What Is a Nomen Dubium? A Species Name Nobody Can Use

Naming a species requires designating a type specimen and publishing a description, and nothing in the rules requires the specimen to be diagnostic. A name based on a single tooth, a fragment of vertebra or a partial limb bone is validly published if the formalities are met, and it becomes doubtful when later work establishes that the material shows no feature distinguishing it from related species. That is common in palaeontology because material is frequently fragmentary and because nineteenth-century practice named species freely from scraps, in several cases competitively. The consequence is not merely untidy. Names have priority by date, so an older doubtful name can take precedence over a later one based on excellent material, which would mean a well-understood animal being renamed after a fragment nobody can identify. The rules therefore provide procedures for setting such names aside, and the governing commission can rule on specific cases, which is a slow process used sparingly.

Several well-known names have been affected and the outcomes differ:

• •Trachodon, named in 1856 from teeth, which was for decades the standard name for duck-billed dinosaurs and is now regarded as undiagnostic
• •Titanosaurus, named from vertebrae in 1877, whose doubtful status left an entire major group named after a genus that cannot itself be identified
• •Coelurus and several other early names, whose material is insufficient to support the genera built around them
• •Deinodon, named from teeth in 1856, which has priority over a much later name for a famous animal and which was set aside precisely to protect stability
• •Manospondylus, named in 1892 from vertebrae, later recognised as probably belonging to the same animal as a name given in 1905, with the earlier name suppressed under rules protecting long-established usage
• •Numerous names from the competitive collecting of the 1870s and 1880s, which produced a taxonomic backlog occupying specialists for a century

Nomenclature exists to allow people to communicate about organisms, so a change that confuses more than it clarifies defeats the purpose. The codes accordingly contain provisions protecting names in prevailing usage against older forgotten ones, generally requiring that the older name has not been used for a specified period while the younger has been used repeatedly by multiple authors. Those provisions are deliberate policy choices rather than logical consequences, and they represent a judgement that the benefit of a consistent name outweighs strict adherence to priority. The alternative was tested in practice and disliked, since researchers who discovered obscure earlier names and applied them produced disruption disproportionate to any gain. The commission can also conserve a name explicitly, and it publishes rulings that function as case law. The result is a system with a strict default and a mechanism for setting it aside when the default would be absurd, which is a recognisable legal structure.

Several kinds of problematic name are distinguished and the terminology is worth separating. A nomen dubium cannot be applied because the type material is not diagnostic. A nomen nudum is a name published without an accompanying description meeting the requirements, so it has no standing at all and is available for later use. A nomen oblitum is a forgotten name set aside under the stability provisions. A junior synonym is a name for something already named, and it falls out of use while remaining part of the record. A chimera is a name based on material from more than one animal, which occurs when a composite specimen was assembled in error and which requires the type to be restricted to one component. Each has a different remedy, and confusing them produces arguments where a procedure already exists. The general lesson is that a discipline dealing with incomplete evidence over two centuries accumulates administrative problems, and solving them is unglamorous work that keeps the literature usable.

A name published from material showing no distinguishing features cannot be applied to any specimen, and priority means an old doubtful name can displace a well-founded later one. The codes therefore include provisions protecting names in prevailing usage and a commission that rules on specific cases. Trachodon and Titanosaurus are both doubtful, the second leaving a major group named after an unidentifiable genus.