Temporal banding

In traditional phylogenies, the age of clades was not taken into account. However, modern techniques allow divergence times to be assessed with reasonable accuracy, although they may still differ among authors. 

Time-calibrated (dated) phylogenies, in which branch lengths are proportional to time, are usually referred to as chronograms or less often as timetrees. I prefer the latter term because of its euphony. 

The availability of reliably dated phylogenies offers the opportunity for clades to be categorised and ranked according to their absolute ages. To establish age-based classifications, temporal frames have to be defined for each category, This approach, first proposed by Willi Hennig (1966), has been termed "temporal banding" by Avise & John (1999). The younger limit of the temporal bands, which is of particular relevance, is referred to as cutoff age, cutoff line, cutoff point, or cutoff. I will use the term „cutoff“ because it’s the shortest. 

The greatest challenge for applying temporal banding to timetrees lies in answering the question to which groups of organisms the same temporal bands shall be applied. While I agree with Kraichach et al. (2017) that „with different groups of organisms having different evolutionary histories and timelines, trying to find one universal cut-off for each taxonomic rank might not be productive“, Naomi (2014) did propose an integrated framework of biological taxonomy for animals, plants, and fungi.

In the following, I will apply the approach of temporal banding to the clade Aves that has been ranked as a “classis” since Linnaeus (1758), thus literally providing a CLASSification. In Wikipedia (as of May 2021), a list of 107 animal classes is given, e.g. Amphibia, Aves, Gastropoda, Insecta, and Mammalia.

To provide temporal information to clades above class level, either timeclips (Avise & Mitchell, 2007), or plain age information (Zachos, 2011) could be used. The combination of temporal banding and timeclipping ensures relative nomenclatural stability within classes as well as temporal comparability among classes. 

Before applying temporal banding to the avian timetree of Kuhl et al. (2021), I defined a continuous series of temporal bands, each of which is associated with a particular Linnaean rank: 

Definition of temporal bands and corresponding ranks that will be applied for the temporal banding of the avian timetree of Kuhl et al. (2021). 

Temporal bands for avian categorical ranks were determined by initially setting the cutoff for avian orders at 55 Ma. The remaining cutoffs were then aligned at 10 Ma intervals for higher, and progressively shorter time intervals for lower categorical ranks in order to roughly conform to the previous results of Holt & Jønsson (2014). In their pioneering study these authors cut phylogenies at ages that returned the same number of clades as found in the original rankings, resulting in cutoffs at 65 Ma for orders, at 37 Ma for families, and at 11 Ma for genera. 

Interestingly, many clade ages recognised by both Avise & Johns (1999, table 2) and Naomi (2014, table 2), who based their temporal bands on geological episodes, are pretty close to the average clade ages established by Holt & Jønsson (2014). 

Holt & Jønsson (2014), who based their temporal banding on the timetree of Jetz er al. (2012), also were the first to convincingly demonstrate that the intrinsic classification of Passeriformes is not at all compatible with other avian orders. For example, they lumped 14 out of 15 families of Passeroidea into a single family. 

Futuristic family-level timetree of extant Aves based on Kuhl et al. (2021), to which temporal banding has 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 banding has 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. 

How to create an age-based CLASSification?

The broader temporal banding approach as a means of deriving age-based CLASSifications from timetrees encompasses the following consecutive steps: 

Step 1: Select a class. 

Step 2: Create or select a timetree. 

Step 3: Determine "Temporal Error Scores" (sensu Holt & Jønsson, 2014) for orders, families, and genera. 

Step 4: Define temporal bands of equal age (e.g. 10 Ma, 20 Ma, 50 Ma), taking into account the results of the previous step.  

Step 5: Apply "Cutoff Collapsing" to each temporal band (see Note: "CLASSification") to transcribe the revised timetree into a formal CLASSification. 

While steps 1-4 have been executed above, step 5 will be implemented in the Note “CLASSification.


Alternative temporal banding of CLASS Aves

Temporal bands are inherently arbitrary and only depend on conventions that taxonomists 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 instead. In this case, temporal bands would have to be arranged quite differently.

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. Comparable young family ages are also found in Charadriiformes, Procellariiformes, and Piciformes. For most avian orders, however, shifting temporal bands towards younger ages would lead to profound changes of current Linnaean ranks. 

These very young family ages would not conform to any previous suggestion (Avise & Johns, 1999; Holt & Jønsson, 2014; Naomi, 2014). 

Alternative recognition of CLASS Reptilia

In their higher-level classification of all living organisms, Ruggiero et al. (2015) treated Aves (as well as Crocodylomorpha, Rhynchocephalia, Squamata, and Testudinata) as a subclass of the CLASS Reptilia. This is a feasible alternative approach. However, it would lead to major rearrangements to conventional avian classifications. 


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