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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.