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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. Under the ICZN, a holotype is one of several kinds of name-bearing types. An isotype is a duplicate of the holotype.
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 fossil. The holotype of Pelorosaurus humerocristatus, a large herbivore dinosaur from the early Jurassic period, is a fossil leg bone stored at the Natural History Museum in London. Even if a better specimen is subsequently found, the holotype is not superseded.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. a new species of capuchin monkey, genus Cebus or the Arunachal macaque Macaca munzala). 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, an additional and clarifying type could be designated, an epitype under §9.7 of the Vienna Code, where the original material is demonstrably ambiguous or insufficient. Great care must be used in speaking of types, as definitions are very precise: a conserved type would be used to clarify a nomen ambiguum or "fix" a name which has come to apply to material that disagrees with its holotype (that is, where the holotype clearly belongs to another taxon under current usage).