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Typical and less typical types
In mid-late July there was a discussion on the meaning of holotype, neotype,
paratype and so on. Here's how I understand the subject:
1) Holo-, syn-, co-, lecto- and neotypes are important. Para-, allo- and
paralectotypes are not.
2) Imagine someone finds one new specimen and designates it the holotype of
a new species. Then a lot of better preserved specimens are found and
referred to the same species. Then it turns out that they are not the same,
or that the holotype is undiagnostic. The normal procedure is to pick one of
the referred specimens as the holotype of a new species. But if this would
produce too much confusion, someone can petition the ICZN. The ICZN (and
_nobody else_, ignoring the ICBN...) can then decide to pick one of the
referred specimens as the neotype of the original species. The original
holotype _loses its type status_ in the process! The neotype _replaces_ it.
A species with a neotype has no holotype anymore. -- This has happened with
*Coelophysis*, and, "one level higher up", with *Iguanodon* (whose original
type species, *I. anglicus*, is a nomen dubium, so the ICZN has made *I.
bernissartensis* the new type species; *I. anglicus* is no longer the type
species of anything).
3) Imagine someone finds 5 new specimens that seem to belong to the same
species (perhaps a growth series). Being in the early 20th century or
earlier, he designates _all of them together_ the type; they are syntypes.
This practice has meanwhile been forbidden; the reason is obvious -- who
guarantees that the syntypes all belong to the same species (as IIRC has
turned out not to be the case with *Trachodon* and *Chienkosaurus*). Lots of
syntypes are, however, still haunting around; this is because a first
revisor is needed for each case. That person picks one of the syntypes and
declares it the lectotype (lectus, Latin: chosen/picked). The others become
paralectotypes in the process; as paratypes, they are not of much further
4) I'm not sure what a cotype is, but I think that's what syntypes are
called when there are 2 of them (perhaps supposed to show sexual
5) An allotype is a special sort of paratype, namely one of the opposite sex
to the holotype. Rare, AFAIK not existing for any Mesozoic dinosaur, and, as
a paratype, not of any nomenclatural interest. (Except that it probably has
a heightened probability of becoming the neotype should one ever be needed.)
I hope all this is correct and helps! :-)