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Re: Sauropod Biology

From: David Marjanovic <david.marjanovic@gmx.at>

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 then
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

Tripodal Massospondylus
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.

Sim Koning