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Segnosaurs are Theropods?
I have never liked the idea of the Segnosauria being placed within
in the Theropoda. The many features that they share with the
prosauropoda are convincing enough to conclude that they cannot be
excluded as ancestors. Also, after having veiwed the original type
specimen of _Alxasaurus_, and looking for the supposed semi-lunate
carpal block which is the seamingly only thing that links segnosaurs
to theropods, I have to say that none of _Alxasaurus'_ wrist bones
look especially like (or anything like for that matter) the obvious and
distinctive semi-lunate carpal block of maniraptoran theropods.
In "The Dinosauria", Barsbold and Maryan'ska listed a group of
features that the segnosauria supposedly share with theropods
1) A lack of a process of the dentary passing dorsal to the external
-This is not a feature that is possesed by all theropods, but it is
possesed by other dinosaurs as well.
2) Presence of pleurocoels in the vertibrae.
-Sauropods have pleurocoelus vertibrae
3) The fibula closely appressed to the tibia.
-The shinbones of segnosaurs are Very strange and bear no close
resemblance to any other dinosaur group; that the bones are close
4) Expanded pubic boot.
-_Nanshiungosaurus_ does not have a booted pubis.
5) A tall ascending process of the astragulus.
-Other dinosaurs have tall astraguli.
There are however, many features that segnosaurs share with
prosauropods, such as:
1) A relatively large premaxillary (compaired to theropods).
2) Elongate vomers.
3) Elongate nares.
4) Deep narial shelf.
5) Deeply recessed anteorbital fossa and fenestra.
6) Downwardly deflected dentary.
7) General tooth structure.
8) Broad tetradactyl pes (which George Olshevsky quite convincingly
shows is not a "reversion").
9) "T" -shaped postorbital.
10) "S" -shaped quadrate.
11) Low occiput.
12) Prearticular running the whole lower and forward margin of the
external mandibular fenestra.
14) The Prearticular-Angular-Splenial complex.
15) General hand structure and articulation.
16) Depressed jaw joint and a small "coronoid" process.
17) Lack of the maniraptoran "pulley-action" elbow-wrist complex
(i.e. the arms don't fold like birds').
18) Lack of the Avetheropodan "double-jointed" lower jaw.
With Ornithischians they share:
1) An opisthopubic pelvis.
2) General tooth structure (as per primitive forms).
4) Downwardly deflected Dentary.
5) Broad tetradactyl pes.
6) Hand articulation (as per _Heterodontosaurus_).
7) Depressed jaw joint and small "coronoid" process.
8) Lack of the maniraptoran "pulley-action" elbow-wrist complex.
9) Lack of the Avetheropodan "double-jointed" lower jaw.
When I went to Vancouver and saw the type material of
_Alxasaurus_ I was very excited. I did not know it was going to be at
the dinosaur exhibit at The Forum. When I looked at the hand I really
wanted to see the semi-lunate carpal block that "proves" its theropod
affinities. I looked and looked and stared ito a small plexiglass box
for upwards of fifteen minutes looking for the semi-lunate carpal
block. I still cannot find it (yes, I know what a semi-lunate carpal
block looks like). I did take a picture of it and I am still looking for it.
Please tell me where it's supposed to be!
These are the many reasons I believe that Segnosaurs (not
Therizanosauroideans) were transitional between prosauropods and
ornithischians and not theropods.
I know this will get responses.
Peter "I really couldn't see the semi-lunate carpal block!" Buchholz
--Boycot Taco Bell's "Texas Tacos" and "Border Light" menu items.