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Re: terminology



 Whether *Titanosaurus* is valid or not has no impact on the validity
 of Titanosauridae, providing the former has been published in
 accordance with the Code. By definition, Titanosauridae includes
 *Titanosaurus*. This is the same problem for Hadrosauridae, and
 Ceratopsidae (and other related names): they are based on dubious
 species (well, today, at last). Which means that they can not be
 used as specifiers of the clade names that they define (!).

Not true. A specifier doesn't need to be distinguishable from other species (whatever a species is) in the clade the name of which it is supposed to define; it only needs to be identifiable as a member of that clade and as not a member of other clades.

The PhyloCode doesn't use the term _nomen dubium_ either.

 Which means that all these names should *naturally* be considered
 invalid as long as their respective type taxon/specimen is considered
 as invalid too.

 Let's say I erected a taxon A with taxon B as a type/specifier. If B
 is now considered as invalid, THEN A is invalid too. It would be a
 complete nonsense to retain A in this case.

Nomina dubia are not invalid under the ICZN; and "undiagnostic" is a matter of degree, not an either-or thing. (For instance, any fossil bone fragment obviously comes from a vertebrate and is therefore allowed to be a type specimen or specifier for Vertebrata.)

 Fortunately, you have three possibilities if you encounter a nomen
 dubium threatening an old-established (='stable') taxonomy: 1)
 request the deletion of this taxon to the ICZN (e.g., *Dynamosaurus*,
 *Rioarribasaurus colberti*) 2) request the transfer of the type
 status to another taxon (e.g., *Cetiosaurus) or specimen (e.g.,
 *Allosaurus fragilis*, *Parasuchus hislopi*, *Coelophysis bauri*) 3)
 change the taxonomy.

Article 75.5 strongly recommends 2). That's why the attempt to solve the *Coelophysis* situation in way 1) was rejected by the ICZN in favor of 2).

 To eliminate Titanosauridae, *Titanosaurus* would have to be a
 junior synonym or (I guess) a nomen nudum.

Sorry!!! I meant homonym, not synonym.