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Deconstructing _Therizinosaurus_ [long]
One of the projects I left in my in-box until the summer (and which
I am now playing with as a means of procrastinating from my nacient sauropod
dataset) is to modify the _Therizinosaurus cheloniformis_ reconstruction from:
-FREE-->Russell, D. A. et Russell, D. E. 1993. Mammal Dinosaur Convergence.
-REF!-->National Geographic Research and Exploration 9(1):70-79
...the somewhat famous and very very cool "chalicothere analog" article.
Part of this modification is to examine the effects of certain of
the assumptions of the authors above, as well as get a little practice in
those cool G.S. Paul reconstructions we all love so much. Following are a
list of concerns I've had about the reconstruction the two Dales Russell
supply, and what I am planning to do about them. I desire comments from any
and all corners (advice from paleoartists appreciated).
Please note that none of this is intended as criticism of the Dr.s
Russell, to whom we are all indebted for this paper. These are simply things
which I need to check on, and on which I do not consider my interpretation
to be of any real scientific value whatsoever. Gouge away!
1) Ribs: I am trying my best to sweep them back, ala Carpenter and
Paul. I also made the scapula a little more horizontal.
2) Dorsal vertebral column: The authors restore it as very short,
shorter than GSP's _Nanshiungosaurus_ (hereafter N.) and their own
_Alxasaurus_ (hereafter A.). Also, the ribs to not taper going backwards as
in A., GSP's N., or at least some theropods. What should I do?
3) Teeth: Mine is getting some when it gets "negatived". The authors
appear to subscribe to what I can only describe as a "trend-based" view of
therizinosauroid evolution, and decided that there was a trend towards
losing all of their teeth. I just don't see it, myself...
4) Speaking of trends. The authors decided to scale the legs down. The
leg is not significantly smaller than A., which is nearly half the size of
T. They say they see a trend in limb reduction, and I guess I'll buy that,
but should the trend be continued? I know that lower limb elements get
smaller with increased femur length (roughly = body size, see Holtz
1994("b")), but should the femur remain the same length with increased size?
5) And more trends. In Russell and Dong 1993, the paper describing A.
elesitaiensis, those authors state that the tail of A. was shortened. This
has always struck me as odd. I realize that this corroborates with the
upward angle they postulate based on hip structure (a similar hip structure
is, however, found in some oviraptorsaurs(?) and _Microvenator celer_, see
below). In any case, I am nixing this "trend", as it makes T. so ungainly
looking it is hard to believe it could walk. Perhaps it walked in the
"tail-up godzilla" pose it is restored in, but that seems rather ungainly as
well. I suppose one could speculate that the tail sported a whip (which
might explain the "lack of tetanuran specializations"). In any case, I can
see no justification for the supposed trend of tail shortening, and I am
restoring the tail at approximately the same relative length as that of A.
(as restored by the two Russells and Dong).
I also added chevrons going down to near the distal portion of the
tail. They are the "hatchet-shaped" tetanuran kind. Comments?
6) So then we get to body stance. The body is restored as being held at
something around a 40 degree angle to the horizontal. As mentioned above,
this is based on observations which led Russell and Dong (1993) to conclude
that the "principle weight-bearing axis" was above the horizontal. Also as
noted above, some of these specializations are also found in other taxa, for
which such a stance has not (yet) been proposed. I guess there shouldn't be
much doubt that such a position was used by the animal, considering the
ecological interpreations of the authors, but would move that way? In any
case, my restoration is of the animal moving, for which I lowered the body
(it just feels right somehow). Of course, at that angle, the shortness of
the tail looks even more ungainly. Perhaps if it directed its arms backwards...
7) And more trendliness. The authors use the hips of N. to fill out the
body, because both are Late Cretaceous forms. I regard N. as highly
specialized (see below) due to the highly derived pelvis (preacetabular
spike, obturator process massive(?), pubic boot absent), amongst other
things. I am substituting the far less derived pelvis of _Segnosaurus_.
8) The authors note that the base of the tail in N. is directed upwards
at a strong angle (and they also note that GSP did not restore it this way
in PDW). They add this to their reconstruction of T. Given that this is not
present in A., nor has it been reported in any other therizinosauroid, I am
inclined to ignore the assumptioon that this would be present in any Late
Cretaceous therizinosauroid and consider this an apomorphy of N. until
proven otherwise. I omitted it from the restoration.
9) Ye olde cervicals: Are very big (whiskey barrels come to mind...),
and the neck is quite long as restored by the Dr.s Russell. Does anyone know
anything about this. It certainly contributes to ungainliness in the
"walking" restoration, but I have no basis for questioning it.
10) Also, the arched neck posture is truly weird. The outline drawings
of the cervical vertebrae appear beveled for at least a moderate S-curve,
but they are restored in a smoth curve from the front of the body. In
playing with the drawings, I got them to easily "articulate" in a shallow
S-curve. Still, nothing like GSP's N. restoration.
11) Is the skull too small?
All of this adds up to an animal which may have been the subject of
"dinosaur-mammal convergence", but looks more like the subject of
"theropod-prosauropod convergence". Fascinating indeed!
Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock TX 79409
"They Airbrushed My Face" - REM