ON THE SPECIAL TREATMENT OF FOSSILS AND TAXONOMIC BURDEN: A RESPONSE TO LOCONTE

Loconte (1990) contends that extinct and extant organisms should be treated differently in cladistics, and he is not alone in this point of view (e.g., Crowson, 1970; Hennig, 1966, 1981; L ~ v t r u p , 1977, 1985; Patterson, 1977, 1981a,b, 1982; Jefferies, 1979, 1986; Rieppel, 1979: 147; Rosen et al., 1981: 178; Fortey and Jefferies, 1982; Gardiner, 1982; Ax, 1985, 1987; Willmann, 1985; Forey, 1986; Craske and Jefferies, 1989). There are two interrelated issues in such a position: (a) fossils do not count as much as living terminal taxa in assessing sister group relationships, and (2) special taxonomic conventions, such as plesion (Patterson and Rosen, 1977), are required to be able to communicate accurately the position of fossils on a cladogram. The importance attributed to different character bearers is usually reflected in the protocol recommended for cladogram construction. For example (Gardiner, 1982), clades of living organisms are individuated first, and the position of fossils, if they are considered at all, is examined in the context of those clades. In the jargon of Ax (1987: 201-231), fossils are added to the stem lineages connecting two monophyla that are adelphotaxa. Extant organisms are supposed to be more important than fossils because they are complete sources of information, are of the same age, and have had greater taxonomic stability (Patterson, 1981a,b; Craske and Jefferies, 1989: 73). The need for special taxonomic conventions is a consequence of the extra burden fossils place on the number of categorical ranks in the Linnaean system because extinct organisms tend to be plesiomorphic sister lineages (Gauthier et al., 1988). Gauthier et al. (1988; see also Donoghue et al., 1989) addressed both of these issues. They demonstrated empirically that fossils can overturn a theory of relationships based only on Recent taxa, and concluded (p. 191) “that extant taxa may be even less informative, or for that matter unimportant in determining amniote phylogeny.” Contrary to Loconte’s claim (p. 187), Gauthier et al. did not “propose to distinguish stem lineages from their respective monophyla”. In point of fact, Gauthier et al. analyzed a single set of all the relevant available evidence, one in which living and extinct forms were coequal as terminal taxa (Table 2, Fig. 3) , and that operation is consistent with the maxim of total evidence (Kluge, 1989). Lastly, Loconte (p. 189) states that “if fossils are unambiguous members of a stem lineage, the fossil record can be utilized as a secondary criterion of character polarization that is capable of overturning a polarization by outgroup comparison between extant taxa, or resolving an equivocal polarization”. These conclusions do not appIy to fossils alone, and are therefore trivial, because “unambiguous” sister relationships of any kind of organism cannot be improved upon. Gauthier et al. ( 1988) dealt with the burden fossils place on taxonomy by simply doing away with higher categorical ranks. They argued that the discovered cladogram serves