stemgroup

A stem group is a term in paleontology and cladistics used to describe a paraphyletic set of extinct organisms that are more closely related to a particular crown group than to any other living group, but are not part of the crown group itself. In this sense, stem groups lie outside the last common ancestor of all living members of a clade (the crown group) and include lineages that diverged before that node, often without possessing all the defining features of the crown group. The crown group includes the last common ancestor of all living members of the clade and all of its descendants.

The stem group concept helps illuminate evolutionary history by framing the sequence of character acquisition and the morphology of transitional forms leading to the modern diversity. Because stem groups are typically extinct, they are known primarily from fossil records and can reveal when and how key innovations arose, even if those innovations are not fully expressed in any living lineage.

Examples commonly discussed in vertebrate and plant evolution include stem tetrapods, such as Acanthostega and Ichthyostega, which are considered stem tetrapods relative to the crown Tetrapoda. Other stem groups include stem amniotes relative to the crown Amniota, illustrating how major transitions occurred before the diversification of living lineages.

The total group of a clade comprises both the stem group and the crown group. The stem group concept emphasizes paraphyly and helps structure interpretations of deep-time evolutionary sequences.