Ghost taxa, ancestors, and assumptions: a comment on Wagner

In a recent paper Wagner (1995) asserts that the stratigraphic distribution of fossils is an independent source of evidence for phylogenetic branching pattern and can be used to test patterns of branching derived from cladistic analysis. Wagner develops a phylogenetic method integrating the superpositional relationships of fossils with cladograms. In some cases his method prefers topologies that are at odds (topologically different or have different lengths) with the most parsimonious tree derived from cladistic analysis of characters alone. As part of his argument, Wagner discusses the concepts of ghost taxa and ghost lineages (Norell 1987, 1992, 1993). This comment focuses on a misunderstanding of these ideas. Wagner (1995: p. 155) asserts that ghost taxa analyses are based on three assumptions: "1) that the phylogenetic hypothesis is correct, 2) that evolution proceeds strictly via bifurcating cladogenesis (see Rensch 1960), and 3) the cladogram contains no ancestral species. " Reading of my original papers clearly shows that none of these assumptions are required or justified. Wagner contends that assumption 1 "cannot be demonstrated. " For any historical phylogeny except those constructed under laboratory conditions, this is obvious, as the "true" phylogeny can never be known. Furthermore, searching for "truth" is a metaphysical pursuit not particularly relevant to modern schools of phylogenetic inference. One current commentary points out that we should not be "obsessed with 'truth' in the sense of correspondence with the really real, but only in the sense that truth is the limit of scientific inquiry" (Frost and Kluge 1994: p. 260). Whether a phylogenetic hypothesis is "correct" in the sense of how well it estimates the unknowable and subjective ""really real" is trivial as long as phylogenetic methods are agreed on and based on logical considerations. Preference of one phylogenetic method over another boils down to testability and performance relative to some optimality criterion. Such optimality criteria (in my case parsimony and in Wagner's a combination of parsimony as modified by a statistical estimate of superpositional relationships) are chosen according to intrinsic merits of how justifiable and relevant their underlying assumptions are regarding phylogenetic inference. Attempts like Wagner's at synthesizing parsimony and stratigraphy (Fisher 1994; Huelsenbeck 1994; see also Siddall 1996) suffer because they mix optimality criteria. In doing so they compound the number of underlying assumptions, instead of reducing them as Wagner suggests. The misconception that cladistic analysis requires a strictly bifurcating history of evolution is common and undoubtedly stems from a literal reading of cladogram topologies. Cladistic analysis presumes no particular mode or pattern, or even definition, of speciation (for a discussion of such relative to paleobiology see Schoch 1986: p. 109-116). Furthermore, multitomous speciation is often recovered as an empirical solution during phylogenetic analysis (see Swofford and Begle 1993: p. 49). In my 1992 paper (Norell 1992) Figure 3.2 clearly demonstrates a scenario of nondichotomous "speciation" and how it is recovered on a cladogram. The crux of Wagner's entire paper is that ancestral species can and should be recognized. Although I would agree that we can discuss "ancestral" diagnoses (but not taxa), Wagner's view of species is logically impaired. He wants to treat species not as replicator continua but as corporeal things, interactors that re-