A holotype (sometimes just type) is the single physical example or illustration of an organism which defines the characteristics of the whole species. It is the definitive member of that species. Other specimens which may be members of the species can be compared with it to determine whether they really are. For example the holotype for the butterfly Lycáeides ídas longínus 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. The concept of holotypes is an important part of Linnaean taxonomy.

Plants are conserved, typically dried and kept in an herbarium. They are the specimen defined by the author as representing the taxon. A holotype may define not only a species or a lower taxon, but it may also define higher taxons, eg Cactus opuntia L. is the holotype for the Opuntia genus. This type species is now known as Opuntia ficus-indica (L.) Mill. This is relevant as it shows that for a type species a reference is made to the relevant description.

The comparison against a holotype is often unnecessary if the organism is well-known and complete. Organisms which were described before the 19th century do not normally have holotypes. The holotype is often, but not always, the first example of the particular species which has been found because it represents the first opportunity to describe it in scientific literature. If more than one specimen was used to describe the species, then the other specimens apart from the holotype are called paratypes, although there is sometimes a risk that a paratype is not in fact of the same species as the holotype but a very similar one and so the description may not be accurate.

Sometimes just a fragment of an organism makes the holotype, for example in the case of a rare fossil. The holotype of Pelorosausus 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 original remains the definitive example of the species because it may eventually turn out that the new example is not of the same species.

There are a range of terms describing other examples of the species - such as an isotype, which is a duplicate specimen for the holotype collected in the same place and the same time. If there is a comparative specimen of the opposite sex to the holotype then it is called the allotype. Should the holotype be destroyed or otherwise lost then a substitute specimen can be selected and which is then called the neotype.

More obscurely, if the author of the original paper describing the species did not select a holotype, then the specimens used are called syntypes. One of them may subsequently be selected as the definitive example and is then called the lectotype and the remainder become paralectotypes. For example, the mosquito Aedes refiki was described in 1928 in terms of several specimens, but the lectotype was only selected from them in 2000 reducing the remainder to the status of paralectotypes.

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