Temporal threshold values (cutoffs) are inherently arbitrary and depend on conventions that taxonomists must agree upon. Thus countless alternatives are possible. Below, I present three alternative sets of cutoffs, one of which, depending on the outcome of refined future fossil calibrations and divergence-times estimates, might be implemented one day for avian chronoclassification: 

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].

For comparison:

• Avise & Johns (1999) proposed the following avian cutoffs: 145 Ma for orders, 65 Ma for suborders, 56 Ma for superfamilies, 33 Ma for families, 24 Ma for subfamilies, 5 Ma for tribes, and 2 Ma for genera.
  
• Holt & Jønsson (2014), based on the timetree of Jetz et al. (2012), determined cutoffs that yielded the same number of taxa as previously recognised in the respective categorial rank: 65 Ma for avian orders, 37.5 Ma for avian families, and 11.4 Ma for avian genera.
  
• To retain family rank for many clades within Passeriformes, ornithologists tend to apply considerably younger cutoffs for passerine families. Thus, 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. Comparatively young traditional families are also recognised in Charadriiformes, Procellariiformes, and Piciformes. [Note: Although probably favoured by many/most avian taxonomists, I dislike the idea of establishing a second, younger series of cutoffs especially for the Passeriformes.]

To provide temporal information for 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 both relative nomenclatural stability within classes as well as temporal comparability among classes.

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