A basal taxon is a group of organisms that is outside a monophyletic group but is more closely related to it than to any other group. This means that the basal taxon is the most primitive or ancestral member of a particular clade. Basal taxa are often paraphyletic, meaning that they do not include all descendants of their most recent common ancestor. For example, the basal chordates are a paraphyletic group that includes all chordates except for vertebrates. This is because the basal chordates are more closely related to vertebrates than they are to any other group of animals, but they do not include all vertebrates.
