Definition: Stem Group / Crown Group

Clade

A monophyletic taxon; a group of organisms which includes the most recent common ancestor of all of its members and all of the descendants of that most recent common ancestor. From the Greek word "klados", meaning branch or twig (UCMP Glossary of Natural History Terms, Volume 1, Phylogenetic Terms).

Crown Group

Those organisms descended from the last common ancestor of all living members of a clade (Runnegar 1992).

The last common ancestor of a living clade plus all of its descendants (Budd 2001, p. 333).

All the taxa descended from a major cladogenesis event, recognized by possessing the clade's synapomorphy (UCMP Glossary of Natural History Terms, Volume 1, Phylogenetic Terms).

Stem Group

All fossils more closely related to a particular crown group than to any other, but more basal than its most basal member (Budd 2001, p. 333).

All the taxa in a clade preceding a major cladogenesis event. They are often difficult to recognize because they may not possess synapomorpies found in the crown group (UCMP Glossary of Natural History Terms, Volume 1, Phylogenetic Terms).

References

Budd, Graham E. 2001: Why are Arthropods Segmented? Evolution & Development 3(5): 332-342.

Runnegar, Bruce (1992):  Chapter 3 – Evolution of the Earliest Animals.  In, Schopf, J. William (ed.) Major Events in the History of Life.  Jones and Bartlett.