[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

RE: tiny-armed theropods



"And yet.... the "community" does follow this convention. By this, I mean the 
convention that nomina dubia shouldn't be used for taxonomic purposes."

Sometimes authors will use it as a reason to support their preferred taxonomy, 
as Russell did for Deinodontidae or Wilson and Upchurch did for 
Titanosauridae.  But sometimes the traditional taxonomy is based on a supposed 
nomen dubium and no one cares enough to disrupt it, like Ceratopsidae.  To 
borrow a phrase from religion, this is salad bar taxonomy.

And yes, I would say send Deinodontidae and (probably) Ornithodesmidae to the 
ICZN for petitions if we want to keep Tyrannosauridae and Dromaeosauridae.  The 
latter would probably win, and I'd be happy since we went through the proper 
procedures.  Atlantosaurus' type species hasn't been shown to be diplodocid 
(though it should be restudied) and Ornithopsis hasn't been shown to be 
brachiosaurid since the latter family was partially smeared out to be a grade 
of basal camarasauromorphs.  As for Titanosauridae, it's complicated now 
because we have so many family-level taxa in the area of the tree where 
Titanosaurus probably goes.  And since everyone's been ignoring Titanosaurus 
since Wilson and Upchurch declared it a nomen dubium, no one's cared exactly 
where it would go.  If we actually can't determine if it's a saltasaurid or 
nemegtosaurid for instance, I'd be fine with dumping Titanosauridae.  But only 
once we do the hard work to show this is true.

"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_."

Ah, now THAT's something actually meaningful to discuss.  The only reason we 
should care about using ANSP 9259 for a family definition is if it's likely 
doing so will lead to a problem.  Its associated family has priority over any 
other which have been proposed for that clade (e.g. Saurornithoididae) and the 
phylogenetic definition is based off the eponymous species, so so far so good.  
BUT if there is a realistic possibility Troodon is a "non-troodontid", then 
that's an actual problem as opposed to a value-based opinion.  And if I thought 
it plausible Troodon could be a "non-troodontid", I would agree let's petition 
the ICZN to move to Saurornithoididae or to set a neotype for Troodon formosus.

But is it plausible?  We can't just say some relatively unrelated taxa have 
similar teeth (Austroraptor and spinosaurines, megalosaurids and sinraptorids, 
etc.) and be done with it, because every case is unique and some taxa really do 
have distinctive teeth.  Our friend ANSP 9533, a syntype of Deinodon horridus, 
is near certainly a derived tyrannosauroid premaxillary tooth for instance.  
I'd bet almost anything on that being true.  Sure we could say it's POSSIBLE 
some other animal happened to convergently evolve such teeth, and just happened 
to be found in a place and time where tyrannosaurids are common, but I don't 
think either of us believes that's a likely possibility.  It would just be 
special pleading to bring about a taxonomic consideration you found more 
favorable.  And that's the way I view ANSP 9259 as well.

In order to show the plausibility of ANSP 9259 being non-troodontid, you'll 
have to provide evidence for some other clade it could belong to.  The evidence 
for it being troodontid is...
1. Constricted root.
2. Large serrations (7-10 per carina).
3. Recurved crown.
4. Short crown (~15% taller than FABL).
5. Apicalmost distal serration makes up apex of crown.
I think the closest you'll get are therizinosaurs, but their teeth are rarely 
recurved (anterior dentary teeth of Segnosaurus are recurved apically, and some 
Beipiaosaurus teeth are slightly recurved), more elongate (especially 
anteriorly, making Segnosaurus' similarity above less relevent) and lack the 
enlarged apicalmost distal serration.  Of course if you have another idea or 
honestly think ANSP 9259 has a reasonable chance of belonging to a 
therizinosaur, I'm open to arguments.  But just saying "well, teeth vary a lot 
even between jaw positions, so I could imagine a Judith River therizinosaur 
that happened to evolve a shorter, more recurved crown and a large apical 
serration somewhere in its jaw" would be more special pleading.

Mickey Mortimer


----------------------------------------
> Date: Mon, 17 Oct 2011 10:46:40 +1100
> From: tijawi@gmail.com
> To: dinosaur@usc.edu
> Subject: 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