Re: Cretaceous therapods of the south

["Jaime Headden" <jaemei@hotmail.com>]

Our estimable Thomas R. Holtz, Jr. (unfortunately not here to reply) 
wrote:

<<carcharodontosaurs (considered by some to be close to abelisaurs, but 
by others to be advanced allosauroid carnosaurs), and various 
coelurosaur groups (dromaeosaurs, etc.).>>

Then Jack Conrad replied:

<I'm going out on a limb again, is it unlikely that carcharodontosaurs 
are closely related to sinraptorines?>

  Not unlikely at all. Now, this is in the archives, but I think it 
bears repeating (and I will not bore everyone by retelling my concept of 
allosaur phyllogeny) that *Giganotosaurus* has a skull that is, in many 
ways, nearly identical to that of the smaller *Carcharodontosaurus*, 
albeit a rounder lower infratemporal fenestra and a longer snout, the 
former being a ceratosaur character.

  The rest of the skeleton is strange. First, the vertebrae are unique 
for theropods in the region of the anterior dorsum, with rostrocaudally 
long neural spines in the first couple bones, and this cannot be used as 
a phyllogenetic character; it's derived, and that's it. The rest of the 
back neural spines are narrow, and highly reminiscent of allosaurs, not 
ceratosaurs like the Abelisauria. The scapula is restored as very short, 
and while I've not seen the bone, just odd views of it from a distance 
and restorations of it, it does seem to be missing about one-third its 
distal length, and would parallel the scapula of *Acrocanthosaurus* very 
well (and Tom Holtz has already proven Acro was an allosaur, and the 
list may well remember my debate with him on the closeness of that 
kinship). The pelvis is unusual for Giganoto, with a very arched ilium 
and thick pubis. I need to _see_ the thing to affirm or deny its 
placement. (Plus, we got that new giant-dino bonebed to see to in 
Argentina that may be a Giganoto gathering spot.)

  Anyway, there are several other features of Giganoto that point 
towards the abelisaurs, including the cnemic crest of the tibia, which 
is very pronounced and superficially similar to ceratosaurs, but it 
should be noted that abelisaurs and carnosaurs share a similar knee and 
this this is not diagnostic.

<It seems that their skulls are rather a lengthened _Sinraptor_ type,>

  Aside from the lower infratemporal fenestra, the skull is nearly a 
dead-ringer for a really big carcharodontonosaur. The chin (like 
Godzilla's) is autapomorphic. It is also an allosaur trait, though not 
all allosaurs have this feature, to have doubled ridges surmounting the 
nasal bones, and Giganotosaurus has this feature and no ceratosaur does. 
Yes, superficially there is a sinraptorine resemblance, however, not 
enough (and no complete Giganoto skull) of the Giant from the South is 
present to perform a full morphological examination as can be done for 
*Allosaurus*, *Acrocanthosaurus*, *Carnotaurus*, and possibly even 
*Ceratosaurus*.

<and the tall neural spines (of which I don't know how much is 
restoration and how much is preserved, no reference handy)>

  Half the dorsals, most of the anterior and scattered medial and 
posterior bones with neural spines are known, and about half the neck, 
of various sections. This from seeing the type material and restorations 
(I wasn't at Dinofest, so didn't get to pop over to the Philedelphia 
Academy to take a peek as the Big G [the _other_ Big G shall remain 
nameless as he is not a dinosaur])

<also seem similar between these two animals and _Yangchuanosaurus_.>

  *Sinraptor* and *Yangchuanosaurus*, (4 species in 2 genera, 2 per) 
both have very tall neural spines that are morphologically similar to 
*Acrocanthosaurus'* and even to *Ceratosaurus*, being twice the height 
of the vertebral centrum or higher, where Bull-Face has shorter neural 
spines relatively and *Allosaurus* has knob-topped neurals. Again, not 
very diagnostic between the two groups.

<Also, as restored by our own GSP in _The Complete Dinosaur_,
_Giganotosaurus_ has less of a sigmoid curve in the neck than
_Allosaurus_.>

  I'm not sure where Greg got the shallow curve, have not seen the 
material too closely, so can't comment on that, but I will tell you that 
*Carcharodontosaurus'* is short and *Acrocanthosaurus'* like 
*Allosaurus'*, very long, and perhaps longer relatively.

  Anyway, your comment was in the buck range. Suffice it to say, we need 
more of *Giganotosaurus'* head and vertebrae (and the manus would help!) 
to form a proper diagnosis.

  Now, this is my couple cents on the matter, not worth much more, since 
I've said it all before.

Jaime A. Headden

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