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[firstname.lastname@example.org: The therizionsaurid pelvis]
[ Jonathan Wagner has just left for the field and unsubscribed before he
left, so I'm forwarding this on his behalf. Please don't hold me
personally responsible for anything after the square bracket! :-)
-- MR ]
Date: Thu, 29 May 1997 21:14:05 -0600 (CST)
From: "Jonathan R. Wagner" <email@example.com>
Subject: The therizionsaurid pelvis
I have asked Mickey to forward this to the list, as I will be off in
Colorado doing Field Camp for the next five weeks. Please remember to cc any
replies to firstname.lastname@example.org so I'll get them. - Wagner
In my previous post, I noted some confusing aspects of Russell et
Russell's reconstruction of _Therizinosaurus_. I would particularly like to
address some aspects of the pelvic morphology. Dunderhead that I am, I did
not thoroughly examine the illustrations in _The Dinosauria_ until after I
sent that post, and review of that work and a discussion with Peter Buckholz
prompted the following revelation:
The "up-angled tail" which Russell et Russell ascribe to
_Nanshiungosaurus_ may not exist.
Upon examination of the pelvis illustrated in _The Dinosauria_, and
comparison with the other therizinosaurid pelves found therein, it is clear
to me that the appearance of an upward angled tail may be due to a
fundamental mis-reconstruction of the articulation of that element by the
two Russells. When oriented as it is in _The Dinosauria_, there is no angle
to the tail worth mentioning.
This more conventional orientation is supported by the angle of the
pubes relative to the horizontal, which is virtually identical to that of
__Enigmasaurus_ and _Segnosaurus_ as given. It also provides for the pubic
peduncle of the ilium to be lower than the ischiadic peduncle to a degree
similar to that seen in other therizinosaurids (although more than
_Alxasaurus_). In this orientation the base of the tail is held horizontal.
The evidence against this orientation stems from the position of the
excavation of the ilial plate immediately cranial to the acetabulum (I
believe this is the preacetabular fossa, and I shall refer to it as such
below). In most theropods, this fossa is short and positioned cranial and a
little dorsal to the pubic peduncle of the ilium.
In the orientation preferred by Russell et Russell, the top of this
fossa is situated in such a conventional position. This results in an
aftward rotation of the distal pelvic elements, an elevated tail, and an
abnormally long pubic peduncle (including the articulation with the pubis),
the greatest thickness of which is cranioventral to the acetabulum.
In the orientation proferred by _The Dinosauria_, the tail is
horizontal, the distal pelvic elements are in a more conventional
orientation, and the thickest portion of the pubic pecuncle is cranial to
the acetabulum. However, the preacetabular fossa then extends far
caudodorsally to the conventional orientation, even to partway over the
acetabulum. I believe Russell et Russell (and Greg Paul before them) used
the top of this fossa as an indicator for the horizontal plane of the
pelvis. I also believe that this mislead them into their peculiar
reconstruction of the tail.
In addition to the largely circumstantial evidence for the alternate
position presented above, I would point out that while other theropods may
show a dorsal narrowing of the pubic peduncle, this condition seems
particularly severe in _Nanshiungosaurus_ and may indicate that the
preacetabular fossa has migrated dorsocaudally (see the figures and you'll
see what I mean).
In that case, the top of the fossa is not a good indicator of pelvic
orientation for _Nanshiungosaurus_, and the "up-tilted tail" may not be
correct. Of course, the true test of this is how the sacral ribs articulate,
and this was not noted in _The Dinosauria_.
As a correction to Russell et Russell, Paul (_PDW_ 1988)
reconstructs the pelvis as they do, and *does* show the tail elevated (as
pointed out to me by Peter Buckholtz), but the effect is not readily obvious
in his reconstruction, as the first caudal and the silhouette of the tail
extend out in a more conventional orientation.
On another matter, does anyone know why the preacetabular process of
the ilium of _Nanshiungosaurus_ viewed from the side in _The Dinosauria_ is
curved, while that illustrated in _PDW_ and the article by the two Russells
is flat? I have been assuming that the latter form shows the true side view,
while the former somehow does not account for the outward flare of the
preacetabular process, but I cannot guess as to why the Barsbold and
Maryanska (in _The Dinosauria_) would reconstruct it this way.
Have a nice summer, y'all.
Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock TX 79409
"They Airbrushed My Face" - REM