Crown group

The concept was developed by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms relative to their extinct relatives in his "Die Stammesgeschichte der Insekten",[1] and the "crown" and "stem" group terminology was coined by R. P. S. Jefferies in 1979.

Extinct side branches on the family tree that are descended from the most recent common ancestor of living members will still be part of a crown group.

An alternative definition does not require any members of a crown group to be extant, only to have resulted from a "major cladogenesis event".

Crown-Aves and Crown-Mammalia therefore differ slightly in content from the common definition of Aves and Mammalia.

Thus, a host of prefixes have been defined to describe various branches of the phylogenetic tree relative to extant organisms.

Pan-Mammalia consists of all mammals and their fossil ancestors back to the phylogenetic split from the remaining amniotes (the Sauropsida).

While often attributed to Jefferies (1979), Willmann (2003)[13] traced the origin of the stem group concept to Austrian systematist Othenio Abel (1914),[14] and it was discussed and diagrammed in English as early as 1933 by A. S.

The following cladogram, based on Benton (2005),[8] illustrates the concept: Crocodilia Pterosauria Hadrosauridae Stegosauria Sauropoda Tyrannosauridae Archaeopteryx Neognathae (including the extinct dodo) Paleognathae (including the extinct moa) The crown group here is Neornithes, all modern bird lineages back to their last common ancestor.

Finally, at the base of the crown group, all traits common to extant birds were present.

Stem arthropods constitute a group that has seen attention in connection with the Burgess Shale fauna.

[23] It is generally taken to mean a side branch splitting off earlier on the phylogenetic tree than the group in question.

Stem groups thus offer a route to integrate unique palaeontological data into questions of the evolution of living organisms.

The application of the stem group concept also influenced the interpretation of the organisms of the Burgess shale.

Their classification in stem groups to extant phyla, rather than in phyla of their own, is thought by some to make the Cambrian explosion easier to understand without invoking unusual evolutionary mechanisms;[21] however, application of the stem group concept does nothing to ameliorate the difficulties that phylogenetic telescoping[24][25] poses to evolutionary theorists attempting to understand both macroevolutionary change and the abrupt character of the Cambrian explosion.

Overemphasis on the stem group concept threatens to delay or obscure proper recognition of new higher taxa.