Stemlineages

Stemlineages are evolutionary lineages that lead to a crown group but are not themselves part of that crown group. They consist of extinct or unsampled lineages that diverged before the most recent common ancestor of all living members of a clade and are ancestral to the crown group. Because they precede the origin of the crown group, stemlineages may display a mix of ancestral and derived traits and often lack the defining synapomorphies of the crown group.

In systematics, the crown group denotes the last common ancestor of all living members and all its descendants. The total group includes both crown group and stem lineages. The concept helps frame when key evolutionary novelties arose and how much of earlier morphology persisted into lineages that produced living descendants. Stemlineages are frequently inferred from fossils, especially transitional forms, and from phylogenetic analyses that include extinct taxa. Ghost lineages are related ideas, referring to lineages implied by molecular divergences that lack fossil representation.

Well-known examples include stem cetaceans, such as Pakicetus and Ambulocetus, which lie on the lineage leading to modern whales, and stem tetrapods like Acanthostega and Ichthyostega, which precede the crown group of living tetrapods. The stemlineage concept is a useful framework for describing deep-time evolutionary history when the fossil record is incomplete or when defining the precise boundaries of crown groups is contested.