Cutoffs for avian chronoclassification

In the following, a comprehensive set of cutoffs is suggested for the chronoclassification of birds (Aves), a clade that has been ranked as "classis" since Linnaeus (see e.g. Kumar et al, 2022).

The proposed avian cutoffs are based on the avian timetree provided by Kuhl et al. (2021). Cutoffs were determined by initially setting the cutoff for avian orders at 55 Ma. Higher-rank cutoffs were then set at 10 Ma intervals to roughly conform to the results of Holt & Jønsson (2014). In their pioneering study (which is based on the taxonomy and phylogeny of Jetz et al., 2012), these authors 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 species often exhibit substantial intraspecific diversification, subspecies and infrasubspecies must also be considered. Infrasubspecies 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".

Newly proposed set of cutoffs to define categorical ranks for comprehensive avian chronoclassification. 

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.

Number of ranks

Personally, I tend to rely on the categorial ranks that are considered 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).

Alternative cutoffs for birds

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, instead of 35 Ma as suggested here. 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).

Alternative recognition of class Reptilia

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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