“Incertae sedis” is a term used in biology to describe a taxonomic group that is difficult to classify, often due to recent discovery, taxonomic challenges, or incomplete fossils. Molecular phylogeny has made classification easier, but many animals remain unclassified. Basal taxa are sometimes placed in a clade without a specific classification. The Ediacaran fauna and Wiwaxia are examples of incertae sedis fossils.
In biology, “incertae sedis” (Latin: of uncertain position (seat)) indicates a taxonomic group that is difficult to place in the larger taxonomic scheme. This could be because the group was recently discovered, taxonomically challenging, or a fossil animal known only from fragments. Thanks to molecular phylogeny, most animals can be taxonomically classified quite easily as long as there is sufficient funding and a tissue sample. However, since there are millions of animal species, many have not yet been definitively 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 a taxon’s place unless it’s obvious. For example, in the snake family Colubridae, there are more than a dozen genera listed as incertae sedis. In taxonomy, there is also a growing tendency to place a basal (primitive) taxa in the clade containing its ancestors, but refrain from giving it a more specific classification. For example, a basal reptile might be considered part of the class Sauropsida with other reptiles, but not have a family or genus designation. These types 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 variety of sack, carpet, circular and cone-shaped forms, often with a distinct tufted pattern on the surface. Some scientists have described the Ediacaran fauna as “a failed experiment in a lifetime.” Characteristic Ediacara fossils such as Dickinsonia costata are some of the earliest examples of bilateral animals and grew to 1.4 m (4.6 ft) in length, nearly the size of an average human.
An iconic example of an incertae sedis fossil from the Cambrian Period is Wiwaxia, an oval-shaped animal covered in spikes. It has been alternately classified as a mollusc and an annelid (segmented worm). The debate about Wiwaxia’s true affinity is ongoing.
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