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The PhyloCode
Article 10. Selection of Clade Names for Establishment
10.1. Clade names are generally to be selected in such a way as to minimize disruption of current and/or historical usage (with regard to composition, diagnostic characters, or both) and to maximize continuity with existing literature. Therefore, except under the conditions described in Art. 10.2, a preexisting name that has been applied to a taxon approximating the clade to be named (see Note 9.8.2), or to a paraphyletic group originating with the same ancestor, must be selected. If there is a preexisting name for a paraphyletic group originating with the same ancestor as a particular clade and that name is much better known than any preexisting name for that clade, or if there is no preexisting name for that clade, the name of the paraphyletic group may be (but need not be) chosen.
Note 10.1.1. Art. 10.1 and Rec. 10.1A are not intended either to encourage or to discourage the application of preexisting names to crown, apomorphy-based or total clades. Because the associations of preexisting names with precisely identified clades commonly are ambiguous, reasonable arguments can often be made for applying a particular name to any one of several nested clades between crown and total (inclusive).
Recommendation 10.1A. If more than one preexisting name has been applied to a particular clade (including those applied to paraphyletic groups originating with the same ancestor), the name that is most widely and consistently used for it should generally be chosen, though a less widely used name may be chosen if it is a panclade name (see Note 10.3.1). Similarly, if a preexisting name has been applied to more than one clade, it should generally be established for the clade to which it has been most widely and consistently applied (but see Note 10.1.1). If the most widely and consistently used name is not selected for conversion, a rationale should be provided.
Note 10.1A.1. In selecting "the name that is most widely and consistently used," considerable discretion is left to the converting author. It is not necessary to choose a name that is slightly more widely used than its closest competitor. As a general guideline, if there is less than a twofold difference in the frequency of use of two or more names, the converting author may choose any of them without providing a compelling justification.
Recommendation 10.1B. The name that is more commonly used than any other name to refer to (e.g., discuss or describe) a particular crown clade should generally be defined as applying to that crown clade, even if the name is commonly considered to apply to a clade that includes extinct taxa outside of the crown. If there is a conflict between Recs. 10.1A and 10.1B, Rec. 10.1B should be given precedence. If the name that is more commonly used than any other name to refer to a crown clade is instead defined as applying to a more inclusive clade (e.g., an apomorphy-based or total clade) that contains that crown, a justification should be provided.
Note 10.1B.1. In older works and in works dealing only with extant organisms, names have sometimes been used as if they apply to particular crown clades, though it is unclear whether the author considered the name to apply to the crown or to a more inclusive clade (i.e., including some or all of the stem). In such cases, the name may be interpreted as applying to the crown for the purpose of this recommendation.
Example 1. If a publication stated that all members of clade X (e.g., Mammalia) exhibit a particular feature M (e.g., lactation), and this feature has only been observed in extant species, the name X would have been used in that publication as if it applied to the crown clade. Given this situation, name X could be interpreted as a candidate name for the crown.
10.2. A new name may be selected for a clade only under one of the following circumstances: (a) the clade has no preexisting name (but see Note 10.2.1); (b) the most widely used preexisting name for the clade has already been established for another clade or is best applied to another clade (see Recs. 10.1A 10.1B), and there are no other preexisting names for the clade; (c) the most widely used preexisting name for the clade has a preexisting homonym that has already been established under this code (see Recs. 10D–F); (d) the group to be named is a total clade, in which case a panclade name (see Arts. 10.3–10.6) may be used instead of a preexisting name; (e) the group to be named is an apomorphy-based clade, and the name of the most inclusive crown clade exhibiting the apomorphy of concern refers etymologically to that apomorphy (see Arts. 10.7, 10.8).
Note 10.2.1. In the absence of a preexisting name for a particular clade, the choice between a new name and a preexisting name for a paraphyletic group originating with the same ancestor as the clade is left to the discretion of the author.
10.3. If a new name (as opposed to a converted name) is to be established for a total clade by adding an affix to the name of the corresponding crown clade, the prefix Pan- must be used. The prefix is separated from the base name, which retains an initial capital letter, by a hyphen. Such names are called panclade names and may only be used to designate total clades.
Example 1. If Testudines is established as the name of a crown clade, the panclade name for the corresponding total clade is Pan-Testudines.
Note 10.3.1. Although most panclade names will be new, some panclade names may have been defined in a publication prior to the starting date of this code, and these qualify as preexisting names (see Note 6.2.1).
Recommendation 10.3A. Informal panclade names may be useful in referring to total clades that may or may not have formal (established) panclade names. In order to distinguish informal panclade names from formal panclade names, the informal names should not be capitalized or italicized (see Rec. 6.1A).
Example 1. The non-capitalized, non-italicized names pan-Rosidae and pan-rosids are informal panclade names for the total clade corresponding to the crown clade Rosidae.
10.4. A panclade name may only be formed from a base name that has a crown clade definition (i.e., a branch- or apomorphy-modified node-based definition or a standard node-based definition in which all of the specifiers are extant; see Note 9.3.1).
Example 1. If the names Trilobita and Tyrannosaurus were established as the names of non-crown clades, then the names Pan-Trilobita and Pan-Tyrannosaurus could not be established as clade names.
Recommendation 10.4A. Some converted clade names will necessarily begin with Pan, but the initial letters Pan should be avoided in new clade names that are not intended as panclade names to reduce the likelihood of confusion between panclade and non-panclade names.
10.5. The definition of a panclade name is branch-based and will take the form "the total clade composed of the crown clade [name of the crown clade] and all extinct organisms or species that share a more recent common ancestor with [name of the crown clade] than with any extant organisms or species that are not members of [name of crown clade]" or "the total clade of the crown clade [name of the crown clade]".
Example 1. The definition of Pan-Testudines is "the total clade composed of the crown clade Testudines and all extinct organisms or species that share a more recent common ancestor with Testudines than with any extant organisms or species that are not members of Testudines" or "the total clade of the crown clade Testudines."
Note 10.5.1. Equivalent to the abbreviations provided in Note 9.3.1 for other kinds of definitions, the definition of a panclade name may be abbreviated "Total (X)", where X is the name of a crown clade.
Example 1. The abbreviated form of the definition in Art. 10.5, Example 1 is "Total (Testudines)".
Note 10.5.2. This format for the definitions of panclade names differs from the other recommended definition formats (see Note 9.3.1) in not listing any specifiers, which are implicit. The internal specifiers of the panclade name are those of the crown clade name on which the panclade name is based. The external specifiers of the panclade name are all extant species or organisms that are not members of the crown clade on whose name the panclade name is based.
Note 10.5.3. Extinction of crown clades after establishment of a panclade name does not affect the composition of the clade to which the panclade name refers. A crown clade that is extant at the time of establishment of a panclade name is forever treated nomenclaturally as though it were still extant. This treatment applies both to the crown clade that provides the base name for the panclade name and to other crown clades whose members are implicit external specifiers (see Note 10.5.2).
10.6. If there is a preexisting name that has been applied to a particular total clade, that name may be converted or a panclade name may be established instead.
10.7. If the name of a crown clade refers etymologically to an apomorphy, and a new name (as opposed to a converted name) is to be established for the clade originating with that apomorphy by adding an affix to the name of the crown clade, the prefix Apo- must be used. The prefix is separated from the base name, which retains an initial capital letter, by a hyphen.
Example 1. If Spermatophyta (meaning "seed plants") were established as the name of a crown clade, the name Apo-Spermatophyta would refer to the most inclusive clade exhibiting the apomorphy "seeds."
Note 10.7.1. Although most names that take the form described in Art. 10.7 will be new, some such names may have been defined in a publication prior to the starting date of this code, and these qualify as preexisting names (see Note 6.2.1).
10.8. If there is a preexisting name that has been applied to a particular apomorphy-based clade, and the name of the most inclusive crown clade exhibiting the apomorphy of concern refers etymologically to that apomorphy, the preexisting name may be converted or a name formed in accordance with Art. 10.7 may be established instead.
10.9. A clade name may not be converted from a preexisting specific or infraspecific epithet (ICBN and ICNB) or from a name in the species group (ICZN). However, a clade name may be converted from a supraspecific name that is spelled identically to a specific or infraspecific epithet or name.
Example 1. A clade cannot take the name Paradoxa if the name was converted from the specific epithet in Oenothera paradoxa Hudziok 1968; however, a clade can take the name Paradoxa if the name was converted from the genus name Paradoxa Mattirolo 1935.
Recommendation 10A. In selecting new clade names, an effort should be made to avoid any name that, under a rank-based code, applies to a non-overlapping (mutually exclusive) group.
Recommendation 10B. In selecting new clade names, an effort should be made to avoid names that are so similar to names that were previously established under this code that they are likely to be confused.
Recommendation 10C. In selecting new clade names an effort should be made to avoid names that have misleading connotations.
Recommendation 10D. In rank-based nomenclature, there are many examples of identically spelled names being applied to different taxa under different codes (cross-code homonyms). Only one member of each set of cross-code homonyms is, after conversion, an acceptable name under this code (Art. 13.3). If the preexisting name that has been most widely used for a particular clade cannot be converted because an identically spelled name has already been converted and established for a different clade, another preexisting name that has been widely and recently applied to the clade concerned (or to a paraphyletic group originating with the same ancestor) may be selected. On the other hand, continuity with existing literature and consistency with rank-based nomenclature are not well served by resurrecting old and little-known names. Therefore, if there is no other name that has been widely applied to the clade in the recent past, a new name should be selected that consists of the most widely used preexisting name with a taxon-related prefix added, such as Phyto- for plants, Phyco- for "algae" (excluding cyanobacteria), and Myco- for fungi (three groups of organisms whose names are governed by the ICBN), Zoo- for animals and other organisms whose names are governed by the ICZN, and Monero- for organisms whose names are governed by the ICNB and cyanobacteria (governed by the ICBN). If there is another preexisting name that has been widely applied to the clade in the recent past, the choice between converting this name and establishing a new name with a taxon-related prefix is left to the discretion of the author.
Example 1. Under rank-based nomenclature, the name Prunella applies to a genus of birds (ICZN) and to a genus of angiosperms (ICBN). If this name were to be established under this code for a clade of birds, the name selected for the clade corresponding in composition to the plant genus Prunella (provided that there is no other preexisting name that has been widely and recently applied to this clade) would be Phyto-Prunella.
Recommendation 10E. In rank-based nomenclature, previously undiscovered homonymy occasionally occurs within a single rank-based code, although only one of the homonyms can be legitimate (ICBN, ICNB) or potentially valid (ICZN) once the homonymy is discovered. Only one member of each set of homonyms is, after conversion, an acceptable name under this code (Art. 13.3). Once a case of homonymy within a rank-based code is discovered, it is generally rectified by replacing the junior homonym with an already existing synonym or a new replacement name. However, if a user of this code is the first to discover a case of homonymy within one of the rank-based codes, the names should be defined in a manner that is consistent with the way in which they will likely be applied under the rank-based code when the situation is rectified. Specifically, the homonym that will likely have precedence under the rank-based code (i.e., generally the one that was published earlier) should be the one that is converted under this code. For the other homonym, the synonym (if one exists) that will likely be applied to this taxon under the rank-based code should be converted, provided that this synonym qualifies as a preexisting name for the clade of concern (see Art. 9.8).
Note 10E.1. In the situation described in Rec. 10E, it is not necessary that an author who converts one homonym (or its synonym) also convert the other one (or its synonym).
Recommendation 10F. Under rank-based nomenclature, the name (or epithet; see below) of a subdivision of a genus that contains the type species must be the same as that of the genus. Only one member of each such pair of names is, after conversion, an acceptable name under this code (Art. 13.3). Furthermore, under the ICBN, names of subdivisions of genera (e.g., subgenera, sections, series) consist of a generic name combined with a subdivisional epithet. These epithets, like specific epithets, are not necessarily unique; the same epithet may be combined with the names of different genera without creating homonyms. Only one member of each set of identically spelled subdivisional epithets is, after conversion, an acceptable name under this code (Art. 13.3). If the preexisting subdivisional name (ICZN, ICNB) or epithet (ICBN) that has been most widely used for a particular clade cannot be converted because an identically spelled name has already been converted and established for a different clade, another preexisting name or epithet that has been widely and recently applied to the clade concerned (or to a paraphyletic group originating with the same ancestor) may be selected. On the other hand, continuity with existing literature and consistency with rank-based nomenclature are not well served by resurrecting old and little-known names. Therefore, if there is no other name or epithet that has been widely applied to the clade in the recent past, a new name should be selected that consists of the most widely used preexisting name or epithet, preceded by the name of the genus in rank-based nomenclature, with both words capitalized and connected by a hyphen. If there is another preexisting name or epithet that has been widely applied to the clade in the recent past, the choice between converting this name or epithet and establishing a new name that combines the preexisting genus name and subdivisional name or epithet is left to the discretion of the author.
Example 1. If one were selecting a name for the plant clade corresponding in composition to Arenaria sect. Parviflorae McNeill, and if the subdivisional epithet Parviflorae could not be converted because a clade name Parviflorae, based on Dracula ser. Parviflorae Luer, had already been established under this code, the name that should be selected is Arenaria-Parviflorae (provided that there is no other preexisting name that has been widely and recently applied to this clade).
Note 10F1.1. This is a hypothetical example in that these subdivisions of genera may not correspond to clades.
Example 2. If one were selecting a name for the animal clade corresponding in composition to the subgenus Crotaphytus of the genus Crotaphytus Holbrook, and if the name Crotaphytus could not be converted for that clade because that name had already been established under this code for a clade corresponding in composition with the genus, the name that should be selected is Crotaphytus-Crotaphytus (provided that there is no other preexisting name that has been widely and recently applied to this clade).
Recommendation 10G. When establishing a name for a crown clade that, under rank-based nomenclature, corresponds to a monogeneric "higher" taxon, the genus name should be converted for that clade rather than any of the suprageneric names that have been applied to it. Doing so will permit the use of the "higher" taxon names for more inclusive clades that extend beyond the crown.
Example 1. In rank-based nomenclature, the names Equisetophyta, Equisetopsida, Equisetales, Equisetaceae, and Equisetum have all been used to refer to the same crown clade, which is widely understood to include only the genus Equisetum. (Most of these names have also been used to refer to more inclusive clades that contain extinct species outside the crown.) When selecting a name to convert for the crown clade, Equisetum should be chosen. The names Equisetaceae, Equisetales, etc. are better applied to clades that are more inclusive than the crown.