Incertae sedis—"of uncertain position (seat)"—is a term used to define a taxonomic group where its broader relationships are unknown or undefined.

Examples

The taxonomy for humans is usually recognized as follows:

• 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 was 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 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

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 neccessary 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.

Stem taxa

There is a growing trend (see PhyloCode) among phylogeneticists to place a stem 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.