subKingdom

Subkingdom is a taxonomic rank used in some biological classifications to subdivide a kingdom. It lies between the rank of kingdom and phylum and is intended to group together related phyla into larger clades within a kingdom. The exact definition and criteria for placing a group at the subkingdom level have varied over time and among systems.

Historically, subkingdoms were more commonly used in the era before widespread adoption of the domain-based framework. Today, many taxonomic schemes do not employ subkingdom as a formal rank, preferring broader divisions or clade-based groupings that are not tied to a fixed rank. As a result, the term is largely conventional or regional rather than universally standardized.

In zoology, some authors have used Subkingdom Parazoa for sponges and Subkingdom Eumetazoa for most other animals, to distinguish animals with true tissues from those without. However, Porifera and other phyla have also been treated at other ranks in different systems, and not all classifications recognize Parazoa and Eumetazoa as subkingdoms. This exemplifies the variable and historical nature of subkingdom usage.

Within modern taxonomy, higher-level categorization often takes the form of domains and clades, and many groups abandon formal ranks such as subkingdom in favor of phylogenetically defined groups. When used, the term subkingdom can help organize large kingdoms into major lineages, but its application remains inconsistent across taxa and publications.