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RE: Nomina Dubia Part II: Rapator



Short answer: Fragmentary specimens should only be named when there is 
compelling reason to do so.  In other words, only if the specimen shows 
striking new autapomorphies or combinations of characters (e.g., 
_Xenoposeidon_, based on an incomplete dorsal vertebra).


Naming a fragmentary specimen because it has a few characters that *might* turn 
out to be unique sounds like a ticking time-bomb.  Somewhere down the line 
there's going to be an issue over whether this specimen is a nomen dubium, or 
whether a newer and better named specimen should be referred to it.  So yes, I 
agree that avoiding potential nomina dubia is  a pragmatic approach that 
follows the status quo.  But to me this seems preferable than a proliferation 
of dodgy names that contribute very little (if anything) to dinosaur 
paleontology.


Cheers

Tim



--- On Fri, 28/5/10, Michael Mortimer <mickey_mortimer111@msn.com> wrote:

> From: Michael Mortimer <mickey_mortimer111@msn.com>
> Subject: RE: Nomina Dubia Part II: Rapator
> To: dinosaur@usc.edu
> Received: Friday, 28 May, 2010, 11:40 PM
> 
> Tim Williams wrote-
> 
> > Yes, and here we come to the crux of why we have
> different opinions. For the vast majority of characters, I
> would tend to treat them as potentially non-diagnostic.
> These include shape or length of horn cores, length and
> depth of sulci, size and orientation of metacarpal
> processes, degree of fusion of metatarsals, and so on.
> >
> > So for our old friend _Ceratops_, I think we can
> safely assume (and I concede that it is *just* an
> assumption) that because of the huge variation in the length
> and orientation of postorbital horn cores known within
> *other* ceratopsian species, that _Ceratops_ cannot be
> diagnosed based on the length and orientation of its
> postorbital horn cores. I think we can say this even in the
> absence of a specimen-level statistical analysis of
> variation in ceratopsian horn cores.
> >
> > Yes, I agree. It's all very warm-and-fuzzy, and so
> often in the eye of the beholder. But I think at a
>
characters that we
> suspect may be prone to intraspecific (including
> ontogenetic) variation. I know there is no standard
> operating procedure for this; your _Australovenator_ is a
> perfect example of this.
> >
> > That's an excellent question. If push comes to shove:
> you can't. Not for most characters anyway. I suspect that a
> taxon based on a partial skeleton gets treated far more
> leniently than a taxon based on just one or a few elements.
> But given we have multiple elements for _Australovenator_,
> the sum total of 'diagnostic' characters at least offers the
> chance of a unique combination of characters.
> >
> > It's perhaps unfair that _Rapator_ is considered a
> nomen dubium when we haven't actually tested the validity of
> its metacarpal characters. Instead we've just *assumed* that
> its characters are not reliable for diagnosing a genus.
> >
> > But let's say we did erect a new name for *every*
> fragmentary specimen that has one or two characters that are
> possibly diagnostic. The result would be a proliferation of
> new genera and species. Such a policy would lead to an
> explosion of highly suspect taxa, and spawn a thousand
> arguments over whether this-or-that genus had priority. Your
> question regarding whether _Australovenator_ should be
> referred to _Rapator_ (based on shared characters mentioned
> by Agnolin &c) would be repeated dozens of times over.
> This sounds like a
> > without us throwing more dodgy names into the mix.
> >
> > Your approach of giving any and all characters the
> benefit of the doubt would justify naming taxa such as
> _Serendipaceratops_, based on a single ulna that was said to
> have unique proportions. I would say that
> _Serendipaceratops_ should probably not have been named,
> because the fact that the ulna is shaped a little
> differently to all other known ornithischian ulnae does not
> stack up as a 'good' diagnostic character.
> >
> > I think the current approach, whereby names like
> _Rapator_ and _Ceratops_ and _Serendipaceratops_ are
> regarded as
lly
> *tested* whether or not their characters are subject to
> intraspecific variation, is an attempt at damage control.
> While it might seem unfair, from a taxonomic perspective
> these names tend to cause more trouble than what their
> worth.
> 
> To me, your approach seems more like following the status
> quo than actually doing science.  You give characters
> the benefit of the doubt too, but only when they're listed
> as being diagnostic by their authors.  If you've ever
> gone through the diagnosis of any taxon, you'd understand
> why I don't value characters listed in diagnoses more than
> others.
>  
> As an example I've studied more than Australovenator, take
> Struthiomimus.  You'd think it was obviously
> diagnostic.  Known from complete specimens and
> such.  However...
>  
> Osborn (1917) erected Struthiomimus for Ornithomimus altus
> because it possessed metatarsal V, which he incorrectly
> thought was absent from Ornithomimus velox. Later authors
> often realized Osborn's error and synonymized the genera,
> though Parks (1926, 1928, 1933) did name four additional
> species in Struthiomimus because they possessed the
> metatarsal (S. ingens, S. currelli, S. brevetertius and S.
> samueli). Russell (1972) revised ornithomimid taxonomy and
> was the first author to use real morphological differences
> to validate the separation of Struthiomimus from
> Ornithomimus edmontonicus (and his new genus
> Dromiceiomimus), though several characters he cites are now
> seen as invalid due to the recent synonymization of
> Dromiceiomimus with Ornithomimus edmontonicus. Notably,
> Dromiceiomimus samueli also has a humerus shorter than its
> scapula, antebrachium length overlaps Dromiceiomimus', and
> preacetabular, tibial, metatarsal and pedal digit length are
> no longer distinct. Furthermore, all the characters proposed
> by Russell to be apomorphies of Struthiomimus are
> symplesiomorphies when viewed in a cladistic context. The
> robust forelimb is plesiomorphic, being seen in all
> ornithomimosaurs except Dr
s
> and Sinornithomimus. The curved manual unguals are also
> plesiomorphic, present in all ornithomimosaurs except
> Dromiceiomimus, Anserimimus and "Gallimimus" "mongoliensis".
> Dromiceiomimus is the only ornithomimosaur known with a
> presacral column not longer than its hindlimb, as opposed to
> Struthiomimus, Anserimimus, Gallimimus and Sinornithomimus.
> The proximal caudal centra are also posteriorly wide (over
> half their length) in Garudimimus, "Grusimimus", Gallimimus,
> Ornithomimus sp. nov. and O? sedens. The transition point is
> also posterior to the fourteenth caudal in Harpymimus,
> Anserimimus and Gallimimus. Metacarpal I is shorter than
> metacarpal II in all ornithomimosaurs except Anserimimus,
> Dromiceiomimus and Ornithomimus. The elongate manual ungual
> I (longer than ungual II) may be primitiv!
> e for orn
> homimids, also being present in "Grusimimus",
> Sinornithomimus and Anserimimus. The elongate manual ungual
> III (longer than phalanx III-3) is here found to be
> synapomorphic of a larger clade also containing Gallimimus
> bullatus, "G." "mongoliensis" and Ornithomimus? sedens.
> Makovicky et al. (2004) proposed an elongate manus (>7%
> longer than humerus) as an additional apomorphy of
> Struthiomimus, and this seems to be true as it is otherwise
> present only in the somewhat distantly related Ornithomimus?
> sedens, Harpymimus and Pelecanimimus. Kobayashi et al.
> (2006) and Longrich (2008) both proposed a small skull
> (<50% of femoral length) is unique to Struthiomimus, but
> this is also found here to be characteristic of the larger
> clade also containing Gallimimus bullatus, "G."
> "mongoliensis" and Ornithomimus? sedens. Longrich also
> proposed many characters to distinguish Struthiomimus from
> Dromiceiomimus (Ornithomimus in his use) and his new large
> Dinosaur Park ornithomimid, but most were not examined in a
> broader context. The convex ventral maxillary edge is also
> seen in Gallimimus bullatus, Sinornithomimus and probably
> "Gallimimus" "mongoliensis", for inst
bital portion of the frontal
> and dorsally flat distal caudal centra (in posterior view).
> The latter may be plesiomorphic however, as it is also found
> in Patagonykus and Aniksosaurus. Broad pedal unguals also
> seem plesiomorphic, being present in Anserimimus, Gallimimus
> bullatus and Garudimimus. Finally, the elongate
> proximodorsal process on its pedal unguals is primitive,
> found in Sinornithomimus, "Grusimimus", Garudimimus and
> Harpymimus as well.
>  
> So of the characters listed by Osborn (1917) and Russell
> (1972), none withstand scrutiny.  Nor did over half of
> Longrich's (2008).  Which ones are we left with as
> valid?
>  
> Diagnosis- (after Longrich, 2008) frontal with an orbital
> rim that is completely convex in dorsal view; frontals
> abruptly expanding posteriorly, with the anterolateral edge
> angled 40 degrees to the midline; strongly flattened dorsal
> edge of distal caudal vertebrae; ventrolateral edges of
> pedal unguals rounded (unknown in most taxa).
> (after Makovicky et al., 2004) manus 8% longer than humerus
> (also found in Ornithomimus? sedens).
>  
> Gee, those seem like exactly the kinds of characters you
> tend to dismiss.  Minor proportions and angles,
> convexities of surfaces that aren't known or examined in
> most taxa.  So unless I want to blindly follow
> professional authority and say "Struthiomimus is valid
> because everybody has thought so, it's complete and has had
> long diagnoses; Rapator is a nomen dubium because the most
> recent reviews say so, it's known from one bone and has no
> explicit diagnosis published", I have to value these as much
> as any other character until they've been shown to be
> invalid.
>  
> You admit that you have no objective reason to value
> Australovenator's supposed diagnostic characters over
> Rapator's, you just 'feel' some characters are better than
> others and figure Australovenator has a better chance at
> validity due to its relative completeness.  Then you
> defend your approach with a pragmatic reason rather than a
> sc
 out fragmentary
> specimens would make my job easier, but I don't study these
> issues because they're easy, I study them because I actually
> care about the identity and relationships of the
> animals.  If it takes hard work to find out which name
> to call them, then so be it.  It makes it more
> interesting and fun.
>  
> Mickey Mortimer    
>         
>           
>   
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