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

When considering ICZN-taxa that include fossil species, producing even subjective synonymies proves more difficult because many ICZN-taxon names have been applied to a swarm of internested clades. For example, Rowe and Gauthier (1992) described how the ICZN-taxon name Mammalia has been applied to a broad range of clades originating anywhere from the Carboniferous to the late Jurassic, and differing only in the extent to which they include the phylogenetic stem of crown mammals. Among turtles, a similar swarm of clades is associated, for instance, with the turtle ICZN-taxon name Dermochelyidae (e.g., Gray, 1825; Seeley, 1880; Lydekker, 1889; Hirayama, 1994). What unifies these usages is the presumably honest intent of any given author to include only fossil stem representatives that they deem anatomically 'mammalian' or 'dermochelyid' in some essential feature(s). That is to say, authors generally referred fossils to 'Dermochelyidae' because their characters were thought to be sufficiently similar to those of extant Dermochelys coriacea. Because we are most interested in documenting the names associated with crown clades, we chose from a list of available stem-associated ICZN-taxon names that name which most nearly circumscribes the crown in its referred composition.

Although ICZN-taxa are often explicitly conceptualized as clades in more recent literature, the connection between a name and a clade is seldom unambiguous (e.g., Laurin, 2002). Typically, names are written next to nodes, thus implying a node-based understanding of the specified ICZN-taxon. Subsequent nomenclatural decisions often turn on characters, however, implying an apomorphy-based conceptualization. To enable a comparison of ICZN-taxon names used in the cladistic literature with names from the traditional neontological and paleontological literature, we decided to consider all names used in cladograms as referring to the node they label.

Using this method of establishing synonymy, several apomorphy-based ICZN-taxa can refer to different nodes at different times. That comes as no surprise to traditional taxonomists because the circumscription of an ICZN-taxon is allowed to vary. For instance, based on a series of apomorphies, Gaffney (1975a, 1975c) assigned the name Eucryptodira to the crown node composed of all cryptodiran turtles. Soon after, several fossil taxa were discovered that also exhibited the defining apomorphies of 'Eucryptodira,' but were situated just basal to the cryptodiran crown. Gaffney (1984) accordingly included them in 'Eucryptodira' Gaffney, 1975c, and proposed a new apomorphy-based name-Polycryptodira-for the collection of species previously called 'Eucryptodira'. Since then, Gaffney (1996) referred to 'Polycryptodira' a new fossil stem species that possessed all these apomorphies, thus leaving crown Cryptodira-one of the primary clades of turtles we presume most herpetologists want to talk about-without a name once again.

Character versus node versus stem: Who gets the name?.-With the advent of modern methods of phylogenetic inference and explicit phylogenetic hypotheses, a discussion quickly emerged about which among a series of internested clades should receive the most widely used name (sensu de Queiroz and Gauthier, 1992). Following the arguments presented in a series of articles (de Queiroz and Donoghue, 1988; de Queiroz and Gauthier, 1990, 1992, 1994; Rowe and Gauthier, 1992; Gaumier and de Queiroz, 2001), we assign all widely used names to crown clades. Although we urge the interested reader to refer to the publications listed above, we will briefly summarize two major arguments that favor this approach below.