Incertae sedis

Incertae sedis (Latin for "of uncertain placement"), abbreviated "inc. sed.", is a term used to define a taxonomic group where its broader relationships are unknown or undefined and which cannot be assigned to a recognized phylum.

Examples

The taxonomy for humans is usually recognized as follows:

• Empire Neomura - along with all the living organisms, except eubacteria.
• Domain Eukaryota - along with all other nucleated organisms.
• Kingdom Animalia - along with all other animals
• Phylum Chordata - along with all other vertebrates and the invertebrate chordates
• Class Mammalia - along with all other mammals
• Order Primates - along with all other primates
• Family Hominidae - along with all other apes
• Subfamily Homininae - along with gorillas and chimps
• Tribe Hominini - along with chimps
• Subtribe Hominina - along with Australopithecus and prehistoric humans
• Genus Homo - along with Homo erectus and other prehistoric humans
• Species H. sapiens - humans

If modern humans were newly discovered or considered to be a taxonomic enigma, they might be given the rank of incertae sedis. For example, if it were uncertain how Homo related to other members of the family Hominidae, a list of the great apes would look like this:

• Kingdom Animalia
• Phylum Chordata
• Class Mammalia
• Order Primates
• Family Hominidae
• Genus Homo incertae sedis
• Subfamily Ponginae - orangutan
• Subfamily Homininae - gorilla and chimpanzees

Likewise, if humans were known to be primates, but no other relationships were clear, a taxonomy of the primates would look like this:

• Kingdom Animalia
• Phylum Chordata
• Class Mammalia
• Order Primates
• Genus Homo incertae sedis
• Family Cercopithecidae - Old World monkeys
• Family Hominidae - apes
• Family Lemuridae - lemurs
• Etc.

Reasons a group might be considered incertae sedis

Poor description

This excerpt from a 2007 scientific paper about crustaceans of the Kuril-Kamchatka Trench and the Japan Trench describes typical circumstances through which this category is applied in discussing:

...the removal of many genera from new and existing families into a state of incertae cedis. Their reduced status was attributed largely to poor or inadequate descriptions but it was accepted that some of the vagueness in the analysis was due to insufficient character states. It is also evident that a proportion of the characters used in the analysis, or their given states for particular taxa, were inappropriate or invalid. Additional complexity, and factors that have misled earlier authorities, are intrusion by extensive homoplasies, apparent character, state reversals and convergent evolution.

Not included in an analysis

If a formal phylogenetic analysis is conducted that does not include a certain taxon, the authors might choose to label the taxon incertae sedis instead of guessing as to its placement. This is particularly common when molecular phylogenies are generated since tissue for many rare organisms is hard to obtain. It is also a common scenario when fossil taxa are included since many fossils are defined based on partial information. For example, if the phylogeny was constructed using soft tissue and vertebrae as principal characters and the taxon in question is only known from a single tooth, it would be necessary to label it incertae sedis.

Controversy

If conflicting results exist or if there is not a consensus among researchers as to how a taxon relates to other organisms, it may be listed as incertae sedis until the conflict is resolved.

Basal taxa

There is a growing trend (see phylogenetic taxonomy) among taxonomists to place a basal taxon in the clade that contains its ancestors, but to refrain from giving it any more specific taxonomic ranks. For example, the ancestor to all primates would be placed in the Order Primates, but would not be placed in a family at all. Placing it in an individual family (such as Lemuridae) would suggest that it is more closely related to members of that family (lemurs) than to other primates when, in fact, it is equally related to all primates.

References