylum, the rank is best understood as a traditional, not universal, level of organization. Its exact definition and position can differ, and in some classification schemes the rank has been abandoned in favor of unranked clades or moved to other hierarchical levels.