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Re: late night thoughts: misunderstand what?

don ohmes writes:

Dann, you are neglecting the issue of balance, which is the key issue w/ large short-armed bipeds. Whether they bog down is not the issue, it is whether they _fall_ down. What can suddenly happen to your 'vertical equilibrium' in situations of uneven, steep, or low density footing has a much wider range of outcomes when you are bipedal.

How many multi-tonne counter-balanced bipeds has anyone had the chance to observe? You can't really compare modern primates or tailess flightless birds to a large theropod (no matter how much lumber you strap to an undergraduate).

It is an area of locomotion that the quadruped has a fundamental advantage in, making a wider range of substrate environments available to them; thus the possibility of refuge. Bipeds are particularly vulnerable to sudden, unpredictable changes in footing (vertical differential, traction, etc).

As are large quadrupeds (ask any elephant). Bipeds at least have the advantage of a better sense of balance.

I feel it probable that the large bipeds not only stuck to firm ground, but probably firm ground that was well-compacted by large quadrupeds. And why wouldn't they?

If only we didn't have so many darn footprints (of both bipedal and quadrupedal dinosaurs) that were made in soft substrates...

I agree intuitively that sink rate in some 'densities' of substrate would be slower w/ the theropod foot, but the sink rate is not the metric that measures the likelihood of traversing a given area, except (possibly) under certain very unlikely and limited conditions. I actually have considerable personal experience in this area, having spent much of my life literally in swamps. This includes working w/ machinery; pickups and tractors; and animals (cows, horses, and dogs). Trust me on this; a cow can go places a man can't go, WITHOUT using his hands, and a mule bred from a Percheron horse can pull logs from a place an ox can't.

Neither cows, horses nor mules are multi-tonne quadrupedal archosaurs. Neither, for that matter, are bipedal humans a good analogy for multi-tonne theropods.

Which is why the old-time loggers used them, or so I am told. Even more to the 
point, I became permanently restricted to crutches some years ago, and have 
experience w/ both modes that very few humans can claim to have. This is 
because I am not very smart, and keep right on going out there... quadrupedia 
is even superior in machines; motorcycles
 can't go where the 4 wheelers can.

Now we're getting somewhere. Motobikes are *much* better analoges for extinct giant vertebrates...

I can cite no studies...

[Reels with shock]

... but can tell you this with confidence. A quadruped can climb steeper slopes, and traverse softer ground than a biped that cannot use it's 'hands'...

Fairly recent studies have shown that elephants evaluate steep terrain to determine whether the energy required to traverse it is worth the quality of food it can hope to get from it (see the 'Elephants don't like steep slopes' thread in the DML archives - wow! I just cited an actual study!). Predators, on the other hand, will tend to follow prey just about anywhere. Meat is a much more concentrated and energy rich source of food than plant matter, hence has the promise of greater rewards. In this matter at least, it would have been the large quadrupeds that avoided steep or otherwise difficult to traverse terrain more often than the large predators.

... therefore locomotive substrate can offer refuge from large bipeds, even if only part of the time. Daily and seasonal cycles would be factors, as would climate cycles. Relative body mass plays into this, I suppose, but not in any linear way. Also, some large theropods had longer arms than others. How long your arms have to be to be of assistance in critically unstable balance situations I don't know, of course. Intuitively, I would say arms much shorter than a humans is too short.

In theropods, the length of the arms was quite secondary to the length of the tail as far as bipedal stability went. Quadrupedal dinosaurs more likely relied on their four-legged stance to keep them stable (with the exception of large ornithopods, which enjoyed the best of both worlds). Bipeds in general evolve to be able negotiate obstacles while maintaining their balance - which would have been even more true for multi-tonne bipeds that couldn't afford any major fall. Therefore I suspect that large well-counterbalanced bipeds would have been better equipped to handle diffucult to traverse terrain than their herbivorous quadrupedal cousins.


Dann Pigdon
GIS / Archaeologist         http://www.geocities.com/dannsdinosaurs
Melbourne, Australia        http://heretichides.soffiles.com