The phylogenetic definition of reptilia.

Naming taxa is an important endeavor in the documentation of life by systematists, whether it is conducted in the context of traditional rank-based classification or within a phylogenetic framework. Proponents of the phylogenetic approach distinguish between the diagnosis of a group and its definition (Ghiselin, 1984; Rowe, 1987, 1988), and this distinction forms the basis for a phylogenetically based method of naming taxa formerly referred to as Phylogenetic Taxonomy (de Queiroz and Gauthier, 1990, 1992, 1994) and now called Phylogenetic Nomenclature (PN; Cantino et al., 1999; Gauthier and de Queiroz, 2001; Bryant and Cantino, 2002). Emphasis in naming has been placed on ancestry using phylogenetic definitions, and the widespread adoption of nodeand stem-based definitions (apomorphy-based definitions have yet to receive as widespread acceptance, but see Gauthier and de Queiroz, 2001; Anderson, 2002; Laurin and Anderson, 2004) has led to a proliferation of new names and definitions. This shift in nomenclatural practice has, unfortunately, fostered a growth in redundant names and definitions for well-known taxa (Benton, 2000). The PhyloCode (Cantino and de Queiroz, 2003) has modified the rule of priority as used in other codes (i.e., International Code of Zoological Nomenclature) to determine which of two or more possible names with equivalent definitions is valid (Brochu and Sumrall, 2001), or which of several definitions for a given name is valid (Cantino and de Queiroz, 2003). Unfortunately, it is now apparent that some of the definitions for wellknown taxon names established early in the emergence of PN were not devised following conventions now widely accepted, by either defining groups in an overly restrictive manner, or via selection of reference taxa without due consideration of the ramifications of differing tree topologies (Anderson, 2002; Laurin and Anderson, 2004). It has become evident in broad-scale amniote taxonomy that the first published definition for Reptilia (Gauthier et al., 1988a), which would have priority under a binding PhyloCode, is problematic because of the dramatic controversies over the affinities of the specifier taxon Testudines (see Zardoya and Meyer, 2001 for review of hypotheses for turtle relationships). Recent morphological and molecular studies have challenged conventional hypotheses concerning the affinities of turtles, and this has led to unexpected and undocumented changes in the composition of the well-known taxon Reptilia, with additional ramifications for the nomenclature of some of its included taxa. We examine the consequences of the application of priority to the nomen Reptilia as our understanding of early amniote interrelationships has progressed over the past two decades, and offer a new definition that brings the phylogenetic concept of this taxon name into line with both currently accepted conventions of PN and historical usage. This new definition corrects an error created by the combination of the selection of a higher taxon (rather than a species) as a specifier, and an unexpected topology. We believe that now is an appropriate time to examine the definitions established when PN was in its earliest stages, and hope to correct what we consider to be a poorly formulated definition upon publication of a binding PhyloCode.