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Why therizinosaurs are theropods (kind of long)



As many list members might remeber I had formerly expressed support for the 
notion that segnosaurs were the sister group to ornithischians.  I have since 
changed my mind by examining several lines of evidence, some of which George 
has wrote about below.

George Olshevsky wrote:
 
> I've not seen a semilunate carpal block in the type specimen of _Alxasaurus_,
> despite examining the type specimen >in person< >specifically< for this
> feature. In Russell & Dong's description of _Alxasaurus_, the carpal elements
> that are supposed to make up this block are described as >separate<--take a
> look at the figure on p. 2117 of Russell & Dong's description and tell me
> just which of the two existing carpal elements is the semilunate. There IS NO
> SEMILUNATE CARPAL in this earliest of known segnosaurs. If the illustration
> of a semilunate carpal in _Therizinosaurus_ by Barsbold 1976 is correct
> (debatable), then it must have developed independently in the group.

I too have examined the type specimen in person.  I did not see a semi-lunate 
carpal then either.  There is a reason for this.  The carpal elements in the 
type of Alxasaurus are disarticulated and flipped over, placed almost 
upsidedown to there natural positions.  The otherwise fully articulated arm 
tricks you into thinking that the carpal elements are in fact articulated 
when the opposite is true.  If you try to make a model of wrist motion 
according to the position of the bones, you will see that the wrist moves in 
all sorts of odd and unnatural ways, indicating that Russel and Dong are 
correct in repositioning the bones to their proper places and showing that 
there is a semi-lunate carpal block in Alxasaurus.
 
> The supposedly birdlike cranial neurology is also completely
> unconvincing, since the description (Clark et al., 1994) of the only
> known segnosaur braincase, that of _Erlikosaurus_, contains
> anatomical errors and is inadequately illustrated. Furthermore, the
> authors of that description do not cite any neurocranial anatomy
> among their supposed theropod synapomorphies, but rather cite
> certain neurocranial features as autapomorphies of _Erlikosaurus_
> and segnosaurs, >not< synapomorphies of birds and segnosaurs.  As I
> have pointed out in previous posts, the palate and basicranium of
> _Erlikosaurus_ as illustrated by Clark et al. bear a >striking<
> overall similarity to those of prosauropods, as illustrated by
> Galton in _The Dinosauria_. (Note that it is the >basisphenoid< that
> is highly pneumatic, and that the >parasphenoid< of _Erlikosaurus_
> is not inflated as in troodontids and ornithomimosaurs. Don't take
> my word for it; compare the illustrations yourself.)

Note also, that they state that ALL the sutures in Erlikosaurus' braincase are 
fused and unrecognizable.  They seem to have called the bulbous lower 
braincase element the basisphenoid because they know that at least one 
element is the basisphenoid and they can't be certain where the parasphenoid, 
or any of the other bones for that matter, begin or end.  I am not sure 
exactly where it was reported, but I do know that the V1 cranial nerve 
anatomy is identical to that of a birds.  Note that only maniraptoriformes 
(and perhaps a few other theropods close to them) have their brains set up 
like this and that every other tetropod has it set up in a kind of "default" 
tetropod arangement.  This would be a remarkable and superbly unlikely 
convergence if therizinosaurs indeed weren't maniraptoriform theropods.

> The manus and forelimb of all segnosaurs are highly derived and there are any
> number of reasons that can explain the absence of digits IV and V besides a
> putative close phyletic relationship between segnosaurs and theropods. The
> manual digits of segnosaurs are >not< markedly elongate--only the claws
> are--and the digits are not completely known in any specimen. So your
> statement that segnosaurs had maniraptoran forelimbs is at best debatable and
> most likely groundless.

I find it odd that you claim the *only* reason theropods could have
lost MC IV-V is because they were learning to fly, but "there are any
number of reasons that can explain the absence of digits IV and V
besides a putative close phyletic relationship between segnosaurs and
theropods."  Why is that George?  First off, you're statement: "The
manual digits of segnosaurs are >not< markedly elongate" is true if
you are compairing it to something with absurdly long fingers like
Archaeopteryx; but the truth is, they are fairly long and theropod
like, and not very short and prosauropod like.  Also, your statement
"and the digits are not completely known in any specimen" is comletely
false.  I, as well as you, saw the totally complete articulated
forelimb of the type specimen of Alxasaurus.  To say that it was
incomplete is being absolutely untruthful.  Yet another thing that
links segnosaurs to theropods are the proportions of phalanges in MC
III.  In at least therizinosaurs and maniraptoriform theropods MCIII 2
and MCIII 3 are fairly short, while MCIII 4 is as long as 2 and 3
combined.  In all known prosauropods and ornithischians MCIII 2-4 are
all about equal in length.  This would be a remarkable and very
useless convergence if segnosaurs weren't theropods now wouldn't it?

Additionally, the lower jaw of Erlikosaurus could very well have been 
mistaken for that of some toothy Ornithomimosaur.  Every bone's shape is 
remarkably similar to those in Ornithomimosaurs; especially those on the 
lingual side.  The dentaries are also downwardly deflected like in 
Ornithomimosaurs (including Pelecanimimus), but not in the manner of 
prosauropods and ornithischians.  Erlikosaurus' teeth are practically 
indistinguishable from those of other theropods, especially bullatosaurs, 
differing only in the jaggedness of their serrations.

I think that an increadibly strong case is made for the theropodan nature of 
therizinosaurs, and a fairly strong case is made for them being bullatosaurs as 
well.  Therizinosaurs are not as prosauropod like as previously assumed, new 
discoveries and reexamination has shown that they are in fact derived 
theropods.

Peter Buchholz
gpb6845@msu.oscs.montana.edu

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