Incertae sedis (Latin for "of uncertain placement") is a term used to define a taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". Uncertainty at specific taxonomic levels is attributed by incertae familiae (of uncertain family), incerti subordinis (of uncertain suborder), incerti ordinis (of uncertain order) and similar terms.
- The fossil plant Paradinandra suecica could not be assigned to any family, but was placed incertae sedis within the order Ericales when described in 2001.
- The fossil Gluteus minimus, described in 1975, could not be assigned to any known animal phylum. The genus is therefore incertae sedis within the kingdom Animalia.
- While it was unclear to which order the New World vultures (family Cathartidae) should be assigned, they were placed in Aves incertae sedis. It was later agreed to place them in a separate order, Cathartiformes.
- Bocage's longbill, Amaurocichla bocagei, a species of passerine bird, belongs to the superfamily Passeroidea. Since it is unclear to which family it belongs, it is classified as Passeroidea incertae sedis.
- HeLa cells, descended from human cervical cancer cells, may be sufficiently genetically divergent from normal human cells to be categorized as a new species with largely incertae sedis taxonomy.
Reason for use
...the removal of many genera from new and existing families into a state of incertae sedis. 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 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.
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.
Lower taxonomic levels
As stated in the International Code of Botanical Nomenclature, only ranks higher than the genus may be assigned incertae sedis; species must be assigned to genera and infraspecific taxa must be assigned to species. For uncertainties at lower levels, question marks may be used to denote a questionable assignment. For example, if a new species with the specific epithet album was discovered by Anton and attributed with uncertainty to Agenus, it would be denoted "Agenus? album Anton (?Anton)"; the "(?Anton)" indicates the author that assigned the question mark. So if Anton described Agenus album, and Bruno called the assignment into doubt, this would be denoted "Agenus? album (Anton) (?Bruno)", with the parentheses around Anton because the original assignment (to Agenus) was modified (to Agenus?) by Bruno.
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