In zoological nomenclature, synonyms are different scientific names that pertain to the same taxon, for example two names for the same species. The rule of zoological nomenclature is that the first name to be published is the senior synonym; any others are junior synonyms and should not be used.
Synonyms are "objective" if they unambiguously refer to the same taxon; this is the case if they refer to the same description or the same type specimen. Otherwise the synonyms are "subjective", meaning that there is room for debate: one researcher might consider the two names to refer to the same taxon, another might disagree.
For example, Werner Janensch published the name Giraffatitan brancai in 1914 for a species of Brachiosaurus, and in 1988, Gregory S. Paul noted that the African form (on which most popular depictions of Brachiosaurus are based) showed significant differences from the North American form (B. altithorax), especially in the proportions of its trunk vertebrae and in its more gracile build. Paul used these differences to create a subgenus he named Brachiosaurus (Giraffatitan) brancai. In 1991, George Olshevsky asserted that these differences are enough to place the African brachiosaurid in its own genus, simply Giraffatitan. This renaming has not achieved widespread acceptance.
Objective synonyms are common at the level of genera, because two researchers may independently arrive at the conclusion that a species is sufficiently different from others in its genus that it needs to be given its own genus. Thus each names a new genus with the same type species; these are objective synonyms.
- Blackwelder, R. A. (1966). Taxonomy: A text and reference book. New York: Wiley.