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Re: tiny-armed theropods



Mickey Mortimer <mickey_mortimer111@msn.com> wrote:

> And there we go with what Tim's argument also boils down to.  You *think* 
> that nomina dubia and other taxa of questionable identity
> shouldn't be used for taxonomic purposes.  Which is an opinion, but not 
> backed by the ICZN.  And the Phylocode isn't enforcable yet,
> and doesn't have anything about nomina dubia in it either.  So while you can 
> have your opinion, it's not something that the community
> has any need to follow,


And yet.... the "community" does follow this convention.  By this, I
mean the convention that nomina dubia shouldn't be used for taxonomic
purposes.  Otherwise, we would have Deinodontidae instead of
Tyrannosauridae, and Atlantosauridae instead of Diplodocidae.
Tyrannosauridae and Diplodocidae have become entrenched - in spite of
what the ICZN says.


However, applying your logic, we should all be using Deinodontidae and
Atlantosauridae, in order to blindly conform to the ICZN.  Also
applying your logic (and strict application of the ICZN), we should
probably use Ornithopsidae instead of Brachiosauridae, and
Ornithodesmidae instead of Dromaeosauridae.  That means we should
probably include the type specimens of _Ornithopsis_ and
_Ornithodesmus_ in phylogenetic analyses.  But _Ornithopsis_ and
_Ornithodesmus_ are questionable genera (probably nomina dubia), so
that would undoubtedly lead to increased instability.


I don't see the point of all this extra effort (and pain) just to jump
through the hoops of an outdated Linnaean nomenclature system, as
advocated by the ICZN.


More recently, Wilson and Upchurch (2003) declared _Titanosaurus_ to
be a nomen dubium.  They accordingly argued that Titanosauridae should
be abandoned.  Now, irrespective of whether or not I believe
_Titanosaurus_ to be a nomen dubium, it does seem prudent to *not*
have a whole family based on a taxon that is questionable (i.e.,
_Titanosaurus_).  Sauropod workers should make the call about what
family-level clades they want to use - not the ICZN.


Yes, even taxa based on "good" material can bounce around a tree - I
can't disagree with you here, Mickey.  But a taxon based on outright
crappy material (such as a pair of vertebrae, or a single tooth) has
little or no chance at all of finding a stable position.


> and doesn't concern me until you or Tim show that basing Troodontidae on ANSP 
> 9259 (as in ANSP 9259 <- Dromaeosaurus, Passer)
> has a realistic possibility of resulting in a clade that does not include 
> troodontids as now conceived.  That's the only thing to be worried
> about when choosing a specifier for a clade (well, that and eponymity).


No, because ANSP 9259 (the type tooth of _Troodon_) and _Passer_
(sparrow) represent very different situations.  We can be confident
that _Passer_ (being based on literally millions of living specimens)
is a crown bird.  However, the phylogenetic position of a single tooth
is always going to be precarious.  Look at how closely the teeth of
the unenlagiine _Austroraptor_ resemble those of _Spinosaurus_ -
solely due to convergence.  It is not too difficult to imagine a
non-troodontid converging on the tooth morphology of _Troodon_.






Cheers

Tim