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
herbivore 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 Even if a better specimen is subsequently found, the holotype is not superseded.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 pre
maxillary
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, an additional and clarifying type could be designated, an epitype under ยง9.7 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 taxa under current usage).
See also
