Geological age of origin
Perhaps the most frequently proposed criterion to rank taxa generally (not at a single taxonomic level) is geological age of origin of clades, and as such, it deserves a more detailed discussion. It was first proposed by Hennig (1966), and supported most recently by Avise and Mitchell (2007). Under that proposal, the geological age of origin of the least inclusive node subtending all terminal taxa of a clade (or even possibly paraphyletic group) would determine the rank of that taxon. For this purpose, geological times would be subdivided into a number of periods that could (but need not) coincide with currently recognized geological periods. The tremendous advantage of this proposal is that taxa of a given rank would have comparable ages of origin, a feature that would vastly improve usefulness of biological classifications in evolutionary studies, as emphasized by Avise and Johns (1999). The main drawback of this proposal is that it would require numerous changes in rank allocations, and this probably explains why this proposal has not been more generally followed, although some studies applied its principles to suggest rank allocation of a few taxa (Lim, 2007; Tinn and Oakley, 2008). The more recent proposal by Avise and Mitchell (2007) is aimed at avoiding this problem; it consists in appending timeclips to taxa (ranked or not). These could take the form of a three-letter code within brackets, such as [G:pa] to denote a clade originating in the Paleocene (the particular temporal divisions adopted and corresponding codes were left open by the authors). Thus, as this temporal information becomes available through molecular (e.g. Hugall et al., 2007) or paleontological dating (e.g. Maranović and Laurin, 2007), it could be incorporated into taxonomies. Under this latest proposal, rank allocation would not need to be changed to reflect the geological age of origin (although it could be done if the systematic community chose to do it), so it would not create taxonomic confusion. However, unless categories were reassigned to reflect age of origin, they would remain ontologically empty. The useful ranking information would reside entirely in the timeclips, rather than in the Linnaean categories.
The geological age of origin would be optimal to determine ranks if taxa originated in a synchronous, periodic (but not necessarily regular) manner. Suppose, for instance, that the first cladogenesis in a lineage occurs at time t. The next cladogenesis in both descendents occurs at time t+c1 (c1 is a constant). The four resulting lineages speciate again at time t+c2 (c2 is another constant). In such a case (Fig. 2a), since several taxa appear simultaneously, their age of origin could be used to determine their rank. Unfortunately, cladogeneses do not appear to be coordinated in such ways (Fig. 2b). Nothing in modern evolutionary theory (Lee and Doughty, 2003; Minelli, 2007; Padian, 2008) predicts that cladogeneses should be simultaneous. The age of origin of taxa is often difficult to determine, but both molecular (Sanderson, 2002; Hedges and Kumar, 2009; Hugall et al., 2007) and paleontological dating (Marjanović and Laurin, 2007, 2008) suggest that asynchronous cladogeneses are the rule. There are periods of intense cladogenesis, for instance when taxa invade new niches (Ward et al., 2006), or after mass extinction has emptied ecological niches (Bromham, 2003), but these presumably represent periods of dense, asynchronous cladogenetic events.
Ranks traditionally attributed to taxa certainly do not reflect geological age, as shown by even a cursory glance at the literature. Extinct organisms of any geological age are usually attributed to taxa of all five ‘mandatory’ categories (genus, family, order, class, and phylum), in addition to a taxon of species rank (Laurin, 2005). Thus, the oldest species is as old as life itself, and so is the oldest genus, family, order, class, and phylum. Even if we exclude extinct organisms (a decision that would be difficult to justify but that might somewhat improve the correlation between taxonomic rank and geological age of origin), the geological age of taxa of any given rank is highly variable. In these comparisons, the age of origin of a taxon will be taken as the age of its basal node (i.e. the age of its oldest fossil member, or the nodal age inferred by molecular dating), rather than the age of its stem, although changing this choice would only make all taxa older without changing the age difference between them much. Sirenidae (ranked as a family), a clade of aquatic salamanders, originated in the Early or Late Cretaceous (about 80 to 110 Ma ago), depending on whether or not some extinct forms are included (Marjanović and Laurin, 2007: fig. 3). Hominidae (also a family under rank‑ based nomenclature) originated about 7 Ma ago in the Miocene, if it is defined as the largest clade that includes Homo sapiens but not Pan troglodytes (Linnaeus, 1758), the chimpanzee (Pilbeam and Young, 2004), so it is at least 10 times more recent. Some lissamphibian genera (Amphiuma, Necturus, Dicamptodon) appeared in the Paleocene (Marjanović and Laurin, 2007: fig. 4), about 60 Ma ago, whereas the genus Homo dates from less than 3 Ma (Semaw et al., 2005). Clearly, the geological age of origin of taxa of a given rank is highly variable; changing rank allocation to improve the correlation between age and rank would result in so many, and so drastic, nomenclatural changes that this solution will surely appear unsatisfactory to most systematists.