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Re: Sauropod Biology

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.

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 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.

The fact that many later (and far larger) sauropods
retained this ability is certainly evidence that Prosauropods were capable
of doing the same.

No, it only means that this ability must have evolved at _some_ point.

Because of the sheer weight of the animal, the tail could get damaged

Most of the weight would still be on the back legs. I'm not saying that these animals put ALL their weight on their tail. The tail was there as a counterbalance and possibly to take some of the weight. Obviously if this animal could stand on two legs, its certainly not going to need to put a whole lot of weight on its tail while upright! The tail would rest on the ground, and at the very least would help stabilize the body.

If it is to stabilize the body, it must carry some weight -- at the very least its own.

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?

A very bad choice of words I admit = P But I consider the early cretaceous
to be the first 1 3rd of the cretaceous, followed by the mid and late

Oh, sorry. I misunderstood.

(There is officially no Middle Cretaceous http://www.stratigraphy.org/gssp.htm.)

My point was simply that the extremely derived, high shouldered
forms seemed to have gone into decline after the Jurassic.

And as Tim Williams wrote

Yes, that seems to be true.

Currently it seems so, yes. But the Early Cretaceous fossil record is probably too bad, or at least too badly known, to tell.