Re: STRANGE THEROPOD SKULLS

["Matthew Troutman" <m_troutman@hotmail.com>]

><It can be of some value when defining a species or seperating a 
>species, but not for a whole group.>
>
>Generally, but of all "allosaurs" like *Marshosaurus*, *Allosaurus*, 
and 
>*Saurophaganax*, there is a slender ischium that is distolaterally 
>expanded into a sort of "boot". There is a dorsal ridge (as stated 
>before) on sinraptorids and *Afrovenator*. "Megalosaurs" like Meggie 
and 
>Torvo have inwardly curved ischia with compressed "feet", as do several 
>other of my basal "carnosaurs" like *Eustreptospondylus* et al.

There is usually a lot of variation in the ischium in animals and it is 
only diagnostic at the genus level. Let's look at primitive birds and 
the dromaeosaurs. In dromaeosaurs there is a lot of variation in the 
proportions that the ischium is compared to the pubis. The dromaeosaur 
Norell an Makovicky described has a proportionally long ischium while 
that of Saurornitholestes is proportionally short. The obturator process 
varies in placement ( either proximal or distal in the taxon ). Some 
dromaeosaurs have a spatulate ischial process whereas others don't. Now 
we can compare this to basal birds starting with the sickle-clawed form. 
Compared to dromaeosaurs the ischium has a thin, compressed proximal end 
and a flaring dital end ( of what is preserved ). The bird also lacks an 
anterodorsal ischial process.

Archaeopteryx has a highly automorphic ischium that is best described as 
"bumpy" anteriorly and there is no clear presense of an obturator 
process. It is proportionally short. And there is an anterodorsal 
ischial process. 

Confuciusornis has a simple, Archaeopteryx-like ischium with less 
rugosities. 

One can now immediately tell that the ischium is varible in design. 
Ridges and rugosities may be useful for deciding where a particular 
allosaur ( with all the main allosaur apomorphies ) fits in the 
Allosauroidea, but not anywhere else.

The example in basal birds and dromaeosaurs shows that some features can 
are consistent in different members ( a way to test polarity and 
relationships among closely related taxon ) the ischium as a whole is 
not diagnostic. 



>
>The femora of most "carnosaurs" I listed are diagnostic are all mostly 
>strait, have a raised fourth trochanter near to the epiphysis, a large 
>medially or forward-set lesser trochanter, and a raised femora head. A 
>few lack some of these characters (e.g., *Gasosaurus*) and these almost 
>all occur in the Jurassic, with the most advanced (or derived) forms 
>appearing in the Cretaceous (carcharodontosaurs, Afro, spinosaurs and 
>dryptosaurs).

    Since when are dryptrosaurs carnosaurs? The femur is also 
susceptible to functional change ( such as from migration of the 
M.caudofemoralis or any other muscles that can be adapted for a special 
locomotion ).

>These femora occur on pelves that have all long ischia with booted 
ends, 
>unlike the short ischia of ceratosaurs (not neoceratosaurs like 
>abelisaurs and *Ceratosaurus*, which may be tetanurans, but thats a 
>different thread) or the short unbooted ischia of coelurosaurs.

    That suggests a function for this correlation, not relationship.



>
>Then maybe I should see how similar the maxillae and dentaries of 
>*Baryonyx* and *Megalosaurus* are to each other. Or Afro and Bary with 
>Spino, like I should have done. Good point, Paul.

    I'm not Paul,  I'm Matt. 




><In what features? Crylophosaurus may be lacking one or two allosaur 
>features, but that does not mean a close relationship.>
>
>But Cryo does occur earlier than most other carnosaurs, and has 
features 
>both primitive and those considered "carnosaurian"; pneumatic 
lachrymals 
>without lachrymal foramen, a combo of primitive and carnosaurian. The 
>pelvis was described, but not figured, and I don't know if the photos 
of 
>the specimen show it, since I haven't seen them. Glut (1997) cites 
>Hammer and Hickerson (1994) in saying the femoral head declines from 
the 
>greater trochanter, where the opposite is diagnostic for carnosaurs, 
the 
>extensor groove on the back of the femur is shallow, and both 
characters 
>are diagnostic for coelophysoids; there is a fusion of the ankle bones 
>resulting in a tibiotarsus, indicative of neoceratosaurs. The posterior 
>ramus of the angular extends to the rear of the mandible, with the 
>ventral process of the surangular invading the angular's lateral 
>surface, creating a "bulge" that is seen only in sinraptorids. The 
orbit 
>is invaded dorsally by processes from both the lachrymal and 
>postorbital. Otherwise, other characters of the skull can be either 
>neoceratosaurian or "megalosaurian"

    Crylophosaurus does show a mix of primitive and advanced features as 
correctly shown, but other carnosaurs show other primitive features ( 
such as a camerate bone structure in Marshosaurus ) that are indicative 
of a more basal standing than Crylophosaurus. Crylophosaurus shows 
features that ally it with the Allosauroidea that are lacking in more 
basal carnosaurs. 

>This dinosaur seems to represent one of the best examples of a 
>transistory taxon, truly between one group and the other.
 
 So its a basal tetanuran now? 



><Ugh! Afrovenator is a torvosaur, related to spinosaurs. It lacks all 
>allosaur apomorphies. Carcharodontosaurs and sinraptorids are closely 
>related.>
>
>That Torvo may be a carnosaur could make this problematic.

    How is Torvosaurus a carnosaur? 

 And I guess I 
>must be a stubborn cuss, but I hold to Afro's carnosaurian nature. The 
>lachrymal foramen is lacking in Torvo and Meggie, the tooth is flat 
>laterally and thus bladelike in a form that is nearly identical to 
>allosaurs (and even carcharodontosaurs, though not "wrinkled" or evenly 
>serrated). The pubis is double-curved as in Eustrepto (another possible 
>torvosaur) and *Neovenator*. There is a maxillary fenestra, nasal 
>participation in the antorbital fossa [assumed: no nasals known]. The 
>cnemic process of the tibia is ceratosaurian, though, and unusual for a 
>Cretaceous (Barremian-Hauterivian?) dino unless it were a Neoceratosaur 
>(which torvosaurs may be). Form of humerus is allosaurian.

     I don't think that these features hold weight. 
Since I have not yet seen Neovenator I'll not mention it. But 
Afrovenator shows more synapomorphies with torvosaurs than it does with 
allosaurs.

 MattTroutman

______________________________________________________
Get Your Private, Free Email at http://www.hotmail.com