Terms in themselves are trivial, but taxonomies revised for a different ordering of thought are not without interest. (Gould and Vrba 1982)

Abstract

Dinosaur systematics provides examples of two approaches to the definition of supraspecific assemblages. The clade, or monophyletic taxon (sensu stricto, Hennig 1966), can be diagnosed by shared derived character states, or synapomorphies, and constitutes a ‘real’ entity in the history of organismic diversity. The grade, or paraphyletic taxon, can be diagnosed only by both the presence and absence of synapomorphies and is delineated on the basis of morphologic distance.

The cladistic method for the characterization of monophyletic groups by synapomorphy is briefly described. A comparable method for qualitative measurement of morphologic distance is not available. Phylogenetic classification, based on monophyletic groups, is compared to traditional classification, based on a combination of monophyletic and paraphyletic groups. A historical trend toward a phylogenetic classification is observed.

Finally, some common, but mistaken, criticisms of cladistic methods are discussed. Parsimony is held to be a fundamental criterion for discriminating among hypotheses, and the construction and maintenance of a phylogenetic classification is presented as both a practical and a heuristic endeavor. Explicit phylogenetic hypotheses proposed recently for the Dinosauria promise a better understanding of the interrelationships within this diverse taxon.