Homonyms are names that are spelled identically but refer to different taxa. In this code, all homonyms are established and identically spelled clade names based on different phylogenetic definitions. However, not all identically spelled clade names based on different phylogenetic definitions are necessarily homonyms because different definitions may refer to the same clade under some phylogenetic hypotheses but not under others.
Suppose that Pedersen defined Lamiaceae as the name of the smallest clade containing Lamium album Linnaeus 1753 and Congea tomentosa Roxburgh 1819, and Ramírez defined Lamiaceae as the name of the smallest clade containing Lamium album Linnaeus 1753 and Symphorema involucratum Roxburgh 1798. If so, these two definitions would refer to the same clade in the context of any phylogeny in which Congea tomentosa and Symphorema involucratum share a more recent common ancestor with each other than either does with Lamium album but not if Congea tomentosa shares a more recent common ancestor with Lamium album than it does with Symphorema involucratum.
Homonyms result when an author establishes a name that is spelled identically to, but defined differently than, an earlier established name. This situation can occur either when an author is unaware of the earlier establishment of an identically spelled but differently defined name (Ex. 1) or when an author knowingly adopts an earlier established name but proposes, either deliberately or inadvertently, a different definition for that name (Ex. 2). Although names in the second scenario can be considered the same name in the sense that one use is derived from the other (see Note 9.15A.2), the identically spelled names in both scenarios are treated as homonyms under this code because they have different definitions.
If Mukherjee defined Prunella as the name of the smallest clade containing Prunella modularis Linnaeus 1758 and Prunella collaris Scopoli 1769 (which are birds), and Larsen defined Prunella as the name of the smallest clade containing Prunella laciniata Linnaeus 1763, Prunella grandiflora Scholler 1775, Prunella vulgaris Linnaeus 1753, and Prunella hyssopifolia Linnaeus 1753 (which are plants), Prunella of Mukherjee and Prunella of Larsen would be homonyms.
Gauthier et al. (1988) defined the name Lepidosauromorpha as referring to the clade composed of Lepidosauria and all organisms sharing a more recent common ancestor with Lepidosauria than with Archosauria (a maximum-clade definition). Laurin (1991) defined the name Lepidosauromorpha as referring to the clade originating with the most recent common ancestor of Palaeagama, Saurosternon, Paliguana, Kuehneosaurus, and Lepidosauria (a minimum-clade definition). If this code had been in effect when these names were published, Lepidosauromorpha of Gauthier et al. and Lepidosauromorpha of Laurin would have been homonyms.
Phylogenetic definitions are considered to be different if either: (1) they are of the same kind (e.g., minimum-clade) but cite different specifiers and/or have different restrictions specified in their qualifying clauses (if any), or (2) they are of a different kind (e.g., minimum-clade vs. maximum-clade).
Alternative wordings of minimum-clade definitions such as those provided in Article 9.5 are not considered to be different, provided they are based on the same specifiers and have the same restrictions (Arts. 11.12–11.14). The same is true of alternative wordings of maximum-clade definitions (Art. 9.6), apomorphy-based definitions (Art. 9.7), directly-specified-ancestor definitions (Note 9.5.1), minimum-crown-clade definitions (Art. 9.9), maximum-crown-clade definitions (Art. 9.9), apomorphy-modified crown-clade definitions (Art. 9.9), maximum-total-clade definitions (Art. 9.10), crown-based total-clade definitions (Art. 9.10), and other types of phylogenetic definitions that are not explicitly mentioned in this code.
If two or more definitions have been established for identically spelled clade names, the only acceptable name (i.e., the combination of name and definition; see Note 12.1.1) is the first one established under this code. A later homonym, unless conserved, is not an acceptable name of any clade.
When two or more homonyms have the same publication date (Art. 5), the one that was registered first (and therefore has the lowest registration number) takes precedence.
If the oldest name of a clade is not acceptable because it is a later homonym, it is to be replaced by the established name that has precedence. If all established names that apply to the clade are not acceptable because they are later homonyms, a replacement name may be explicitly substituted for the earliest-established name that applies to the clade. A replacement name must be established, following the procedures in Articles 7, 13.6, and 13.7. The definition of a replacement name for a clade is the definition of the name it replaces.
In order to be established, a replacement name must be clearly identified as such in the protologue where the replacement is published, by the designation “replacement name” or “nomen substitutum.”
In order for a replacement name to be established, the replaced name on which it is based must be clearly indicated by a direct and unambiguous bibliographic citation (see Art. 9.16) that includes its author, date, and the journal or book in which the name was originally published. The registration number of the replaced name must also be cited.