infrakingdoms

An infrakingdom is a proposed taxonomic rank used by some biologists to subdivide a kingdom into smaller, more inclusive groups. The term derives from infra- meaning below and kingdom, and it has appeared in various classification schemes as a way to describe deep organizational levels within a kingdom. Because taxonomy has varied across periods and groups, the exact position of an infrakingdom relative to other ranks is not standardized. In some historical schemes, infrakingdoms lay between the kingdom and subkingdom, while in others the usage is more loosely defined as a way to highlight major evolutionary splits within a kingdom. The lack of consistent criteria for defining infrakingdoms—such as diagnostic characters, monophyly, or genetic distance—has contributed to its limited and uneven adoption.

In contemporary practice, infrakingdoms are rarely used in mainstream taxonomy. Modern classifications tend to favor higher-level clades or use established ranks such as subkingdom or infraphylum, with increasing emphasis on phylogenetic relationships rather than fixed rank-based divisions. However, the term may still appear in older literature or in niche classifications. In fiction and worldbuilding, infrakingdoms are sometimes invoked as a convenient device to describe broad, prehistoric divisions of life or to organize imagined biotas.

See also: taxonomic rank, kingdom, subkingdom, clade, phylogeny.