DEVELOPING A PROTOCOL FOR THE CONVERSION OF RANK-BASED TAXON NAMES TO PHYLOGENETICALLY DEFINED CLADE NAMES, AS EXEMPLIFIED BY TURTLES

Journal of Paleontology, Sep 2004 by Joyce, Walter G, Parham, James F, Gauthier, Jacques Armand

In traditional rank-based taxonomy, authors conceive increasingly inclusive classes of organisms that are united by defining characteristics (differentia), such as synapomorphies and/or symplesiomorphies. These classes of organisms are then assigned a categorical rank and a type species or specimen, thus becoming ICZN-taxa. Finally, each ICZN-taxon is given a diagnosis that circumscribes its composition by reference to its differentia, and, using the rules of the ICZN (1999), is assigned a certain name based on its rank and type species or specimen. Naturally, given that the ICZN (1999) does not govern taxonomic practice per se for the most part, this description of taxonomic practice should be understood as a simplification, not an authoritative account of traditional taxonomic practice.

The emphasis on rank allows any subsequent taxonomist to retrieve efficiently the rank of an ICZN-taxon and its relative placement within any ICZN-taxonomic system (at least for some levels in the taxonomic hierarchy). Names of the family group also efficiently refer to their type genus (ICZN, 1999, Articles 25-34); the type species, however, can only be retrieved after consulting the relevant literature (e.g., King and Burke, 1989, for turtles). Using the associated differentia, systematists are able to approximate the limits of the circumscription of an ICZN-taxon, but they will typically not be able to do so with precision. Furthermore, because names are not tied to groups of organisms, but rather to ranks and type species, any ICZN-taxon name can refer to a large number of different groups (as long as they include the type), thus impeding cognitive efficiency. Because it is our intention to document the history of names associated with groups of turtles, most of the sections below are devoted to establishing methods that allow approximation of the groups to which traditional taxon names might refer.

Establishing synonymy.-One of the main objectives of this paper is to provide complete lists of all names that have been associated with any given set of species or apomorphies of turtles in order to document the history of name association and to guide the choice of crown clade names. We consider names that apply to the same group of organisms synonymous. This usage of the word synonymy contrasts that of the TCZN (1999, Article 23.3), where two names are considered synonymous only when they refer to ICZN-taxa of equal rank and with the same type species (see above). This usage also differs somewhat from that of the PhyloCode (2003, Article 14) where two names are considered synonymous only when they refer to the same phylo-taxa (i.e., clades), because not all historical groupings considered herein are necessarily clades.

Assessing the group of organisms to which a name refers is difficult in traditional taxonomy (see above). This is because the limits of the circumscription of an ICZN-taxon can only be approximated using the species and diagnostic characters listed by the author (Gauthier and de Queiroz, 2001). Only in very few cases, especially in pre-Darwinian times, did authors clarify how they conceptualized the circumscription of their ICZN-taxa, and how their proposed names were to be tied to those circumscriptions, thus leaving this decision to the inclinations of individual reviewers. All synonymies of circumscriptions of traditional taxon names, consequently, are based on approximations and must be considered subjective. For instance, Linnaeus (1758) made no explicit statement as to the nature of his genus Testudo. II' it is circumscribed in terms of explicit composition, Linnaeus (1758) was referring to what later came to be known as Cryptodira Cope, 1868 (hide-necked turtles), because his species list does not include a single representative of Pleurodira Cope, 1865 (side-necked turtles). Testudo, consequently, could be considered the senior synonym of Cryptodira. However, if Linnaeus (1758) intended to apply a name to any set of species, so long as those species (or organisms) possessed the appropriate differentiating characteristics, we must conclude that pleurodircs are included in 7eif�(/o because they display the relevant character differentia (viz., "corpus tctrapodum, caudatum, testa obtectum"-body four-legged, with a tail, covered by a shell). Last, but not least, because not a single fossil species is included in the Systema Naturae, it is not clear, if Linnaeus' Testudo is to be restricted to the crown, or if it is to include any fossils from along the extinct Testudo stem. As such, how can Testudo Linnaeus, 1758 be objectively synonymized with any other ICZN-taxon name? Correspondingly, if it is unclear to what an author was referring, how can traditional name usage be inferred unambiguously?