Incertae sedis (Latin for 'of uncertain placement')[2] or problematica is a term used for a taxonomic group where its broader relationships are unknown or undefined.
Their reduced status was attributed largely to poor or inadequate descriptions but it was accepted that some of the vagueness in the analysis was due to insufficient character states.
Additional complexity, and factors that have misled earlier authorities, are intrusion by extensive homoplasies, apparent character state reversals and convergent evolution.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 its placement.
This is particularly common when molecular phylogenies are generated, since tissue for many rare organisms is hard to obtain.
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 necessary to label it incertae sedis.