Incertae sedis
A term for taxa with unknown or undefined broader relationships.
Incertae sedis (Latin for "of uncertain placement"), also called problematica or "enigmatic taxa," describes a taxonomic group whose broader relationships are unknown or undefined. In open nomenclature, specific levels of uncertainty are marked with terms like *incertae familiae* (uncertain family), *incerti subordinis* (uncertain suborder), or *incerti ordinis* (uncertain order).
Examples include the fossil plant *Paradinandra suecica*, placed *incertae sedis* within the order Ericales because it could not be assigned to any family. The fossil *Gluteus minimus* could not be placed in any animal phylum, so it remains *incertae sedis* within Animalia. New World vultures (Cathartidae) were once listed as Aves *incertae sedis* until they were moved to their own order, Cathartiformes. Bocage's longbill, formerly *Amaurocichla bocagii* and now *Motacilla bocagii*, was classified as Passeroidea *incertae sedis* until a 2015 study placed it in Motacillidae. *Parakaryon myojinensis*, a single-celled organism distinct from prokaryotes and eukaryotes, has a completely unknown position in the tree of life. Biological dark matter—genetic material from unidentified microorganisms—also has an unknown position. *Metallogenium* is a bacterium that forms star-shaped minerals. Circothecidae, a Cambrian family, is sometimes linked to Hyolitha but not consistently. The frosted phoenix moth (*Titanomis sisyrota*) is so rare it cannot be placed in any lepidopteran family. The Ediacaran biota’s position within Eukaryota is unclear, with proposals ranging from cnidarians and fungi to an extinct kingdom.
In formal nomenclature, uncertainty at higher ranks is manageable, but the International Code of Nomenclature for algae, fungi, and plants requires species and subdivisions of genera to be assigned to genera. Ranks above genus may be left *incertae sedis*.
**Reason for use** Poor description often leads to *incertae sedis* status. A 2007 study on Kuril–Kamchatka and Japan Trench crustaceans noted that many genera were moved to *incertae sedis* due to inadequate descriptions, insufficient character states, inappropriate characters, homoplasies, character reversals, and convergent evolution.
If a taxon is not included in a formal phylogenetic analysis—common for rare organisms lacking tissue or fossils known only from partial remains (e.g., a single tooth)—authors may label it
- field
- Taxonomy, biological nomenclature
- known_for
- Designating taxa of uncertain phylogenetic placement
- usage
- Formal and informal taxonomic classification
- related_terms
- Incertae familiae, incerti subordinis, incerti ordinis, problematica, enigmatic taxa
Lore & Background
The term incertae sedis has been applied to a wide range of organisms, from fossils to living species. For example, 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, so the genus is 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 until later agreement placed 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. Parakaryon myojinensis, a single-celled organism apparently distinct from prokaryotes and eukaryotes, is the only identified species with a completely unknown position within the tree of life. Biological dark matter, genetic material from unidentified microorganisms that matches no known species, also has its producers' positions in the tree of life completely unknown.
In fungal taxonomy, incertae sedis is applied to taxa whose position in the hierarchy cannot be stated with confidence. The 2022 'Outline of Fungi and fungus-like taxa' reported about 3,185 genera as incertae sedis, including roughly 2,680 in Ascomycota and about 420 in Basidiomycota. Causes include limited taxon sampling, discordant signals from different genetic markers, missing links between sexual and asexual morphs, and cases where a taxon simply does not match any recognized family or order.
Reader's Guide
Incertae sedis serves a critical function in biological classification by providing a formal placeholder for taxa whose phylogenetic relationships are unresolved. This term allows taxonomists to avoid premature or erroneous assignments while acknowledging that further research is needed. The reasons for applying incertae sedis are varied: poor or inadequate descriptions of taxa, insufficient character states in analyses, inappropriate or invalid characters, homoplasy, convergent evolution, or simply because a taxon was not included in a formal phylogenetic analysis. Controversy among researchers or conflicting results can also lead to a taxon being listed as incertae sedis until the conflict is resolved.
The term is distinct from nomen dubium (a name of unknown or doubtful application) and species inquirenda (a species requiring further investigation). In formal nomenclature, the International Code of Nomenclature for algae, fungi, and plants stipulates that species and subdivisions of genera must be assigned to genera, but ranks higher than the genus may be assigned incertae sedis. In zoological nomenclature, incertae sedis is not a nomenclatural term per se but is used by taxonomists in classifications to mean 'of uncertain taxonomic position'. The concept remains essential for managing the vast number of organisms—especially rare fossils, poorly preserved specimens, and microorganisms—that resist easy classification, and it highlights the provisional nature of taxonomic knowledge.
Did You Know?
- The fossil Gluteus minimus, described in 1975, could not be assigned to any known animal phylum and is incertae sedis within the kingdom Animalia.
- Parakaryon myojinensis is a single-celled organism apparently distinct from prokaryotes and eukaryotes, with a completely unknown position within the tree of life.
- The 2022 'Outline of Fungi and fungus-like taxa' reported about 3,185 genera as incertae sedis, including roughly 2,680 in Ascomycota.
- In zoological nomenclature, incertae sedis is not a nomenclatural term per se but is used by taxonomists to mean 'of uncertain taxonomic position'.
Frequently Asked Questions
What is Incertae sedis?
It is a Latin taxonomic label meaning 'of uncertain placement,' applied when a species or group cannot yet be confidently assigned to a higher-ranking category such as a family or order. In plain terms, it is taxonomy's formal way of saying, 'We know this exists, but we haven't worked out where it belongs yet.'
What does the name Incertae sedis actually mean, and are there other names for it?
The phrase is Latin for 'of uncertain seat' or 'of uncertain placement,' evoking the idea that the taxon lacks a confirmed spot on the classification tree. In everyday field usage it is also referred to as 'problematica' or 'enigmatic taxa.'
How is Incertae sedis different from a taxon simply being 'unclassified'?
Incertae sedis is a deliberate nomenclatural statement that the broader phylogenetic relationships are genuinely unresolved, not merely overlooked. More precise variants exist to flag exactly which rank is in question, such as incertae familiae (uncertain family), incerti subordinis (uncertain suborder), or incerti ordinis (uncertain order).
Can you give a real example of a taxon designated Incertae sedis?
The fossil plant Paradinandra suecica was placed incertae sedis within the order Ericales because no existing family could accommodate it. The fossil Gluteus minimus is another case, landing in the same unresolved category because it could not be fitted into any known animal grouping.
Why does Incertae sedis matter for understanding evolution?
It honestly marks gaps in the tree of life rather than forcing a lineage into a potentially incorrect branch, which keeps the record scientifically honest. Resolving these 'enigmatic' placements often reshapes entire chapters of evolutionary history, making them high-priority targets for future research.
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