Phylogenetic taxonomy, that component of phylogenetic systematics concerned with the verbal representation (rather than the reconstruction or estimation) of phylogenetic relationships, was developed by de Queiroz and Gauthier (1990, 1992, 1994). Under phylogenetic taxonomy, all taxon names are names of clades (i.e., an ancestor and all of that ancestor's descendants). De Queiroz and Gauthier (1990, 1992, 1994) described three possible ways of denning clade names within the phylogenetic taxonomic system: 1) node-based definitions (Figure 1.1), of the form “the most recent common ancestor of Taxon A and Taxon B, and all of that ancestor's descendants”; 2) stem-based definitions (Figure 1.2), of the form “Taxon A and all taxa sharing a more recent common ancestor with Taxon A than with Taxon B”; and 3) apomorphy-based definitions (Figure 1.3), of the form “the first taxon with derived character X and all of that ancestor's descendants.” Bryant (1994) noted that, while the first two definition types will always be stable, apomorphy-based definitions are potentially confusing if that derived character is found to occur in more than one lineage (i.e., is homoplastic). Under the phylogenetic system of taxonomy, definitions of taxon names are independent with respect to previous diagnosis (as particular character states may be found to occur in other lineages or in more inclusive clades) and composition (as particular member taxa may subsequently be found to lie outside the defined clade boundary). This paper is the initial work in an ongoing study by Holtz and Padian (1995, in preparation) to clarify the phylogenetic taxonomy of major clades of theropods and related taxa.
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