Incertae sedis

Incertae sedis is a Latin term meaning "uncertain position" or "uncertain rank". It is used in taxonomy when a group or species has been identified but its exact placement in the classification tree is unknown—for instance, which family or order it belongs to.

This label is a provisional solution. It allows researchers to include the species in an official list without forcing it into an uncertain category. When new evidence emerges—such as genetic analysis or additional specimens—a species can be removed from incertae sedis status and assigned a clear position.

Notation

The term is typically written alongside the name or below the rank in which uncertainty exists, usually in italics, and abbreviated as "inc. sed.". If uncertainty is within the family, the species is placed at the order level to indicate that the family is unconfirmed. Likewise, a group of uncertain order rank may be held at the class level.

Causes

The most common causes are missing or damaged specimens, particularly fossils lacking important distinguishing parts; overlapping characters making multiple groups similar; and conflicting results from measurements of body structures and DNA analysis. New research that revises earlier work can also move species out of incertae sedis into newly established groups.

Uses

The category is most frequently encountered in paleontology, botany, and taxonomy of microorganisms such as bacteria and protists—fields where visible characters are limited. It is also used in formal classification lists and databases, where every species must be assigned a rank, even if that rank is uncertain.