What Is a Holotype? The Single Specimen a Species Name Is Attached To
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A species name does not refer to a description or to an idea of the animal. It is anchored to one physical specimen held in a collection, and if the definition of the species later changes, the name follows whichever group contains that object.
Why a physical anchor
Descriptions are ambiguous and change as understanding improves, so anchoring a name to words would mean the name shifting meaning every time somebody revised the definition. Anchoring it to an object removes that. The holotype is the single specimen designated when a species is named, and the name is permanently attached to it, so any later dispute about what the species includes is resolved by asking which group that specimen belongs to. That has a consequence people find counterintuitive: if a species is split in two, the original name stays with whichever half contains the holotype and the other half needs a new name, regardless of which half is more typical or more numerous. The rules governing this are codified internationally, separately for animals, plants and bacteria, and they specify what must be published, how, and what obligations attach to the specimen, including that it must be deposited in a collection where researchers can examine it.
The related terms
Several kinds of reference specimen exist and the distinctions matter in practice:
- •Holotype, the single designated specimen carrying the name
- •Paratypes, additional specimens listed in the original description, which show variation and do not carry the name
- •Syntypes, where an older description designated several specimens without choosing one, which later workers resolve by selecting a lectotype from among them
- •Neotype, a replacement designated when the original is lost or destroyed, which requires justification and publication
- •Type species and type genus, applying the same anchoring principle at higher ranks, so a genus name is tied to one species and a family name to one genus
- •Ichnotypes and other specialised categories for trace fossils, which are named separately because the maker is unknown
The problems with fossil types
Palaeontology applies the system to material that is frequently poor, which creates specific difficulties. A holotype may be a single fragment, and several dinosaur genera rest on material too incomplete to distinguish them from relatives, which makes them nomina dubia, meaning names of doubtful application that cannot be assigned confidently. That is not an academic problem, since a dubious name with priority can block a better-founded one, and formal petitions to the governing commission are sometimes required to resolve it. Specimens are also lost, and the destruction of the original spinosaur material in the bombing of Munich in 1944 removed the holotype of one of the most famous dinosaurs, leaving only drawings and photographs until new material was found decades later. Holotypes in private hands are a serious concern, since the rules require accessibility and a specimen sold to a collector may become unavailable, which is why most journals will not publish descriptions of privately held material.
Where the specimens are
The requirement that a type be accessible has produced a global infrastructure that most people never see. Natural history museums hold collections running to tens of millions of specimens, of which the types are a small and disproportionately important fraction, stored separately and handled under restricted conditions. Loans between institutions move them to researchers, which is a long-established practice with its own conventions and risks. Digitisation has changed access substantially, with high-resolution photography, computed tomography and three-dimensional models now available online for many important specimens, which reduces the need to travel and the need to handle fragile material. Colonial history runs through the collections, since a great many types were collected in countries that now have no access to them, and repatriation and shared access arrangements are under active negotiation in several cases. Losses are permanent and have occurred, through war, fire and neglect, with the Brazilian national museum fire in 2018 destroying an enormous quantity of irreplaceable material including numerous types.
What the system does well
For all its awkwardness the arrangement solves a genuine problem and its benefits are visible when it is absent. It makes names stable across languages, centuries and changes in theory, since the anchor is an object rather than a concept. It makes disputes resolvable, since two researchers disagreeing about a species can examine the same specimen. It creates an obligation to preserve material, which is why natural history collections hold tens of millions of specimens that are consulted continuously and which are among the most heavily used research infrastructure in biology. It also creates a record that can be re-examined with new techniques, and holotypes described in the nineteenth century are now scanned, sampled for ancient proteins and analysed in ways nobody anticipated, which is only possible because somebody kept them. The rule that a name follows the object rather than the description looks pedantic and is the feature that makes the whole system work.
The takeaway
A name is anchored to one physical specimen rather than to a description, so when a species is split the name goes with whichever half contains that object regardless of which half seems more typical. Fossil types are frequently fragmentary, which leaves some famous names doubtfully applicable, and one major dinosaur's original material was destroyed by bombing in 1944. Specimens in private hands cannot serve, which is why journals refuse them.