Incertae sedis or problematica are terms used for a taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty at specific taxonomic levels is indicated by incertae familiae, incerti subordinis, incerti ordinis and similar terms.
Examples
The fossil plantParadinandra suecica could not be assigned to any family, but was placed incertae sedis within the order Ericales when described in 2001.
The fossilGluteus 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 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, Motacilla bocagii, previously known as Amaurocichla bocagii, is a species of passerine bird that belongs to the superfamily Passeroidea. Since it was unclear to which family it belongs, it was classified as Passeroidea incertae sedis, until a 2015 phylogenetic study placed it in Motacilla of Motacillidae.
...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.
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.
In botany, a name is not validly published if it is not accepted by the author in the same publication.Article 36.1 In zoology, a name proposed conditionally may be available under certain conditions.Articles 11 and 15 For uncertainties at lower levels, the system of open nomenclature suggests that question marks be used to denote a questionable assignment. For example, if a new species was given the specific epithetalbum by Anton and attributed with uncertainty to Agenus, it could be denoted "Agenus? album Anton "; the "" indicates the author that assigned the question mark. So if Anton described Agenus album, and Bruno called the assignment into doubt, this could be denoted "Agenus? album ", with the parentheses around Anton because the original assignment was modified by Bruno.
