15.1
Conservation of names and emendation of definitions are means of overriding precedence based on date of establishment (Art. 12.2) in the interest of stability (e.g., in terms of composition and/or diagnostic characters).
15.2
Conservation of names is possible only under extraordinary circumstances and requires approval of the Committee on Phylogenetic Nomenclature (CPN; see Art. 22).
15.3
Once a name has been conserved, the entry for the affected name in the registration database is to be annotated to indicate its conserved status relative to other names that are simultaneously suppressed. The entries for suppressed names are to be similarly annotated.
15.4
In the case of heterodefinitional synonyms, the earlier name may be conditionally suppressed so that it may be used when not considered synonymous with the later name. In the case of homonyms and homodefinitional synonyms, suppression is unconditional.
15.5
When a name is unconditionally suppressed, there are no conditions under which it has precedence with regard to either synonymy or homonymy. Therefore, if a homodefinitional synonym has been suppressed, that name can be established subsequently with a different definition as an acceptable name.
15.6
When a conserved name competes with names against which it has not been explicitly conserved, the earliest established of the competing names has precedence.
15.7
Although names are normally suppressed only when a synonym or homonym is conserved, the CPN may unconditionally suppress a name if it is nomenclaturally disruptive, without necessarily conserving an alternative. An unconditionally suppressed name can be established subsequently with a different definition as an acceptable name.
15.8
An emendation is a formal change in a phylogenetic definition. A restricted emendation requires approval by the CPN (see Art. 22), while an unrestricted emendation may be published without CPN approval.
15.9
All emendations must be published (Art. 4) and registered (Art. 8).
15.10
A restricted emendation (see Art. 15.8) is intended to change the application of a particular name through a change in the conceptualization of the clade to which it refers. Restricted emendations may involve changes in definition type, clade category, specifiers, and/or qualifying clauses.
Note 15.10.1
A restricted emendation is a mechanism to correct a definition that fails to associate a name with the clade to which it has traditionally referred, even in the context of the reference phylogeny adopted by the original definitional author.
Example 1
Suppose the name Angiospermae had been defined as the smallest clade containing Zea mays Linnaeus 1753 and Gnetum gnemon Linnaeus 1767. By including Gnetum, this definition specifies a more inclusive clade than the one to which the name Angiospermae traditionally refers. Correcting the definition would qualify as a restricted emendation (i.e., it would require approval by the CPN).
15.11
An unrestricted emendation (see Art. 15.8) is intended to preserve the application of a particular name in terms of the conceptualization of the clade to which it refers. Unrestricted emendations may involve changes in specifiers or qualifying clauses, or clarification of the meaning of “extant”, but must retain the same clade category (i.e., crown clade, total clade, or neither) if category was specified in the protologue, the same definition type (minimum-clade, maximum-clade, or apomorphy-based) except as specified in Note 15.11.2, and the same clade conceptualization as interpreted from the protologue.
Note 15.11.1
An unrestricted emendation is a mechanism to prevent undesirable changes in the application of a particular name (in terms of clade conceptualization) when the original definition is applied in the context of a revised phylogeny.
Example 1
Several recent phylogenetic analyses suggest that Amborella trichopoda is sister to the rest of Angiospermae, but evidence for this position of Amborella was not discovered until the late 1990s. If, prior to this discovery, Angiospermae had been given a minimum-crown-clade definition that did not include Amborella trichopoda as an internal specifier, Angiospermae would not include Amborella according to its currently inferred relationships. However, the definitional author would presumably have intended for Amborella to be included in Angiospermae because it has always been included in the taxon designated by that name. In such a situation, an unrestricted emendation that adds Amborella trichopoda to the list of internal specifiers would avoid an undesirable change in clade composition and would be consistent with the clade conceptualization of the original definitional author and with historical use.
Note 15.11.2
In the context of this article, minimum-crown-clade, maximum-crown-clade, and apomorphy-modified crown-clade definitions are all considered the same definition type, so it is permissible for an unrestricted emendation to change from one to another of these three variants of the crown-clade definition, provided that all internal specifiers are extant.
Example 1
In the situation described in Note 15.11.1, Example 1, as an alternative to adding Amborella trichopoda to the list of specifiers, it might be preferable to change the original minimum-crown-clade definition to a maximum-crown-clade definition such as “the largest crown clade containing Zea mays Linnaeus 1753 but not Cycas circinalis Linnaeus 1753, Gnetum gnemon Linnaeus 1767, Ginkgo biloba Linnaeus 1771, and Pinus sylvestris Linnaeus 1753.” Such a definition avoids the need for further emendation if some other species (i.e., other than Amborella trichopoda) or subclade is inferred in the future to be sister to the rest of the angiosperms.
Note 15.11.3
If it is specified in the protologue that the name refers to a crown clade or a total clade, this clade category cannot be changed through an unrestricted emendation. If the clade category is not specified in the protologue, the category may still play a role in determining the author’s conceptualization of the clade (see Note 15.11.4 and Art. 15.13, Ex. 1). The category of crown clade is considered to be specified in the protologue if any of the specified formulations of crown-clade definitions or their standard abbreviations (see Art. 9.9) is used. The category of total clade is considered to be specified in the protologue if a panclade name (Art. 10.3) or any of the formulations of total-clade definitions or their standard abbreviations (Art. 9.10) is used.
Note 15.11.4
Interpretation of the original definitional author’s clade conceptualization is based on the definition and all other information in the protologue. Important components of the definitional author’s conceptualization of the clade include (but are not necessarily restricted to) composition, apomorphies, clade category (e.g., crown versus non-crown; see Art. 15.13, Ex. 1), the existence of a basal dichotomy into two particular subclades (see Art. 15.13, Ex. 2), and conceptualization of a clade as an entire branch regardless of composition (see Art. 15.13, Ex. 3).
15.12
The protologue of an unrestricted emendation must provide evidence that the conceptualization of the clade is the same as that of the original definitional author. The protologue must also explain why the emended definition is preferable to the definition being emended.
15.13
If conflicting evidence from the protologue (see Note 15.11.4) makes it unclear whether a proposed emendation is consistent with the original conceptualization of the clade, the emendation must be considered by the CPN (i.e., it must be a restricted emendation). Disagreements within the systematics community as to whether a published unrestricted emendation changes the conceptualization of a clade (i.e., whether the emendation should have been restricted) are to be resolved by referring the issue to the CPN for a decision (see Art. 22).
Example 1
Suppose that the name Mammalia had first been defined phylogenetically as the smallest clade containing Ornithorhynchus anatinus (Shaw 1799) and Homo sapiens Linnaeus 1758, both of which are extant, without explicitly stating that the name refers to a crown clade. Further, suppose that the definitional author(s) had considered Mammalia to include Multituberculata (a wholly extinct group). If, under a newly proposed phylogenetic hypothesis, Multituberculata is no longer included in Mammalia under the stated definition, and if the definitional author(s) did not indicate whether reference to a crown clade or inclusion of Multituberculata was more fundamental to their use of the name Mammalia, conflicting evidence exists concerning the original conceptualization of that taxon. Therefore, adding a member of Multituberculata to the set of internal specifiers in the definition of Mammalia, or otherwise modifying the definition of Mammalia so that it refers to a clade that includes Multituberculata, would require consideration by the CPN.
Example 2
Suppose that the name Dinosauria had first been defined phylogenetically as the smallest clade containing Megalosaurus bucklandii Mantell 1827 (Saurischia) and Iguanodon bernissartensis Boulenger in Beneden 1881 (Ornithischia). Further, suppose that the definitional author(s) had considered Dinosauria to include Herrerasauridae. If, under a newly proposed phylogenetic hypothesis, Herrerasauridae is no longer included in Dinosauria under the stated definition, and if the definitional author(s) did not indicate whether inclusion of Herrerasauridae or application to the clade whose basal dichotomy is represented by Saurischia and Ornithischia was more fundamental to their use of the name Dinosauria, conflicting evidence exists concerning the original conceptualization of that taxon. Therefore, adding a species of Herrerasauridae to the set of internal specifiers in the definition of Dinosauria, or otherwise modifying the definition of Dinosauria so that it refers to a clade that includes Herrerasauridae, would require consideration by the CPN.
Example 3
Suppose that the name Saurischia had first been defined phylogenetically as referring to the largest clade containing Allosaurus fragilis Marsh 1877 but not Stegosaurus armatus Marsh 1877 (Ornithischia). Further, suppose that the definitional author(s) had considered Saurischia to include Herrerasauridae. If, under a newly proposed phylogenetic hypothesis, Herrerasauridae is no longer included in Saurischia under the stated definition, and if the definitional author(s) did not indicate whether inclusion of Herrerasauridae or application to the sister clade of Ornithischia was more fundamental to their use of the name Saurischia, conflicting evidence exists concerning the original conceptualization of that taxon. Therefore, adding a species of Herrerasauridae to the set of internal specifiers in the definition of Saurischia, or otherwise modifying the definition of Saurischia so that it refers to a clade that includes Herrerasauridae, would require consideration by the CPN.
15.14
Although anyone may publish an unrestricted emendation, it is preferable that the emendation be authored or coauthored by the author or authors of the original definition. If one or more of the original definitional authors are still alive, another worker who thinks that an unrestricted emendation is warranted must provide evidence when registering the emendation that the first author of the original definition (or the second, third authors, etc., if the first author is deceased or otherwise unable to respond) was contacted and offered the opportunity to co-author the emendation.
Note 15.14.1
Minimal evidence required for registration of an unrestricted emendation includes the e-mail address or phone number of the original definitional author(s) contacted and the date when the contact was made. If all of the original definitional authors are deceased or otherwise unable to respond, this information must be submitted to the registration database as well. Supplementary information such as the text of the definitional author(s)’ response may also be submitted.
Note 15.14.2
Although the author or authors of the original definition must be offered the opportunity to co-author an emendation of the original definition, it is not necessary that they be offered the opportunity to co-author the entire publication in which the emendation appears (see Art. 19.2).
15.15
Within a phylogenetic context in which the original definition and an unrestricted emendation apply to the same clade, the original definition has precedence.