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Re: bloated T. rex



<Jaime,
Thank you for your extended response. This is definitely helping.>

[snip]

<Okay, ignoring the ton issue until we can get that cleared up, I can 
see this form of compensation working IF the rex was capable of rotating 
the front half of the body up (and thus, toward the center of gravity) 
while keeping the tail horizontal (as otherwise, the tail rotates down 
toward the center of gravity in the same proportion as the front, 
accomplishing almost nothing--and this doesn't even get into the problem 
of tail dragging.) This raises two obvious questions.  1) Was the rex 
spine in the pelvic region flexible enough to bend like this? and 2) Was 
the musculature above the spine strong enough to accomplish this bend?>

It wasn't the spine, but the rotating femoral head within the acetabulum 
to perpedicular the body's long axis, and the very strong knees flexing 
to help angle the body up; the ilium was huge and aided this muscular 
tension well enough, and the muscles that supported the lower back along 
the spine and the ribs against the legs would be monstrously huge to 
provide the kind of support needed as can be seen in the back muscles of 
an elephant, fully capable of rearing on the hind legs (though I've 
never seen one _walk_ this way!). The back was relatively inflexible [in 
the tyrannosaur], but there was a fair degree of curvature possible, 
however this is mostly downward. Upward is about fifteen to twenty 
degrees, enough to accomplish a one-legged rearing mode, but the belly 
would not have been full or this would be impossible.

<I'm assuming the rex spine was spring loaded to resist gravity so that 
the rex would not have had to expend much energy to maintain it's flat 
cantilevered body position (I've always assumed this spring loading is 
what causes the classic therapod death pose) but I'm guessing the 
"springs" involved were ligaments which would have slacked off as the 
spine bent upward-->

Spring loaded? Yes. As much every theropod had in the spine, for this is 
a common feature among them; coelophysids had more flexible spines in 
the up-down range, but not much more than small tetanuran theropods like 
carnosaurs and maniraptors. Arctometatarsalians all had rigid spines 
that had little flexion possible, or even aduction, and this helps 
support the better running or large-headed-ness this group had.

The ligaments do not go slack, but rather contract as moisture is drawn 
from them during dehydration, and the famous *Coelophysis*- 
*Dilophosaurus* death-pose is achieved.

Jaime A. Headden

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