Sister group

Taxa A and B, together with any other extant or extinct descendants of their most recent common ancestor (MRCA),[Note 1] form a monophyletic group, the clade AB.

Taxa A, B, and C, together with all other descendants of their MRCA form the clade ABC.

In cladistic standards, taxa A, B, and C may represent specimens, species, genera, or any other taxonomic units.

An example is birds, whose commonly cited living sister group is the crocodiles, but that is true only when discussing extant organisms;[3][4] when other, extinct groups are considered, the relationship between birds and crocodiles appears distant.

Although the bird family tree is rooted in the dinosaurs, there were a number of other, earlier groups, such as the pterosaurs, that branched off the line leading to the dinosaurs after the last common ancestor of birds and crocodiles.