Class (biology)

For other uses, seeClass (disambiguation).

This article is about the taxonomic grouping of related organisms. Not to be confused with the ecological grouping of unrelated plant taxa inphytosociology.

The hierarchy ofbiological classification's eight majortaxonomic ranks.Aphylumcontains one or more classes. Intermediate minor rankings are not shown.

Inbiological classification,class(Latin:classis) is ataxonomic rank,as well as a taxonomic unit, ataxon,in that rank. It is a group of related taxonomic orders.Other well-known ranks in descending order of size arelife,domain,kingdom,phylum,order,family,genus,andspecies,with class ranking between phylum and order.

History

The class as a distinct rank of biological classification having its own distinctive name – and not just called atop-level genus(genus summum)– was first introduced byFrenchbotanistJoseph Pitton de Tournefortin the classification of plants that appeared in hisEléments de botaniqueof 1694.

Insofar as a general definition of a class is available, it has historically been conceived as embracing taxa that combine a distinctgradeof organization—i.e. a 'level of complexity', measured in terms of how differentiated their organ systems are into distinct regions or sub-organs—with a distincttypeof construction, which is to say a particular layout of organ systems.This said, the composition of each class is ultimately determined by the subjective judgment oftaxonomists.

In the first edition of hisSystema Naturae(1735),Carl Linnaeusdivided all three of hiskingdomsof nature (minerals,plants,andanimals) into classes. Only in the animal kingdom are Linnaeus's classes similar to the classes used today; his classes and orders of plants were never intended to represent natural groups, but rather to provide aconvenient"artificial key" according to hisSystema Sexuale,largely based on the arrangement of flowers. In botany, classes are now rarely discussed. Since the first publication of theAPG systemin 1998, which proposed a taxonomy of theflowering plantsup to the level of orders, many sources have preferred to treat ranks higher than orders as informalclades.Where formal ranks have been assigned, the ranks have been reduced to a very much lower level, e.g. class Equisitopsida for the land plants, with the major divisions within the class assigned to subclasses and superorders.

The class was considered the highest level of the taxonomic hierarchy untilGeorge Cuvier'sembranchements,first calledPhylabyErnst Haeckel,were introduced in the early nineteenth century.

Subdivisions

As with the other principal ranks, classes can be grouped and subdivided.

Name	Meaning ofprefix	Example 1	Example 2	Example 3	Example 4
Superclass	super:above	Tetrapoda
Class		Mammalia	Maxillopoda	Sauropsida	Diplopoda
Subclass	sub:under	Theria	Thecostraca	Avialae	Chilognatha
Infraclass	infra:below		Cirripedia	Aves	Helminthomorpha
Subterclass	subter:below, underneath				Colobognatha
Parvclass	parvus:small, unimportant			Neornithes	-

See also

• Cladistics
• List of animal classes
• Phylogenetics
• Systematics
• Taxonomy