In systematics, it is necessary to distinguish between crown clades and total clades. While total clades include all extant and extinct members of a given clade, crown clades are restricted to the last common ancestor and descendants of only the extant representatives of that clade. In other words, crown clades do not encompass representatives of the extinct stem group. 

The logical demand for a distinction between crown clades and total clades poses the question how to separate them linguistically. Creating entirely new names for total clades, as both ICZN and ICPN suggest or tolerate (Louchard et al., 2013; de Queiroz, 2007; Cantino & de Queiroz, 2020; de Queiroz et al., 2020), might lead to a confusing plethora of new names. 

Therefore, I strongly recommend to follow the proposal of my deceased teacher Karl-Ernst Lauterbach (1989) to simply add the hyphenated prefix "pan-" to the crown-clade name to create an informal taxonomic name for the total clade, e.g. pan-Aves. This approach should be applied rigorously, without exceptions. Although implemented in the PhyloCode (chapters 10.3 to 19.7), it was missed to make the use of the prefix „-pan“ mandatory. As a consequence, Chen & Field (2020) suggest the total-clade names Pan-Apodiformes and Pan-Trochilidae on the one hand, but Caprimulgimorphae (instead of Pan-Strisores) on the other hand. 

Unfortunately, the Lauterbach publication introducing the pan-clade concept is written in German and published in a journal that isn't digitally accessible. To alleviate this shortcoming, in the following I present the English abstract of the original paper as well as a redrawing (including a translation of the figure legend) of Fig. 1 in Lauterbach (1989). [Note that the term ‘monophylum’, coined by Hennig (1953) and resumed by Ax (1984/1987), is synonymous to the term ‘clade’ that was coined some years later by Huxley (1957). The term ‘monophylum’ never gained acceptance outside of Germany.]

Abstract

Apart from its lowest hierarchical rank (a pair of sister species) the supraspecific taxon of phylogenetic systematics, the monophylum, displays two subcategories which are directly following one another in time: stem lineage and monophylum s. str. (= crown group). Together only both subcategories mean the complete supraspecific taxon of phylogenetic systematics. Ignorance of the existence of the stem lineages of the monophyla is one of the major faults of traditional typological systematics. Unequivocal handling of the monophylum as the complete taxon (including the stem lineage) or as the monophylum s. str. (without the stem lineage) only presents difficulties in practice since both ranges of a monophylum bear the same taxonomic name. To prevent misunderstandings in writing and particularly in Speech it seems of importance to characterize precisely if the complete monophylum is under discussion or the monophylum s. str. only. Previous attempts to solve this problem could not offer a suitable method for practice. Then it seems of particular importance to look for such a method at this time of transition from traditional typological systematics to phylogenetic systematics. With the introduction of the pan-monophylum for the complete monophylum, and thus emphasizing the existence of its stem lineage, such a method is suggested. It remains to strengthen that the pan-monophylum does not mean the introduction of new taxonomic names but only a means for better and more precisely handling the supraspecific taxa of phylogenetic systematics in practice whilst otherwise the taxonomic names remain unaltered.  

Fig. 1 Structure of the supraspecific taxon (= monophylum) with stem lineage in phylogenetic systematics in chronological order. Note the encircled total monophylum in time (pan-monophylum) in its relation to the monophylum s.str. (= crown group) and the position of the stem line of the latter. The sister taxon of monophylum A, indicated by an arrow (also a monophylum), is of course to be thought of in the same way as the monophylum A. The black triangle with the tip pointing towards the ground-pattern representative of monophylum A represents the progeny of the ground-pattern representative, resulting from the splitting of species. Since it is a monophylum with recent members, it ends in the present time plane as shown (base of the triangle). Further explanations in the text (translated and redrawn after Lauterbach, 1989: Abb.1)

References/Literature

Ax P (1987), The Phylogenetic System. The Systematization of Organisms on the Basis of their Phylogenesis, John Wiley & Sons, Chichester and New York. XI + 340p. [translated by Jefferies RPS, original edition: Das Phylogenetische System, 1984). 

Cantino PD, and de Queiroz K (2020), International code of phylogenetic nomenclature (PhyloCode), version 6. (link)

de Queiroz K (2007), Toward an integrated system of clade names, Syst. Biol. 56, 956-974. (free pdf)

de Queiroz, K, Cantino PD, and Gauthier JA (2020), Phylonyms – a companion to the PhyloCode. CRC Press, Boca Raton, FL, USA. (link)

Haug C and Haug JT (2015), The ambiguous use of the prefix ‘Pan’ in arthropod systematics, RRJZS 3, 19-24. (pdf)

Hennig W (1953), Kritische Bemerkungen zum phylogenetischen System der Insekten, Beitr. Ent. 3, 1-85. (pdf)

Huxley JS (1957), The three types of evolutionary process Nature 180, 454-455. (abstract)

Joyce WG, Anquetin J, Cadena EA, Claude J, Danilov IG, Evers SW, Ferreira GS, Gentry AD, Georgalis GL, Lyson TR, Perez-Garcia A, Rabi M, Sterli J, Vitek NS, and Parham JF (2021), A nomenclature for fossil and living turtles using phylogenetically defined clade names, Swiss J. Palaeontol. 140, e:5. (pdf)

Lauterbach KE (1989), Das Pan-Monophylum: Ein Hilfsmittel für die Praxis der phylogenetischen Systematik, Zool. Anz. 223, 139-156. 

Louchart A, Viriot L, and Dubois A (2013), The use of the prefix Pan- and other problems in zoological family-level nomenclature, Zootaxa 3750, 197-200. (pdf)

Meier R, and Richter S (1992), Suggestions for a more precise usage of proper names. Ambiguities related to the stem lineage concept, J. Zool. Syst. Evol. Res. 30, 81-88. (abstract)