Definition and Associated Terms
When Carl Linnaeus introduced the rank-based system of nomenclature into biology, the highest rank was given the name "kingdom" and was followed by four other main or principal ranks. Later two further main ranks were introduced, making the sequence kingdom, phylum or division, class, order, family, genus and species. In the 1960s a rank was introduced above kingdom, namely domain (or empire), so that kingdom is no longer the highest rank.
Prefixes can be added to "kingdom" to derive intermediate minor ranks. Thus, superkingdom is a rank immediately above kingdom, whereas subkingdom and infrakingdom are the two ranks immediately below kingdom. Superkingdom may be considered as an equivalent of domain or empire or as an independent rank between kingdom and domain or subdomain. In some classification systems the additional rank branch (Latin: ramus) can be inserted between subkingdom and infrakingdom (e.g. Protostomia and Deuterostomia in the classification of Cavalier-Smith). Because of its position, branch can be considered as a minor rank of the kingdom group even if it is not etymologically derived from it.
Read more about this topic: Kingdom (biology)
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