[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Sauropod Biology

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?

Philip Senter & Matthew Bonnan: Evidence for obligate bipedality in the basal sauropodomorphs *Plateosaurus* and *Massospondylus*, Society of Vertebrate Paleontology Meeting Abstracts 2005, supplement to Journal of Vertebrate Paleontology 25(3), 114A

"Basal sauropodomorph dinosaurs are often portrayed as habitual quadrupeds. Evidence cited in support of this hypothesis includes supposed prosauropod trackways, trunk to hindlimb length ratios, and vertebral dimensions. However, it has generally been assumed that basal sauropodomorph forelimb morphology allowed efficient quadrupedal locomotion. Utilizing a standardized measurement and photography protocol we examined the range of forelimb movement in two basal sauropodomorphs, *Plateosaurus engelhardti* and *Massospondylus carinatus*, and several outgroup taxa: *Varanus*, *Alligator*, *Anser* [goose], *Struthio* [ostrich], *Acrocanthosaurus*, and *Apatosaurus*. We show that the range of humeral extension and antebrachial flexion were limited and probably prevented the forelimb from moving in a dynamically similar pattern to the hindlimb. Moreover, the palmar surfaces of the manus [ = palms of the hands] faced medially because the forelimbs were unable to achieve pronation. In this orientation, carpal and digital flexion would have produced a medially directed force, instead of the caudally directed propulsive force typically observed in habitual and obligate quadrupeds. Our results also suggest that the use of the pollucal [ = thumb] claws as defensive weapons was unlikely: the limited forelimb movements would have only allowed the pollucal claws to be used on objects beneath the chest of the animal, and were therefore of little use in defense. We conclude that the proposed prosauropod trackways could not have been produced by basal sauropodomorphs and that limb ratios and vertebral dimensions provide only equivocal data on limb and body posture."

"Pronation" means to twist the forearms in such a way that the palms face the ground. Like theropods, "prosauropods" were, according to the above, incapable of this rotation. Their palms faced each other, as happens to be correctly illustrated here http://rainbow.ldeo.columbia.edu/courses/v1001/plateosaurus.gif. They were therefore unable to use a keyboard or to walk quadrupedally -- except maybe if they sprawled like an alligator, but their shoulders may not have allowed this, and their forelimbs look too short for this to work anyway.

I didn't say they had long arms, I said large, by that I meant heavy and

That's of course not enough...

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.

To me much of the tail appears to be much thicker than the neck...

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.

Of course.

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 depends on the weight of the tail.

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.

Sorry, they certainly wouldn't break. But the soft tissue underneath the chevrons could have become a lifelong wound -- depending on the absolute forces involved and on the shape of the chevrons.

The tail and legs form a nice tripod, with the heavy, long and wide body
balanced over the hips.

The hips, however... http://www.senckenberg.de/images/content/museum/daueraustellungen/dinos/plateosaurus.jpg has the weight of the body resting on the part of the hips that looks least apt to withstanding stress in this direction. It also makes me worry about the musculature.

Here http://www.amen.org.uk/vorwelt/leben6/leb606.jpg we even have a specimen whose tail was disarticulated to allow its back a more vertical posture. Again note where the weight is transmitted to the leg.