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Re: Why therizinosaurs are theropods (kind of long)



In a message dated 96-12-11 15:17:58 EST, gpb6845@msu.oscs.montana.edu
writes:

> 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.

Methinks you've changed your mind too hastily. But let's continue...
 
> 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 ...etc.
>
> 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... etc.

I was going by Russell & Dong's figures and descriptions, not by the
disposition of the carpal elements in the specimen. _Alxasaurus_ had _five_
carpal elements altogether: radiale, intermedium, ulnare, a large distal
carpal (the so-called semilunate), and a small distal carpal. In a
maniraptoran such as _Deinonychus_, the carpus comprised just >two< elements:
the radiale and the ulnare, of which the >radiale< is described as
semilunate; it articulates with the radius proximally and the first two
metacarpals distally. Such carpal anatomy (2 elements) is also found in
_Archaeopteryx_, but it is completely different from that of _Alxasaurus_.

In _Alxasaurus_, the large distal carpal articulates with metacarpals I and
II, like the semilunate of _Deinonychus_, but it is probably >not< homologous
with it because in _Deinonychus_ it is almost certainly the smaller distal
carpals that were lost, not the larger proximal carpals.

> 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....

The basisphenoid lies at the >back< of the basicranium, below the occipital
condyle; the parasphenoid at the >front< of the basicranium below the
laterosphenoids and whatnot. Fusion obscures the boundary between the two
bones (or regions, in the case of _Erlikosaurus_), but not their relative
locations! There is no inflated bullatosaur parasphenoid capsule or anything
like it in _Erlikosaurus_, and the basicranial anatomy as figured by Clark et
al. is much more like that of prosauropods than of theropods.

>  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.

I don't know about "remarkable," but convergence it is, if indeed this is the
case (I'm not yet convinced that >any< of the braincase of _Erlikosaurus_ is
particularly avian-looking or theropod-looking).

> 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?

In theropods, loss of the fourth and fifth digits was, I assert, the result
of the development of the forelimb into a wing. In segnosaurs, which I assert
weren't descended from small, volant dino-birds, the loss of those digits
probably occurred for different reasons. I never said that the >only< reason
for the loss of digits IV and V in >all< kinds of dinosaurs (or any other
vertebrates) was the conversion of the forelimb into a wing!

By the way, the notion that segnosaur ancestors were slothlike tree-climbing
insectivores is by no means as way out as it might seem. It could account, in
some way, for the derived segnosaur forelimb, the pneumatization of the
skeleton (retained from small prosauropods such as _Anchisaurus_ and
_Ammosaurus_, whose hollow bones once prompted Huene and others to classify
them as coelurians), and the absence of the early segnosaur lineage from the
fossil record (until a few kinds came down out of the trees like megatheres).

>  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...etc.

The manus is not complete in any specimen of _Alxasaurus_; check out the
tables of measurements in Russell & Dong's paper, where the manual phalanges
are listed as "identifications uncertain" and "only ends present." Lengths
are provided for only a couple of complete phalanges, and the restoration of
the manus in the "complete" skeletal reconstruction is hypothetical (and
based on the notion that _Alxasaurus_ was a theropod: circular reasoning
here). The >first thing< I checked for in Russell & Dong's paper was the
pattern of the manual digit IV phalanges, and I was mightily irritated when I
discovered that these key bones were either missing or incomplete. The manus
of the type specimen of _Alxasaurus_ seems to have been partially restored
before exhibition(!).
 
> 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.

>Practically indistinguishable< from the teeth of _Tyrannosaurus_ and
_Allosaurus_??  :-)

The teeth of _Erlikosaurus_ as well as the interdental plates are remarkably
similar to those of prosauropods such as _Plateosaurus_ and _Anchisaurus_. So
is the lower jaw. The lower jaw of _Segnosaurus_ looks even less like that of
ornithomimids than does the lower jaw of _Erlikosaurus_. I >am< talking about
the >lingual< side of the jaw as well as the lateral side. Look at the huge
mandibular fenestra and the way the prearticular wraps around the bottom of
the fenestra, as in _Plateosaurus_ but >not< as in ornithomimids. It's nice
that the dentary is ventrally deflected in segnosaurs and prosauropods, but
that's not the only similarity between their jaws.
 
> 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.

Not a chance.