In the future, one of the following alternative sets of temporal cutoffs (depending on the outcome of refined future divergence-times estimates) should be implemented in avian taxonomy to chronoclassify birds:

Alternative sets of serial temporal cutoffs defining categorical ranks for avian chronoclassification. Ranks and their respective endings follow the pioneering study of Sibley et al. (1988). [black: class-group names are not regulated, blue: order-group names and endings are not regulated, green: family-group names and endings covered by ICZN-code, red: genus-group names covered by ICZN-code, grey: species-group names covered by ICZN-code].

Based on the timetree of Jetz et al. (2012), Holt & & Jønsson (2014) cut phylogenies of class Aves at ages that returned the same number of clades as found in traditional ranks, resulting in cutoffs at 65 Ma for avian orders, at 37.5 Ma for avian families, and 11.4 Ma for avian genera.

As putative species often exhibit substantial intraspecific diversity, subspecies and infrasubspecies should also be considered. The latter are not regulated by the ICZN-Code, but article 45.5 states that "a fourth name published as an addition to a trinomen automatically denotes an infrasubspecific entity".

To provide temporal information to clades above class rank, either timeclips (Avise & Mitchell, 2007), or plain age information (Zachos et al., 2011) could be used. The combined use of temporal thresholds and timeclipping provides relative nomenclatural stability within classes, as well as temporal comparability among classes.

Personally, I tend to rely on the categorial ranks described in the ICZN-Code. This traditional ranking system represents a well-balanced trade-off between phylogenetic resolution on the one hand and taxonomic manageability on the other. Moreover, ICZN-ranked clades are recognised by standardised endings (-oidea, -idae, -inae, -ini, -ina).

Temporal threshold values (cutoffs) are inherently arbitrary and depend on conventions that taxonomists must agree upon. Thus countless alternatives are possible. For example, some scientists might want to retain the current classification of Passeriformes and adjust other avian orders accordingly. For example, Jønsson et al. (2016) assigned family rank at 21.5 Ma, Cai et al. (2019) at 18 Ma, and Cai et al. (2021) at 15 Ma. Relatively young family ages are also found in Charadriiformes, Procellariiformes, and Piciformes. For most avian orders, however, shifting temporal cutoffs towards younger family ages would not conform to previous suggestions (Avise & Johns, 1999; Holt & Jønsson, 2014; Naomi, 2014).

In their higher-level classification of all extant living organisms, Ruggiero et al. (2015) treated Aves (as well as Crocodylomorpha, Rhynchocephalia, Squamata, and Testudinata) as a subclass of class Reptilia. This is a feasible alternative approach that would, however, lead to major rearrangements to conventional avian classifications.

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