A phylogenetic tree represents a graphical illustration of evolutionary relationships among taxa. The assignment of categori(c)al ranks for classification, however, is not an inherent attribute of a phylogenetic tree but is based on downstream subjective decisions. 

Classifications introduce artificial categories (bands, grades, sections) into continuously evolving biological systems. This poses the question where to draw the lines to separate neighboring categories. The International Commission on Zoological Nomenclature (ICZN) proudly claims to ensure the freedom of scientists to classify animals according to their personal philosophy. In other words, the ICZN doesn´t provide any rules for assigning categorial ranks to taxa beyond the demand for monophyly. Dissatisfied with this situation, a group of taxonomists developed a rank-free alternative taxonomic system, the PhyloCode (De Queiroz & Cantino, 2020; Laurin, 2024). 

Personally, I favour the idea first proposed by Hennig (1966) to maintain categorial ranks provided that their assignment is strictly tied to taxon age. Contrary to the principles of "integrative taxonomy", no other features (like reproductive isolation, phenotype, niche differentiation, vocalisation etc.) should be taken into account.

Timetrees offer the opportunity for clades to be ranked according to their absolute ages. To establish age-based classifications, temporal thresholds (cutoffs) must be defined. For supraspecific ranks, this approach was applied by Sibley et al. (1988), Sibley & Monroe (1990), Monroe & Sibley (1993), Avise & Johns (1999), Holt & Jonsson (2014), Naomi (2014), Jønsson et al. (2016), and Fjeldså et al. (2020).

For classifications that are based on clade age (as reflected by the amount of genetic difference) the term "chronoclassification" is introduced here. Age-based classifications provide simplified phylogenies at the expense of resolution: the broader the temporal bands (i.e. the fewer the cutoffs), the fewer the ranks and the poorer the resolution. Irrespective of the scale of resolution, age-based categories guarantee "categorical equivalence" among clades of the same rank (Sibley et al., 1988, page 410).

Graphic illustrating the approach of applying rank-defining cutoffs (series of coloured broken lines) to timetrees. The number of taxa ascribed to a category of a given rank (figures in brackets behind rank names) depends on the position of the respective cutoffs. Note that the selection of categorial ranks and terminal taxa is arbitrary and has no particular meaning.

Before applying appropriate temporal cutoffs, chronotaxonomists have to decide to which major groups of organisms (e.g. traditional domains, kingdoms, phyla, classes) the same set of cutoffs shall be applied. Naomi (2014), for example, proposed an extremely broad chronotaxonomic framework that covers all animals, plants and fungi. On the other hand it is certainly not advisable to define cutoffs that are restricted to subordinate taxa (e.g. traditional orders, families etc.). Personally, I advocate applying cutoffs to taxa traditionally ranked as "class" (as is the case for Aves), because it is already part of the name "Classification".

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.

Arguments against a universal age-based yardstick approach for species delimitation have been put forward (e.g. Halley et al., 2017), but I do not consider the arguments convincing.

Caveat: The future prospects for chronoclassification are critically dependent on advances in divergence-times estimation and fossil calibration. At present, divergence-time estimates are mostly based on a limited number of mitochondrial genes. In the future, long UCEs or universal single-copy orthologs (USCOs) might serve as nuclear markers for classification (Dietz et al. 2023a,b; Musher et al. 2024). In the meantime, it is not advisable to produce a multitude of conflicting preliminary versions.

References

Avise JC, and Johns GC (1999), Proposal for a standardized temporal scheme of biological classification for extant species, Proc. Natl. Acad. Sci. 96, 7358-63. (pdf)

De Queiroz K, and Gauthier J (1992), Phylogenetic taxonomy, Annu. Rev. Ecol. Syst. 23, 449-480. (link)

De Queiroz K, and Cantino P (2020), International Code of Phylogenetic Nomenclature (PhyloCode), 190 pages. CRC Press. Boca Raton, FL. (free online access)

Dietz L, Eberle J, Mayer C, Kukowka S, Bohacz C, Baur H, Espeland M, Huber BA, Hutter C, Mengual X, Peters RS, Vences M, Wesener T, Willmott K, Misof B, Niehuis O, and Ahrens D (2023a), Standardized nuclear markers improve and homogenize species delimitation in Metazoa, Methods Ecol. Evol. 14, 543-555. (pdf)

Dietz L, Mayer C, Stolle E, Eberle J, Misof B, Posiadlowski L, Niehuis O, and Ahrens D (2023b), Metazoa-level USCOs as markers in species delimitation and classification, Mol. Ecol. Resour. 00, e:13921. (pdf)

Fjeldså J, Christidis L, Ericson PGP, Stervander M, Ohlson LI, and Alström P (2020), An updated classification of passerine birds, In: The largest avian radiation (Fjeldså, J, Christidis L, and Ericson PGP, eds.), pp. 45-63. Lynx Edicions, Barcelona. (link)

Hennig W (1966) Phylogenetic systematics, University of Illinois Press, Chicago, IL. (online book) (pdf)

Holt BG, and Jønsson KA (2014), Reconciling hierarchical taxonomy with molecular phylogenies, Syst. Biol. 63, 1010-17. (pdf)

Jønsson KA, Fabre PH, Kennedy JD, Holt BG, Borregaard MK, Rahbek C, and Fjeldså J (2016), A supermatrix phylogeny of corvoid passerine birds (Aves: Corvides), Mol. Phylogenet. Evol. 94, 87-94. (abstract)

Laurin M (2024), The Advent of PhyloCode – The Continuing Evolution of Biological Nomenclature, 226 pages, CRC Press, Boca Raton, FL. (link) 

Monroe BL, and Sibley CG (1993), A World Checklist of Birds, Yale University Press, New Haven, CT. (link)

Musher LJ, Catanach TA, Valqui T, Brumfield RT, Aleixo A, Johnson KP, and Weckstein JD (2024), Whole-genome phylogenomics of the tinamous (Aves: Tinamidae): comparing gene tree estimation error between BUSCOs and UCEs illuminates rapid divergence with introgression, bioRxiv (pdf)

Naomi SI (2014), Proposal of an integrated framework of biological taxonomy: a phylogenetic taxonomy, with the method of using names with standard endings in clade nomenclature, Bionomina 7, 1-44. (pdf)

Ruggiero MA, Gordon DP, Orrell TM, Bailly N, Bourgoin T, Brusca RC, Cavalier-Smith T, Guiry MD, and Kirk PM (2015), A higher level classification of all living organisms, PLOS ONE 10, e:0119248. (free pdf)

Ruggiero MA, Gordon DP, Orrell TM, Bailly N, Bourgoin T, Brusca RC, Cavalier-Smith T, Guiry MD, and Kirk PM (2015), Correction: a higher level classification of all living organisms, PLOS ONE 10, e:0130114. (free pdf)

Sibley CG, Ahlquist JE, and Monroe BL (1988), A classification of the living birds of the world based on DNA-DNA hybridization studies, Auk 105, 409-423. (pdf) 

Sibley CG, and Monroe BL (1990), Distribution and Taxonomy of the Birds of the World, Yale University Press, CT. (link)