Stability in meaning and content of taxon names: an evaluation of crown-clade definitions

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Abstract
Most well-known taxon names have traditionally referred to groupings that contain fossil organisms lying outside the diversity of living forms, e.g. Aves has traditionally included Archaeopteryx, and Mammalia, Morganucodon. Many systematists have recently advocated applying well-known taxon names to more restricted groups termed `crown-clades': monophyletic groups bounded by extant forms. These definitions change substantially the contents and diagnostic characters of many of the most familiar taxon names, e.g. under crown-clade definitions, Archaeopteryx is excluded from Aves, and Morganucodon from Mammalia. It has been suggested that crown-clade definitions are more stable than traditional, more-inclusive definitions, in terms of both meaning (the association of a particular taxon name with a clade stemming from a particular ancestor) and content (determination of which organisms belong in a particular named clade). Crown-clades have also been suggested to be more highly corroborated than traditional, more-inclusive clades. These arguments are here found to be poorly supported. All phylogenetic (ancestor-based) definitions of taxon names (e.g. `ancestor X, and all its descendants') are equally stable in meaning, in that the ancestor specified, and thus, the boundaries of the clade, will always remain the same regardless of changes in phylogenetic hypotheses. It does not matter whether the specified ancestor is the most recent common ancestor of living forms (a crown-clade definition), or the common ancestor of a more-inclusive group. Crown-clades are also not more stable in content, or highly corroborated, than more-inclusive clades. This is because the position of certain fossil forms within or outside both types of clades is often uncertain. Many workers have interpreted soft anatomical features as synapomorphic of crown-clades, but not of more-inclusive clades, thus arguing that crown-clades are more highly corroborated. However, interpreting these soft anatomical features as synapomorphies of the crown clades is merely an arbitrary convention: these soft anatomical features might diagnose a more-inclusive clade. Thus, crown-clade definitions of taxon names are not superior to more-inclusive definitions in terms of stability in meaning or content. There is therefore no compelling reason to radically redefine familiar taxon names such as Aves, Mammalia and Tetrapoda to apply to crown-clades. Instead, it is possible (and probably less confusing) to attach such well-known taxon names to more-inclusive groups that closely approximate `traditional' usage.