Sauropod Rearing (was SVP On the Missisippi)

[MarkSabercat@aol.com]

Dear Michael,

I'm glad you enjoyed your first experience at SVP St. Paul, which I agree was 
very enjoyable and informative. At the end of your post you invited responses 
to the sauropod rearing talk (Rothschild B., et al, Behavioral Implications 
of Sauropod Stress Fractures, given at 9:45 on Saturday morning, 10/18) and 
here is mine.

The authors attempt to establish their hypothesis that sauropods did not 
habitually practice bipedal or tripodal rearing because of an absence of stress 
fractures within the metacarpal and carpal bones of selected macronarians 
(Camarasaurus, Brachiosaurus) as well as some diplodocoids (Apatosaurus, 
Diplodocus, 
Nurosaurus), which are known in extant large mammals such as elephants and 
others. They further assert that because bipedal human ballet dancers have 
these 
because of the stress exacted by their type of activity, we should expect to 
see this in sauropods if they were also in the habit of assuming bipedal poses 
for feeding or other reasons.

The first weakness of this argument lies in the fact that they do not 
consider the fact that sauropods did not possess highly ossified, tightly 
interfacing 
metacarpal (or tarsal) units like megamammals. The rugose surfaces of 
sauropods' articular surfaces probably served as attachment surfaces for thick, 
elastic cartilage that would have absorbed a great amount of compressive force 
when 
one of these animals went from a quadrupedal back  to a bipedal position, 
thus avoiding the direct stress that might result in a stress fracture for a 
highly ossified metacarpal unit.

They also assert that "resuming normal (sic) posture from a tripodal stance 
would exert extreme forces on the thoracic and lumbar vertebrae". Again, I know 
of no known stress fractures in sauropod dorsals-these animals had among the 
strongest and most well-engineered spinal osteology of any known vertebrates, 
well able to take the stresses of upright rearing. This in itself doesn't 
prove that rearing took place, but it does force a question: if diplodocoids 
like 
Barosaurus, for example, didn't rear, why did they have taller dorsal, sacral 
and caudal neural spines, and bigger, deeper pelves for their size than other 
sauropods (like brachiosaurids) that were presumably quadrupedal and made do 
with smaller ones?

Finally, there is the illogical comparison with human ballet dancers, whose 
skeletal systems and referred stresses are completely different: as humans we 
suffer such stress fractures because we are pushing parts of our bodies to 
perform functions they weren't evolved for. Sauropods, on the other hand, 
especially diplodocoids and some other taxa, had backwardly shifted centers of 
gravity, in some cases reduced forelimb lengths and dorsally or ventrally 
braced 
cervical vertebrae that made them supremely adapted for upright bipedal and 
tripodal feeding.

The authors make a basic error in attributing the stress fractures found in 
some extant large mammals to the same forces that produce these problems in 
humans (sudden, chronic compressive and shear forces in certain bones), without 
certainty that that the formers' condition was created by the same activity; 
the shear forces that result in such fractures could just as easily occur as 
the 
result of other, unknown kinds of behavior, and actual biomechanical studies 
of living large animals and humans should be carried out. To deny sauropods 
the capability of habitual rearing based on such non sequiteurial, fallacious 
reasoning for me makes no sense, and this hypothesis is far from explaining 
what 
sauropods did--or did not--do.

--Mark Hallett