Crown Clades in Vertebrate Nomenclature: Correcting the Definition of Crocodylia

Extract

A crown group is defined as the most recent common ancestor of at least two extant groups and all its descendants (Gauthier, 1986). Despite criticism, crown-group definitions are widely used, especially for certain clades of vertebrates. As an example, crown-group Crocodylia was established by Clark (in Benton and Clark, 1988), and there has been increasing use of crown Crocodylia rather than traditional or total Crocodylia since that date. Originally, the Crocodylia embraced forms dating from the Late Triassic to the present. These were divided into three classes, Protosuchia, Mesosuchia, and Eusuchia, the first two of which were accepted as probably or certainly paraphyletic. The new convention was cemented by Brochu (2003), who gave a new definition of crown Crocodylia according to the conventions of phylogenetic nomenclature (PN), as the last common ancestor of Gavialis gangeticus, Alligator mississipiensis, and Crocodylus niloticus, and all of its descendents. This led to an interesting reversal in the hierarchy, so that crown-clade Crocodylia is a subset of Eusuchia, rather than the other way round, as had been the case.