Introduction

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 each categorical rank. For supraspecific ranks, this approach was first proposed by Willi Hennig (1966) and has repeatedly been advocated since, e.g. by Avise & Johns (1999), Holt & Jonsson (2014), Naomi (2014), Jønsson et al. (2016), and Fjeldså et al. (2020). 

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. In this way, sets of serial cutoffs introduce categories (grades, sections) into an otherwise continuous system.

The derivation of a chronoclassification from a timetree is performed by assigning the same rank to the pan-clade of each lineage traversing the same cutoff line: 

Section of an imaginary timetree to which rank-defining temporal thresholds (cutoffs X, Y) have been applied. The time interval between two neighbouring cutoffs is referred to as temporal band. Pan-clades of evolutionary lineages that traverse the same cutoff line ("basic clades" of Naomi, 2014) are attributed the same categorical rank (highlighted in red). 

Once a comprehensive set of cutoffs is defined, the corresponding ranks can be assigned to all clades in timetrees:

Exemplary order-to-species-level timetree to which a set of cutoffs is applied to assign categorical ranks to clades. In contrast to other cutoffs, the genus cutoff should be applied flexible to allow preservation of well-established genera. The age should span from 8 Ma to 12 Ma.

Rank-assigned timetrees serve as templates for chronoclassifications: 

Chronoclassification, derived from the rank-assigned order-to-species-level timetree presented above.

The taxonomic range of chronoclassifications

Chronotaxonomists have to agree on the question 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 a chronotaxonomic system that covers all animals, plants and fungi. Although this is an extremely broad framework, it is certainly not advisable to define different cutoffs for a plethora of subordinate taxa (e.g. traditional orders, families etc.). I prefer applying sets of rank-defining cutoffs to taxa traditionally ranked as a class because it is part of the name CLASSification. 

References

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

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

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)

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)