Stem group

A stem group comprises extinct taxa more closely related to a crown group than to its living sister group, but outside the crown group. Its members lie along the evolutionary lineage leading toward the crown group.

Crown group: A group containing the last common ancestor of all living members of a lineage and all its descendants. Stem-group taxa fall outside this group, whose living members set its boundary.
Cladistics: A method of classifying organisms by shared derived traits and common ancestry. Cladistic analysis tests whether a fossil lies outside or inside a crown group.
Tiktaalik: A Devonian fossil vertebrate with both fish-like features and adaptations associated with early tetrapods. Its mix of traits illustrates a fossil near the transition to the tetrapod crown group.
Total-group concept: A phylogenetic grouping that includes a crown group and all extinct taxa closer to it than to its living sister group. It names the whole assemblage, whereas a stem group excludes the crown itself.
Total group: A crown group together with all extinct taxa more closely related to it than to its living sister group. The total group combines the crown group with its stem.
Synapomorphy: A shared derived character inherited from the most recent common ancestor of a group. Shared derived traits help place fossils on the branch leading toward a crown group.
Archaeopteryx: A Late Jurassic fossil animal combining feathers and wings with skeletal traits shared by non-avian theropod dinosaurs. Its placement near the bird crown group makes it a classic example in discussions of stem birds.
Crown-group concept: A way of defining a group by its last common ancestor among living members and all descendants of that ancestor. It contrasts with stem membership, which lies outside the living group's defining ancestor.
Last universal common ancestor: The most recent common ancestor of all living organisms. Common-ancestor reasoning underlies the branching relationships used to define crown groups.
Character matrix: A table recording observed traits for taxa, used in phylogenetic analysis. Researchers compare fossil and living taxa by coding traits in a character matrix.

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