In traditional classifications, the age of clades is not taken into account. However, modern techniques now allow divergence times to be assessed with sufficient accuracy, although they may still differ widely among authors.
Divergence-time estimates are based on molecular dating techniques, which in turn depend on reliable fossil calibrations, i. e. correctly dated and placed fossils. To some extent they also depend on the selection of fossils and the maximum age constraint set by investigators.
Time-calibrated ( dated) phylogenies, in which branch lengths are proportional to time, are usually referred to as chronograms or less often as timetrees. I prefer the latter term because of its euphony.
Dated phylogenies allow clades to be ranked according to their geological ages. Clades recognised at the same rank are then of comparable age. To establish age-based classifications, cut-off timelines (also called cut-off ages, limits, lines, or points) have to be defined for each categorical rank. This approach, first recognised by Willi Hennig (1966), has been termed „temporal banding“ by Avise & Johns (1999). The term refers to the time period between consecutive cut-off timelines.
The ultimate challenge for applying the temporal banding approach to timetrees will be answering the question to which groups of organisms the same temporal cut-off timelines shall be applied. Here, I will apply the approach to the clade Aves that has been ranked as a „class“ since Linnaeus (1758), thus literally providing a CLASSification. In Wikipedia (as of Max 2021), a list of 107 animal classes is given, e.g. Amphibia, Aves, Gastropoda, Insecta, and Mammalia.
To provide temporal information to clades above class- level, either so-called timeclips (Avise & Mitchell, 2007), or just age information could be used (Zachos, 2011). The dual approach would combine nomenclatural stability within individual classes, and comparability among different classes.
In the following, the approach of temporal banding is applied to the avian timetree of Kuhl et al. (3021). To provide the basis for assigning a categorical rank to each clade, a continuous series of cut-off timelines has been defined:
The choice of cut-off timelineswas initially determined by setting the age of avian orders at 55 mya. The remaining cut-off timelines were then aligned at 10 myr intervals for higher, and at 2,5 myr intervals for lower categorical ranks in order to roughly conform to the results of Holt & Jønsson (2014). In their pioneering study the authors cut phylogenies at ages that returned the same number of clades as found in the originalrankings, resulting in cut-off timelines of 65 mya for orders, 37 mya for families, and 11 myr ago for genera.
It is stressed that cut-off tinelines are fully arbitrary (though of course not haphazardly) and only depend on the consensus of taxonomists.
Holt & Jønsson (2014), who based their study on the timetree of Jetz et al. (2012), also demonstrated that the intrinsic classification of Passeriformes is not compatible with the other avian orders. For example, they lumped 14 out of 15 families of Passeroidea into a single family.
To preserve traditional passeriform families, Jonsson et al. (2016) in their study on Corvides had to assign family rank to clades at onl 21.5 mya. In another study on Passeriformes, Cai et al. (2019) assignes family status at only 18 mya. However, these very young family ages to not comply with avian classification in general.
Futuristic family-level timetree of extant Aves based on Kuhl et al. (2021), to which temporal banding has been applied here. A number of clades that are traditionally considered families will have to be downgraded to subfamily or tribal rank, or even below (red, orange and yellow family names, respectively). In Passeriformes, however, the situation is quite challenging, as more than one hundred traditional families will have to be merged to only seven families. On the other hand, some traditional families will have to be split, e.g. Cuculidae and Scolopacidae.
Futuristic order-level timetree of extant Aves based on Kuhl et al. (2021), to which temporal banding has been applied here. The cut-off timelines at 55 myr and 65 myr were chosen to define oders and superorders, respectively. As a result, several traditional orders had to be split (blue and green order names), while only two traditional orders, flamingos (Phoenicopteriformes) and grebes (Podicipediformes), had to be merged (red order name). It would have been possible to set the superordinal cut-off timeline at 65,5 myr to retain loons (Gaviiformes) in traditional Aequornithes. However, I found it tempting to set cut-off timelines in steps of 10 myr. The new taxonomy distinguishes 43 orders and 10 superorders. Note that crown-group ages derived from Kuhl et al. (2021, suppl.) are indicated by blue lines, whereas crown-group ages derived from other sources are indicated by green lines.
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