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weighing big theropods

At SVP Therrian and Henderson gave a talk in which they pegged T. rex at 10 
tonnes, and Giganotosaurus at 13-14 t. Yikes! Say it ain't so. Those masses are 
70 to 100% higher than what I've modeled from skeletal restorations. What 
makes it all the more strange is that T & H are using my skeletal drawings for 
their computer generated mass estimates! 

Well somebody is way off here. Way, way off. And I was sure it was not me, 
since I've been massing dinosaurs for years using the old dunking the 
model in the water method since the 1980s and getting consistent results. In 
the DinoFest II volume I noted that my clay-based mass estimate of the famed 
elephant Jumbo, derived form a skeletal figure drawn from the mounted skeleton 
at the AMNH, was just a few percent off the correct figure, and I had similar 
success with a rhino (T & H claim the same success with their modern animals). 

So I redid the AMNH Tyrannosaurus. This time I measured out the amount of 
plasticine needed to obtain the volume that would equal 10 tonnes, rather than 
the 5.7 tonnes I modeled ~20 years ago, after lungs and air-sacs were taken 
account (about 15%). My intent was to try to pile on all the clay and see if 
it was realistic. But as I modeled the body volume over the skeletal outlay 
with the lateral, volume marking cross bulkheads in place I realized it was a 
hopeless task, a big lump of clay was laying there and there was no way it was 
going to fit on. In the end, even though trying to be generous, I ended up with 
a volume only 10% higher. Applying 10 tonnes worth of volume would be 
laughable, there simply is not enough volume encompassed by the skeleton to 
accomodate so much mass. No way Jose. After all, these are chase predators, and 
predators are inherently lightly built to enhance speeds and agility. Because 
my original mass estimate was less biased and probably more accurate I am 
going to stick with it, no need to change it in a science that is not precise 
anyway. Besides, Seebacher 2001 in JVP used a digital method based on my 
restoration arrived at a volume mass estimate of 6650 liters, scarily close 
to the 6670 liters my first model produced! If Seebacher uses digital methods 
to almost precisely verify my clay model, and T & H use digital methods to 
arrive at an estimate that is 70% higher, which am I going to go with, duh?

I do not know how T & H are getting such extremely higher volumes for the 
same restoration. Looking at their digital models they appear a little bulky in 
the hip region, but that should not cause such a large disparity.  

The boy named Sue was about 6.1 t according to my previous results. Likewise, 
the mounted Giganotosaurus weighed somewhere in the area of 6.8 t -- it was 
bigger than any known T. rex but not by that much. There are fragments that 
suggest even larger giganotosaurs but not enough is known to produce reliable 
estimates. Although I am not a big fan of using long bone circumference to 
estimate dinosaur masses because of the very large plus-minus variation that 
in living animals, they produce estimates in the range of 5 tonnes, even less 
than my models. Similarly low results will be produced for giganotosaurs. If 
these theropods were as massive as T & H estimate then they would have had 
mass/limb strength ratios far lower than seen in any large animal, worse than 
those of elephants, and would have been so weak limbed it is questionable that 
they could have stood up and taken a step without damaging something. All lines 
of evidence reject masses significantly higher than the volume of my skeletal 
restoration derived sculptures indicate.  

An interesting thing is that I measured the mass of the hindlimb separately. 
It is not really possible to precisely estimate the mass of a specific body 
part because of the plus-minor error. Even so, the limbs muscles (including 
caudofemoralis) made up about 30% of total mass. This is similar to the 25-40% 
seen in ratites. By no means can it be ruled out the the giant tyrannosaurs has 
as much as 40%, or as little as 25%, of total mass dedicated to limb muscles. 

The more reasonable mass estimates are also compatible with the flexed legs 
of giant tyrannosaurs. At the meeting Gatesy and Hutchinson presented what they 
label constraint-based exclusion to determine the mid-stride limb pose of 
Tyrannosaurus. A major premise was that the center of gravity was forward of 
hip joint in the biped. In the end the only pose allowed in the plausible 
exclusion space was almost exactly the same as I restored in 1988. Aside from 
liking the verification of my conclusions which were based on hip joint 
arc, knee morphology (BTW, in their recent Paleobiology and Nature papers G & 
H completely misarticulated the proximal fibula by attaching it to the cnemial 
crest, this was corrected in the SVP presentation), muscle extension factors 
and a dash of does it look plausible, I found it interesting that G & H came 
up with additional evidence for flexed legs I had not thought of. In bipeds the 
knee cannot be aft of the center of gravity, if it is the foot is going to be 
too far aft and the creature will fall forward: we humans can have straight 
knees because the center of gravity is directly above the legs. That there is, 
it appears, now a consensus that even the biggest theropods had strongly 
flexed knees explains why they retained a basically avian-type knee joint 
morphology that only works when the leg is flexed.