Re: late night thoughts: misunderstand what?4

[don ohmes <d_ohmes@yahoo.com>]

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. 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). 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?

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. 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.

I can cite no studies, 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', 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. Again, give it a try!-- DO.

----- Original Message ----
From: Dann Pigdon <dannj@alphalink.com.au>
To: dinosaur@usc.edu
Sent: Thursday, June 21, 2007 2:16:05 AM
Subject: Re: late night thoughts: misunderstand what?4

don ohmes writes: 

> It is reasonable to assume the large bipeds had to be careful about their 
> footing. Eg, how steep a grade can a 2m human climb without using the hands? 
> Not very. Also, soft ground must have been a huge problem. If you don't 
> believe it, go for a hike in a swamp, without using your hands. There may 
> have been large areas of refugia that were available to quads in many 
> environments. In other words, a basic quadruped vs biped advantage.

I suspect that theropod feet (with their large spreading toes) were much 
better suited to 'soft ground' than the compact feet of sauropods (of any 
size). Whether you're a biped or a quadruped doesn't really come into it. 
Sure, a quadruped may be spreading it's weight over more points of contact, 
but a sauropod foot would have plunged into soft ground at a much faster 
rate than the foot of a large theropod would. As long as the biped lifts 
it's feet before they sink too far, it should be fine (and having just two 
feet decreases the amount of time between steps). 

A sauropod (even with less weight on each foot) would have plunged to the 
knees/elbows well before it could have lifted each foot. In fact, if the 
sauropod walking style was like that of an elephant (with at least three 
feet in contact most of the time) then they'd be even more prone to sinking, 
since the time lag between placing a foot down and lifting the same foot up 
again is even longer. 

___________________________________________________________________ 

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