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Re: Alamosaurus as biggest North American sauropod



 I think in these situations the correct thing to do is designate a
 neotype. If T. rex is non-diagnostic relative new new taxa A and B,
 as long as both are from roughly the same stratigraphic horizon and
 geographic location, then pick one--it doesn't really matter which.
 This is what the ICZN recommends be done for all nomina dubia

Only when necessary, and only by the Commission.

Brackets are in the original, all emphasis is italics in the original. Note "may request" in Art. 75.5 and "should [...] request" in Art. 75.6.

>
75.5. Replacement of unidentifiable name-bearing type by a neotype. When an author considers that the taxonomic identity of a nominal species-group taxon cannot be determined from its existing name-bearing type (i.e. its name is a _nomen dubium_), and stability or universality are threatened thereby, the author may request the Commission to set aside under its plenary power [Art. 81] the existing name-bearing type and designate a neotype.

Example. The holotype of the ammonite species *Cycloceras laevigatum* M'Coy, 1844 lacked important diagnostic features. Upon request the Commission under its plenary power set aside the type status of this specimen and designated a neotype (Opinion 1720 (1993)).

75.6. Conservation of prevailing usage by a neotype. When an author discovers that the existing name-bearing type of a nominal species-group taxon is not in taxonomic accord with the prevailing usage of names and stability or universality is threatened thereby, he or she should maintain prevailing usage [Art. 82] and request the Commission to set aside under its plenary power [Art. 81] the existing name-bearing type and designate a neotype.

Example. On discovering that the only existing type specimen of *Aradus caucasicus* Kolenati, 1857 (Heteroptera) was a specimen of another species, Kerzhner & Heiss (1993) proposed that the prevailing usage of the names of both species should be conserved by the designation of a neotype for *A. caucasicus* under the Commission's plenary power, and this was accepted in Opinion 1783 (1994).
<<

http://www.nhm.ac.uk/hosted-sites/iczn/code/includes/page.jsp?article=75

If you're not the Commission, you may only designate a neotype for a species or subspecies if and only if the previous type has been destroyed or lost _and_ if and only if that species has "an exceptional need" (Art. 75.3) for a type. And even then Recommendation 75B says you "should be satisfied that the proposed designation does not arouse serious objection from other specialists in the group in question" before you publish.

Most of what most people, likely including myself, believe they know about the ICZN is wrong.

 and I can't think of any better alternatives. However, I have seldom
 ever seen this applied, except in the case of Coelophysis bauri. It's
 often mis-applied, as in the case of Iguanodon, where rather than
 assign a neotype for the species I. anglicus, a new type species was
 chosen, probably unrelated to the original, and from a different
 geographic location and stratigraphic horizon.

When the Commission does it, it's not a misapplication.

:-)

 In related cases like Titanosaurus, the new, better specimens from
 the same horizon, if they could not be differentiated from T.
 indicus, should have been made the neotype of that species, rather
 than become the basis of a new taxon which is almost certainly but
 unprovably its synonym. If, later, this is contradicted by evidence
 that the original material WAS distinct, or not really referable to
 the neotype, well then so be it (IIRC this happened with
 Ceoplophysis as well).

The story of the silesaurid *Eucoelophysis* is more complicated than that, but I'm not sure I remember the details and can't look them up now.