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Re: "No Name Saurus," the Case of NGMC 91



Jaime Headden wrote-

> [HP] Mickey Mortimer writes that the possible reference of NGMC
> 91 to *Sinornithosaurus* should have forced the authors to
> ascribe it to a named taxon or just simple establish a new
> taxon.

I said I think it should be a new taxon, only if most of the differences I
listed are real.  In the meantime, I recommended referring to it as cf.
Sinornithosaurus, to emphasize the similarity to Sinornithosaurus, without
the certainty of it being in that genus.  Similar to how the Australian
neoceratopsian ulna is cf. Leptoceratops because it is closest to
Leptoceratops in morphology, but might be a different genus.

>   First, the specimen is most likely a juvenile. It is
> reasonable to expect that, as discussed on this list, only an
> adult taxon can justly provide characters on which to establish
> taxonomy, as juveniles of similar taxa can share almost _every_
> feature of each other (sensu Centrosaurinae and Lambeosaurinae)
> whereas only through ontogeny can the taxa be differentiated
> substantially.... Thus the possible juvenility can provide a
> basis for resemblance, and any features relating to proportions
> should be forgiven; features found in later onteogeny are not
> known, and *Sinornithosaurus* _could_ have looked like this, but
> so could any other species of the mophological trend that
> Mickey, Pete Buchholz, and I have [all independently] found in
> *Bambiraptor*, *Microraptor*, *Sinornithosaurus*, *Rahonavis*,
> and *Unenlagia* from "typical" Dromaeosauridae [*Droameosaurus*
> + *Velociraptor*]. All dromaeosaurids, for instance, have
> enormously large heads, and most have large orbits. The temporal
> region in this specimen is almost completely obfuscated and thus
> not telling in some intra-dromaeosaur relationships....
> Triangular snouts with ovate nares (even restricted caudally)
> occur in juveniles of a multitude of maniraptoran taxa,
> including [esp.] troodontids, purportedly forming a mono- or
> paraphyletic Deinonychosauria. Juveniles of closely related taxa
> _will_ resemble one another....

The specimen is probably younger than the Sinornithosaurus holotype, but 63%
of adult size is hardly juvenile.  Then again, Sinornithosaurus' holotype
may not be adult, so that doesn't mean much.  In any case, it's ridiculous
to think only an adult will show autapomorphic characters.  In the
Centrosaurinae and Lambeosaurinae, even adults can only be told apart based
on cranial ornamentation (or so they say, I've never looked at them and have
a feeling postcranial autapomorphies would be found if rigorously looked
for).  As cranial ornamentation isn't developed until adulthood, subadults
are difficult to tell apart.  In eumaniraptorans, there are many cranial and
postcranial differences (arctometatarsus, constricted tooth bases, elongate
caudal prezygopophyses and chevrons, etc.) that would be expected to be
developed in a subadult.  As NGMC 91 exhibits many differences from other
eumaniraptorans and resembles Sinornithosaurus in many details with only a
few non-controversial differences, I think it is quite clear that
eumaniraptorans of this age can be distinguished from each other.  Of
course, it is possible all subadult eumaniraptorans resembled
Sinornithosaurus more than their adult forms, but I consider this extremely
unlikely.

>   It would be all fine and that should the authors of the
> _Nature_ paper have referred the specimen to a "genus" (as if
> that meant anything) but they dis ascribe it to the most
> reasonable taxon they could without providing assumptions on an
> ontogeny they did not know, which was the safest bet they could
> offer.... Better to get rid of all the *Cheneosaurus*es of the
> world than continue to describe juveniles whose natures and
> relationships depend on final ontogeny, and we cannot be sure of
> their true [or most likely, adult] affinities.

Dromaeosauridae gen. et sp. indet. is not a reasonable designation.  That
indicates it cannot be distinguished from other "dromaeosaurids" sensu Xu et
al.(1999), whereas in reality it is quite distinctive from all but one.
Calling it cf. Sinornithosaurus or Sinornithosaurus sp. is hardly a case
like Cheneosaurus.

Mickey Mortimer