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RE: Gargantuavis a bird, not a pterosaur
While I no longer have my refs regarding *Gargantuavis philoinois*, I was
able to peek at some images, and they did do the refreshing of some of my
opinions regarding this when I first started thinking about whether or not it
might be oviraptorosaur-y almost 15 years ago.
1, All oviraptorosaur and therizinosauroid sacra have ribs that are separate
for nearly their entire length on each vertebrae, forming large lacunae between
ribs at each position. In *Gargantuavis philinois,* the sacral ribs often
coalesce in several positions forming a more-or-less solid plate in large
sequences of the pelvis. This has the additional quality of fusing the ilia to
the sacrum, which is not necessarily known in oviraptorosaurs, but I am unsure
in therizinosauroids _at the moment_. I'm tired, or I'd go do some digging or
turn this into a blog post or something. It'd be easier to use this feature to
argue the pelvis belongs to an ankylosaur than a nonavian theropod.
2, As I noted before, the zygapophyses of the sacrum of *Gargantuavis
philoinos* has an unusual form compared to oviraptorosaurs, but this holds true
for therizinosauroids as well: they are virtually consumed into the sacral
"yoke" of the rib, and thus are not present above the surface of the sacrum's
dorsal surface, but are flush with it. This leaves only the neural spines in
their fused series above the sacral "table." In oviraptorosaurs and
therizinosauroids, as in other theropods, the zygapophyses are distinct and
raised from the level of the ribs and the transverse process, while in the
therizinosauroids they may typically be fused into a "rod-shaped" structure on
each side of the neural "lamina." This feature is known in other birds, as
well, but is otherwise unknown in nonavian theropods.
3, Therizinosauroids and oviraptorosaurs possess a supracetabular crest, albeit
sometimes a small one. *Gargantuavis philoinos* lacks one entirely.
4, The femoral caput of *Gargantuavis philoinos* is spherical, whereas that in
oviraptorosaurs and therizinosauroids are cylindrical; it is set apart from the
body of the femur by a narrow neck that gives the caput a "ball-shaped" aspect,
unlike that in virtually all nonavian theropods. The caput is also highly
elevated relative to the fragment of the femoral shaft that is know which
indicates a distinct offset for the femoral attitude, something that does not
occur to this degree in oviraptorosaurs (even in the _relatively_ similar
*Avimimus portentosus*) or therizinosauroids.
5, The ilia face dorsal in *Gargantuavis philoinos* as they do in virtually all
ornithothoracean birds, and in *Avimimus portentosus*; this face is such that
the entire lateral face of ilium when viewed from above is visible. In
therizinosauroids, this is true only for the preacetabular ala, but not
anywhere posterior to above the acetabulum, and this is particularly notable
because the preacetabular ala in therizinosauroids resemble strongly the
condition of graviportal mammals is being deep, thick, and laterall deflected
(so that the lateral face is also viewable from the poaterior as well as the
dorsal). This doesn't happen in any oviraptorosaur or bird [qualifier: that I
There's a few other oddities, but I don't think they are as important. For
example, the size difference between the first and last sacral centrum, both
its length and its face, distinguish the sacrum as a bird, but there's a
similar effect in length, but not centrum face diameter, in therizinosauroids.
On the other hand, the centra in the middle of the sacrum are very, very
shallow and are essentially 3x wider than deep (or so, guesstimating), while
the middle sacra do not become enormously shallow in therizinosauroids or
oviraptorosaurs. I'd say this arching of the sacrum in the latter is due to the
arrangement of the sacra alone, while in birds like the pelvis in question, the
arching is largely due to the relative size and depth of the centra.
I do not believe this partial hip and limb have anything to do with nonavian
Jaime A. Headden
The Bite Stuff (site v2)
"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)
"Ever since man first left his cave and met a stranger with a
different language and a new way of looking at things, the human race
has had a dream: to kill him, so we don't have to learn his language or
his new way of looking at things." --- Zapp Brannigan (Beast With a Billion
> Date: Mon, 4 Jul 2011 18:14:00 -0400
> From: firstname.lastname@example.org
> To: email@example.com
> CC: firstname.lastname@example.org
> Subject: RE: Gargantuavis a bird, not a pterosaur
> Thank you Mickey.
> The ten sacral vertebrae do seem to be the character cited to diagnose
> Gargantuavis as a bird. Buffetaut and le Leouff cite Chiappe(1996), when
> they state that no non - avian dinosaur has that many sacral vertebrae.
> That leaves open three areas of doubt in my mind.
> 1) The authors were not certain of the vertebral number in Gargantuavis.
> To quote Buffetaut and le Loeuff, 1998:
> "...the exact number of vertebrae in the synsacrum is not easy to determine"
> "...it appears that ten vertebrae are present...".
> 2) Have any non - avian maniraptorans been found since 1996 that have more
> than 8 sacral vertebrae?
> 3) If this type of broad, highly fused, synsacrum with neurapophyses fused
> into a low median dorsal ridge is seen in therizinosaurs such as
> Nothronychus, can we rule out the possibility that gargantuavis is a
> previously unknown species of therizinosaur?
> To be fair, Nothronychus' pelvis is described as dorsoventrally crushed,
> so I may be seeing more superficial similarity than there really was.