Re: Shuvuuia

["Jaime A. Headden" <qilongia@yahoo.com>]

Mickey Mortimer (Mickey_Mortimer111@msn.com) wrote:

<The pes is quite obviously troodontid, and the forelimb is different from
the derived morphology in alvarezsaurids. The forelimb resembles other
troodontids except for the elongate phalanx II-2 and subequal III-1 and
III-2.>

  While I do not deny the skeleton is troodontid, I will offer a bit of
warning that the skull me not be associated with it, which is why I stated
that it was in the display at the AMNH _next_ to skeleton. The skull also
has features of the orbit (extremely tall and elongate: over 1/4 skull
length and almost as high as long) and relatively large braincase to snout
length that suggest, contra either troodontids or described skulls of
*Shuvuuia,* that it may be juvenile. This will then reflect on the length
of the snout bones. There is a very large antorbital fenestra, with what
seems to be a tiny premaxillary fenestra almost enclosed by the anterior
rim of the antorbital fossa, unlike troodontids except for *Sinovenator,*
and it IS found in *Shuvuuia.* The quadrate is more anteriorly inclined by
nearly 1/4 the length of the quadrate, something resembling *Shuvuuia,*
and not at all caudally inclined. The extreme small size of posterior
maxillary teeth is at odds with EVERY troodontid, as this specimen lacks
any apparent size gradation along the jaw for teeth, but this is not too
important: basal troodontid teeth are virtually indistinguishable from
alvarezsaur teeth apart from relative size. There is an elongate, tapering
process of the retroarticular; the post-dentary bones are incredibly
shallow; the post-dentary bones become shallower caudally until they taper
into the retroarticular process -- all unlike troodontids. The dentary is
gently curved upwards and it would appear gently _inwards_. Finally, like
*Byronosaurus,* the skull possesses an antorbital fossa that lacks a
distinct ventral rim, so that the fossa and the lateral margin of the
maxilla are very nearly confluent. Most distinctive of all, perhaps, is
that the braincase is deep, at least half the depth of the skull caudally
(unlike *Shuvuuia*), and the frontal is separated from the parietal by a
dorsal convexity of the telecephalic lobes interally, as in troodontids
for the most part (see *Sinovenator*) but less so in the "more primitive"
alvarezsaurs. The frontals are fully more than twice the length of the
parietals, unlike troodontids, however, and at odds with the juvenile
assignment it would seem to be more parsimonious to assume.

  Contra Mickey, the base of the ascending ramus of the jugal is not
apparent, though the jugal does get deeper caudally (this is true in
*Shuvuuia* as well). The foramen magnum hardly appears to be complete, as
it's angle from the photo taken gives it both a quadrangular aspect, and
the look of erosional defects that suggests it is not as complete as it
could be, especially in regards to the supraoccipital and left
exoccipital. I have no idea what Mickey is talking about when he refers to
a sagittal keel on the parietals, as the fossa that converge medially
appear to leave a platform, rather than a compressed keel; but, that is
fine: a keel is absent in some skulls attributed to *Troodon* as well. The
presence of a T-shaped lachrymal is largely pointless: it has long been
held that the prefrontal had been incorporated into the troodontid
lachrymal, as in other taxa, and the separate prefrontal/preorbital
ossification in *Shuvuuia* can easily be accounted as part of the
prefrontal-lachrymal complex. The two sides of the skull on the AMNH site
are not consistent in their preservation, and while the projecting bone
can be part of the prefrontal, as in *Shuvuuia,* it is likely this is a
neomorph, and hardly consitent with either hypothesis; the left side of
the skull shows a deeper, perhaps dissarticulated, element that
corresponds in shape to the left side, but could also be a scleral plate,
which even the _right_ side element could pertain to. Unlikely, but
possible.

 The skull should be considered apart from the skeleton, in any extent,
for its combination of troodontid-like and alvarezsaur-like features,
which seem at odds with the skeleton's clear troodontid or
deinonychosaur-like affinities. This does not mean then that the skull's
alvarezsaur features can be dismissed, because the association of the
skull and skeleton in a display (especially when the website refers to the
skull as *Shuvuuia*) cannot be considered consistent of a phylogenetic
inferrence or that the skull belongs to the skeleton.

  Cheers,

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)


        
                
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