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Re: Sauropod Biology
From: David Marjanovic <firstname.lastname@example.org>
Plateosaurus, Massospondylus and other Prosauropods
were very front heavy animals,
They do look like they were.
which is why they walked on all fours in the first place.
An SVP meeting abstract from last year says _all_ "prosauropods" so far
investigated were incapable of walking on their forelimbs.
Who investigated this and what was their evidence?
They hand long necks, large heavy arms and big guts.
I agree on the guts, but the necks may have been quite light, and the arms
were often rather short.
I didn't say they had long arms, I said large, by that I meant heavy and
robust. The neck of Massospodylus was as long or longer than its body and
the tail doesn't appear to be much thicker than its neck.
I would imagine running around on
two legs, with the torso pitched forward, would be even more of a stress
than standing upright.
Why? This is the basic dinosaur posture. Neatly balanced across the hips,
the tail acting as a counterweight to the rest of the body.
To be neatly balanced on hind limbs as you put it, you need your center of
gravity above your feet. The long neck, heavy arms and massive fluid filled
guts would have made them front heavy, meaning the center of gravity would
not be over the hinds legs. This would explain the need to support the torso
with the front limbs. I should also point out that the bodies of
prosauropods were rather elongate, which would make balancing on the hind
legs even more difficult.
If it is to stabilize the body, it must carry some weight -- at the very
least its own.
Yes, that was my point. I think it's safe to say their tails were not so
delicate as to break from a little bit of weight, unless they were made out
of glass that is.
A totally different adaptation that needs to be present is the ability to
bend the tail upwards. If the tail is fixed in a straight position (as in
many ornithischians), a tripodal stance is clearly impossible. This is
rather easy to test in fossils. (Ossified tendons, present in most
ornithischians in varying amounts, are absent from sauropodomorphs.)
Your argument does not make much sense when you consider the
fact that the animal would be under less stress in an upright position
it would be in a horizontal position.
Whether this is a fact depends on its hip anatomy. What if the hip joints
were not mobile enough to allow vertical legs and an oblique body at the
same time? What if the bones allowed it but some muscles (many of which can
be reconstructed with reasonable ease) would get impossibly overstretched
in the process?
There are several mounts of actual articulated prosauropod skeletons in
upright positions, with the tail bent and resting on the ground, with no
signs of dislocation of the caudal vertebrae. The legs do not appear to be
bent in any extreme angle and the position looks quite natural and balanced.
The tail and legs form a nice tripod, with the heavy, long and wide body
balanced over the hips.
A list of mounted specimens in this or a similar position.
Upright walking Plateosaurus
Paleontology and Geology Institute, Tubingen
South African Museum, Cape Town.
Crouched Bent Tailed Plateosaurus
American Museum of Natural History
There are others, but I don't have the names of the museums to go with them.
I want to stress that this is not just some idea that I made up. I am simply
going off the real mounted skeletons and osteological evidence available.
Most museums, paleontologists and artists seem to agree on this point. Since
they are the experts, not me, I tend to agree with them.