Carrier et al. 2000 on Rotational Inertia in Theropod Dinosaurs

["Nick Gardner" <ratites637@hotmail.com>]

I recently got a copy of this paper and wanted to invoke some discussion on 
the matter-

The citation is-

D.R. Carrier, R.M. Walter, & D.V. Lee, (2000).  Influence of rotational 
inertia on turning performance of theropod dinosaurs: clues from humans with 
increased rotational inertia.  The Journal of Experimental Biology 204: 
3917-3926

===============

Abstract-

The turning agility of theropod dinosaurs may have been severely limited by 
the large rotational inertia of their horizontal trunks and tails.  Bodies 
with mass distributed far from the axis of rotation have much greater 
rotational inertia than bodies with the same mass distributed close to the 
axis of rotation.  In this study, we increased the rotational inertia about 
the vertical axis of  human subjects 9.20-fold, to match our estimate for 
theropods the size of humans, and measured the ability of the subjects to 
turn. To determine the effect of the increased rotational inertia on maximum 
turning capability, five subjects jumped vertically while attempting to 
rotate as far as possible about their vertical axis. This test resulted in a 
decrease in the average angle of the turned to 20% of the control value.  We 
also tested the ability of nine subjects to run as rapidly as possible 
through a tight slalom course of six 90 degree turns. When the subjects ran 
with the 9.2-fold greater rotational inertia, the average velocity through 
the course decreased to 77% of the control velocity.  When the subjects ran 
the same course but were constrained as to where they placed their feet, the 
average velocity through the course decreased to 65% of the control 
velocity.  These results are consistent with the hypothesis that rotational 
inertia may have limited the turning performance of theropods.  They also 
indicate that the effect of rotational inertia on turning performance is 
dependent on the type of turning behavior.  Characters such as retroverted 
pubes, reduced tail length, decreased body size, pneumatic vertebrae and the 
absence of teeth reduced rotational inertia in derived theropods and 
probably, therefore, improved their turning agility.  To reduce rotational 
inertia, theropods may have run with an arched back and tail, an S-curved 
neck and forelimbs held backwards against the body.

===============

The paper is very interesting and worth getting a copy of.  After reading 
it, I'm left with a few thoughts.  Taking into consideration this data and 
earlier messages on the Dinosaur Mailing List by various authors (especially 
Dr. Holtz).  Refer to the following list messages-

http://www.dinosauria.com/jdp/dromey/dromey.htm

Honestly, it appears that these findings are in-line with Dr. Holtz's 
statement about the sacrificing of speed for agility.  I'd like to bring up 
a point though, on pg. 3918, Carrier et al. state that "to correct for the 
low density of the lungs, we assumed a lung volume of 8% of body volume and 
removed this volume from the thoracic region of the model".   Would the 
removal of the estimated volume of air sacs throughout the axial column of 
_Allosaurus_ and/or other theropods (if others were used in a future study) 
have resulted in the removal of a lung volume far greater than 8% of the 
body volume?  Has anyone considered pursuing such an avenue in their 
research, that is, examining the extent of air sacs and how much volume they 
would take up?

In figure 6, _Allosaurus_ is illustrated in what the authors refer to as the 
"jack-knife posture".  The drooping of the toes seems a little strange and 
doesn't GSP discuss how the toes should not be drooping in PDW?  Could 
theropods (those without the lateral, or near lateral orientations of the 
scapulocoracoid glenoid) have lifted their arms in such an odd position, 
especially tyrannosaurids?  Did the lateral, or near lateral, orientation of 
the scapulocoracoid glenoids in maniraptorans possibly develop for this 
behavior?  Perhaps running in this posture, especially during the juvenile 
stages of life, was useful as a pre-WAIR behavior in which the individual 
would suddenly shift direction and go up a vertical or near-vertical surface 
to evade a predator.  Carrier et al. point out the robustness of the 
iliopubic articulation, isn't this a character convergently developed in 
abelisauroids and tetanurans, perhaps as an accomodation for this behavior.  
 I noticed also that on pg. 3925, Carrier et al. advocate that larger 
theropods were not as common as smaller theropods in mountainous terrains.  
This seems rather similiar to Bob Bakker's "Raptor Red" where the 
Utahraptors find it easier to live in the uplands which are not home to the 
acrocanthosaurs (which didn't live in Utah anyway...) but instead to the 
little deinonychs and troodonts (which also don't live in Utah anyway...) 
which were much smaller than the Utahraptors... (maybe with all of these 
non-Utah taxa, they were really in Montana, which might explain the absence 
of _Acrocanthosaurus_, and perhaps the iguanodonts were _Tenontosaurus_?).  
Just a thought among many.


Nick Gardner

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