In the figure below, the avian timetree of Kuhl et al. (2011) will be broken up into a number of consecutive temporal thresholds as a prerequisite for transcribing the timetree into a CLASSification.

Futuristic family-level timetree of extant Aves based on Kuhl et al. (2021), to which temporal thresholds have been applied here. A number of clades that are traditionally considered families have been downgraded to subfamily or tribal rank, or even below (red, orange and yellow family names, respectively). In Passeriformes, the situation is particularly challenging, as more than one hundred traditional families will have to be merged to only seven families. On the other hand, only few traditional families will have to be split, e.g. Cuculidae, Falconidae, and probably Scolopacidae.

Futuristic order-level timetree of extant Aves based on Kuhl et al. (2021), to which temporal thresholds have been applied here. The cutoffs at 55 Ma and 65 Ma 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 cutoff at 65,5 Ma to retain loons (Gaviiformes) in traditional Aequornithes. However, I found it tempting to set cutoffs in steps of 10 Ma. 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. 

The transcription process itself is performed by assigning the same rank to all terminal pan-clades of each temporal band according to the following scheme:

Diagram depicting an arbitrary temporal band as part of a timetree that shall be transcribed into a formal CLASSification. The intermediary nodes and corresponding clades have to be disregarded, taking into account only the terminal pan-clades (“basic clades“ of Naomi, 2014). The number of clades corresponds to the number of branches traversing the cutoff. 

Collapsing nested (i.e. hierarchical-encaptic) clades by ignoring intermediary nodes within a given temporal band results in a series of pan-clades of identical ranks. The reiterated application of this procedure (temporal band by temporal band) eventually leads to a formal CLASSification. For this way of transcribing timetrees into CLASSifications the term "cutoff collapsing" is proposed here. Applying this approach to the temporally-banded timetree of Kuhl et al. (2021) yielded a revised family-level CLASSification of birds: 

Futuristic family-level timetree (left) and corresponding CLASSification (right) of extant Aves after applying temporal banding to the timetree of Kuhl et al. (2021). 

Ultimately, classifications should cover all taxonomic ranks, including tribes, subtribes, genera, and subgenera. At present, however, the data provided by Kuhl et al. (2021) only allow birds to be classified to subfamily level: 

Futuristic subfamily-level CLASSification of extant birds based on the timetree of Kuhl et al. (2021) to which temporal banding has been applied. [Please note that given rank names might nor comply with the specifications of the “ICZN code”; the names should therefore be considered preliminary]

Overall, the new CLASSification shows a high level of agreement with the conventional avian classification. Passeriformes, however, are a notable exception. While more than one hundred families are traditionally recognised in this order, the new CLASSification recognises just seven families, which still is the highest number of any avian order.

The pros and cons of temporal banding have been extensively discussed elsewhere (e.g. Kraichach et al., 2017; Lücking, 2019). Most authors have reservations against a strictly age-based classification. While the usefulness of the approach as an accessory tool is generally acknowledged, most taxonomists seem to fear that easily diagnosable and long-established clades of their primary research area might be split or lumped. While I consider most of their arguments unconvincing, it is certainly true that the reliable reconstruction of divergence times is still a major scientific challenge.  

However, even critics of the temporal-banding approach concur that obvious temporal outliers (like Mirandornithes and Passeriformes) should be adjusted to better fit the classification of the clade as a whole. Last but not least, implementation of temporal banding might pave the way for proponents of the PhyloCode to return to Linnaean classification.

Finally the question has be addressed how to integrate the fossil record of extinct clades into the new CLASSification. An elegant answer has been proposed by Lücking (2019), who suggested to apply "relative temporal bands", by considering the lifspan of lineages rather than the absolute geological time of existence. 

Why CLASSifications?

CLASSifications provide simplified phylogenies that fascilitate communication by waiving graphical tree reconstructions. The simplification is achieved at the expense of temporal resolution. The rule is simple: the broader the temporal bands, the poorer the resolution. Temporal bands introduce distinct “grades” into an otherwise continuous system.