In biology, "incertae sedis" (Latin: of uncertain position (seat)) signifies a taxonomic group difficult to place in the larger taxonomic scheme. This may be because the group is newly discovered, taxonomically challenging, or a fossil animal only known from fragments. Thanks to molecular phylogeny, most animals can be taxonomically categorized quite easily as long as there is sufficient funding and a tissue sample. However, because there are millions of animal species, many have not yet been definitely classified using molecular techniques.
Since molecular phylogeny became widely available in the 1970s, more animals have been classified as incertae sedis, because taxonomists are less willing to guess about a taxon's place unless it is obvious. For instance, in the Colubridae family of snakes, there are more than a dozen genera listed as incertae sedis. In taxonomy, there is also a growing trend to put a basal (primitive) taxa in the clade that contains its ancestors, but refrain from giving it a more specific classification. For instance, a basal reptile might be considered part of class Sauropsida with other reptiles, but not given a family or genera designation. These sorts of animals are relatively rare, and most are fossils.
Some of the most fascinating organisms classified as incertae sedis are fossils from the Ediacaran period known as the Ediacaran fauna. Many of these fossils are said to have "uncertain affinity" (a term related to incertae sedis) because they are so cryptic. The Ediacaran fauna consists of a number of bag, carpet, circular, and cone-shaped forms, often with a distinct quilted pattern on their surface. Some scientists have described the Ediacaran fauna as "a failed experiment in life." Distinctive Ediacaran fossils like Dickinsonia costata are some of the earliest examples of bilateral animals, and grew as large as 1.4 m (4.6 ft) in length, almost the size of an average human.
One iconic example of an incertae sedis fossil from the Cambrian period is Wiwaxia, an oval-shaped animal covered in spikes. It has been alternatively been classified as a mollusk and an annelid (segmented worm). The debate over Wiwaxia's true affinity is ongoing.