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Re: Bipedal apatosaurs and stegosaurs



On Thu, 24 Oct 1996, Robert J Meyerson wrote:

> Nick Longrich has demonstrated quite effectively that diplodicids
> and stegosaurs carry most of their weight on their hind limbs (and
> kind of threw me for a loop ... for a while).  This demonstration is
> then used to suggest the idea that these animals reared while
> feeding, either habitually or consistantly.  I suggest this is not
> necessarily the case.
> 
> The fact that these animals carried their weight on their hind limbs
> is no real suprise.  They both used their tails as weapons, and to
> be able to wield them effectively they would need to shift their
> weight onto their hind limbs (something that was reinforced by
> evolution).

Does Euoplocephalus show:
        a) enormous difference between humerus and femur length?
        b) very high neural spines over hips decreasing rostrally?
        c) extreme distance between rib articulations and vertebrae  centra?
        d) shortening of rib cage?
The answers are no, no, no, and no, but all of these features are present
in Diplodocus and Stegosaurus. Although ankylosaurs used their tails as
weapons, they do not resemble Diplodocus or Stegosaurus in structure
nearly so much as they resemble each other. 
        And while there may be an obvious connection between dinosaurs
proposed as tripod feeders and tail weaponry, the reasons are debatable-
certainly this cannot just be dismissed as the result of tail weapons.
It could be that tail weaponry tends to develop after dinosaurs
develop large, powerful tails for rearing, for instance, or that the
large, powerful tails necessary for rearing were originally developed
as weapons. But the use of a tail as a weapon, by itself, seems
inadequate to explain structural similarity between stegosaurs and
diplodocids but not ankylosaurs.

> To use stegosaurus again, if the animal was feeding
> bipedally, it's tail would be used for balance.  Since the tail is
> otherwise occupied, it would be unavailable for the animal's defence,
> leaving it vulnerable to a hungry theropod.  If theropods were primarily
> ambush predators, then precious seconds would be lost in reorienting the
> stegosaur for combat. 

        Yet gerenuks and groundhogs commonly rear. Some animals make a
habit of it for the precise reason that it decreases predation:
meerkats and prarie dogs. A rearing sauropod could probably spot an
allosaur from miles away. This might not work in heavily forested
areas, but if I recall, the Morrison formation was not heavily wooded,
but was instead floodplain, desert and semiarid environment. While
trees did exist, they may have existed in conditions other than dense
forest- something like the savannas today.  In such conditions, a
rearing sauropod would be able to see for long distances.