Meaning of the name tetrapoda in the scientific literature: an exchange.

This exchange stems from an on-going debate between advocates of crown clade (e.g., Gauthier et al., 1988; Rowe, 1988; Laurin, 1998) and those of apomorphybased (e.g., Ahlberg and Clack, 1998; Lee, 1999; Padian et al., 1999; Anderson, 2001) definitions of widely used taxon names (see Table 1 for a glossary of terms). Both types of definitions have advantages: apomorphy-based definitions usually retain a composition of taxa similar to that proposed in most paleontological studies (Lee, 1999), whereas crown clade definitions correspond more closely to the usage of these names by comparative biologists working on extant taxa (de Queiroz and Gauthier, 1992; but see Bryant, 1994; Lee, 1996). Here, we concentrate on the name Tetrapoda, which has been the subject of recent discussion (Anderson, 2001, 2002; Laurin, 2002), but many of the arguments presented here could be applied to other widely used names, such as Mammalia, Aves, and Vertebrata. Laurin (1998, 2002) adopted the crown clade definition of the name Tetrapoda (the last common ancestor of amniotes and lissamphibians, and all its descendants) proposed by Gauthier et al. (1988). Anderson (2002) objected to the crown clade definition of Tetrapoda and, following Lee (1999), advocated adoption of an apomorphy-based definition of this name (the first sarcopterygian to have possessed digits homologous with those in Homo sapiens, and all its descendants). The following exchange is aimed at clarifying the respective advantages of both types of definitions. Established usage of taxon names is important because the systematic community will soon define many widely used names, and the PhyloCode (Cantino and de Queiroz, 2000) recommends, when defining a name, to disrupt current and historical usage as little as possible (Recommendations 10A, 11A). Furthermore, as stated by Anderson (2002: 824), “the PhyloCode provides mechanisms for the amendment (Article 13) or suppression (Article 15) of definitions with priority if they should contravene long accepted usage and thus create instability in nomenclature.” These considerations are important because name definitions under the PhyloCode delimit taxa (i.e., determine their inclusiveness when applied in the context of a particular phylogenetic hypothesis), whereas definitions under traditional rank-based codes do not. Within the traditional rankbased nomenclature system, debates about the inclusiveness can go on indefinitely, whereas within the PhyloCode system, such debates will be minimized (given a particular phylogenetic hypothesis) after the International Committee on Phylogenetic Nomenclature (ICPN) determines which of two conflicting phylogenetic definitions, or names with the same definition, should be retained. The ICPN will have ruling authority (although we expect such actions will be rare if the rules of the PhyloCode are followed, similar to current practice under the International Code of Zoological Nomenclature), but it is more productive to discuss various competing definitions prior to the implementation of the PhyloCode. By ensuring that the most appropriate definition becomes established at the initiation of the PhyloCode system, much future controversy and ICPN action can be avoided. The case of the name Tetrapoda is not unique. Similar disagreements between proponents of crown clade and apomorphy-based definitions have involved several other widely used names, such as Aves (reviewed by Gauthier and de Queiroz, 2001) and Mammalia (Rowe, 1988; Desui, 1991; Rowe and Gauthier, 1992; Wible and Hopson, 1995; Ji et al., 1999; Sidor, 2001). Thus, the current debate on the meaning of the word Tetrapoda can serve as a case study that may help with the resolution of similar disputes in the future. In this point–counterpoint, we debate four key issues related to the definition of the name Tetrapoda.