Holotype

A holotype is one of several possible biological types. A type is what fixes a name to a taxon. A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several such, but explicitly designated as the holotype.

For example, the holotype for the butterfly Lycaeides idas longinus is held by the Museum of Comparative Zoology at Harvard University, and the holotype for the extinct mammal Cimolodon is at the University of Alberta.

A holotype is not necessarily 'typical' of that taxon, although ideally it should be. Sometimes just a fragment of an organism is the holotype, for example in the case of a rare fossil. The holotype of Pelorosaurus humerocristatus, a large herbivorous dinosaur from the Early Jurassic period, is a fossil leg bone stored at the Natural History Museum in London. Under unusual circumstances even a good quality photograph can be submitted as holotype. Even if a better specimen is subsequently found, the holotype is not superseded.

In the absence of a holotype (e.g. it was lost) another type may be selected, out of a range of different kinds of type, depending on the case. Note that in the ICBN and ICZN the definitions of types are similar in intent but not identical in terminology or underlying concept.

For example in both the ICBN and the ICZN a "neotype" is a type that was later appointed in the absence of the original holotype. Additionally, under the ICZN the Commission is empowered to replace a holotype with a "neotype", when the holotype turns out to lack important diagnostic features needed to distinguish the species from its close relatives. For example, the crocodile-like archosaurian reptile Parasuchus hislopi Lydekker, 1885 was described based on a premaxillary rostrum (part of the snout), but this is no longer sufficient to distinguish Parasuchus from its close relatives. This made the name Parasuchus hislopi a nomen dubium. Texan paleontologist Sankar Chatterjee proposed that a new type specimen, a complete skeleton, be designated. The International Commission on Zoological Nomenclature considered the case and agreed to replace the original type specimen with the proposed neotype. .

The procedures for the designation of a new type specimen when the original is lost come into play for some recent, high-profile species descriptions in which the specimen designated as the holotype was a living individual that was allowed to remain in the wild (e.g., ). In such a case, there is no actual type specimen available for study, and the possibility exists that - should there be any perceived ambiguity in the identity of the species - subsequent authors can invoke various clauses in the ICZN Code that allow for the designation of a neotype. Remarkably, the Code explicitly states that the designation of a neotype must be based upon an actual physical specimen that is "the property of a recognized scientific or educational institution", but there is no such requirement for a holotype.

Under the ICBN, also, a replacement type could be appointed by such a procedure, but this would be called a "conserved type". However, a conserved type would not be appointed in the case of a type that is insufficiently clear: in that case an additional and clarifying type could be designated, a so-called "epitype". Great care must be used in speaking of types, as definitions are very precise.