Re: Bipedalism

[rjmeyer@ix.netcom.com (Rob Meyerson)]

>In a message dated 98-04-03 03:17:17 EST, rjmeyer@ix.netcom.com writes:
>
><<  I merely suggest that the bipedalism *trait* is most likely to develop on
>the ground. >>
>
>If so, we'd see a lot more extant obligatory bipedal mammals and reptiles than
>we do. Most of these animals' ancestral forms lived "on the ground."

Okay, how about this for an idea [modification of hypothesis ahead]:

It has already been stated that extant mammalian bipeds have recent arboreal 
ancestors.  What if arboreal life preadapts a quadreped for a bipedal 
existance?  In watching the resident gray squirell (I CAN'T SPELL!!!) climb the 
trunk of a tree, I have to admit that it's vertical-technique is basically a 
bipedal climb.  Now, when I watch the squirell on the ground, it is pretty 
ungainly as a quadreped, tending to travel by bounding rather than walking.  
Now, if the trees were to disappear, or if selection pressure encouraged ground 
life, I could see this bounding habit developing the critter into a kangaroo 
like animal.

I do have one problem with the idea of bipedalism evolving completely in the 
trees: for an arboreal animal to become completely bipedal, it will have to let 
go of the branch with it's front limbs.  It's hands would have to let go, if it 
is to stand up.  Even if we can allow for a simultaneous evolution of a bipedal 
posture, in association with foot adaptations to hold the branch better, it 
seems like we are asking too much.  To me, it seems far easier for the animal 
to move to a cursorial existance, and then evolve full bipedalism.  Does the 
fossil record let us see a bipedal animal adapt to ground life?  What happens 
next?

Therefore, I envision a yo-yo scenario for theropod/bird evolution.  
Prototheropods evolve to life in the trees; these animals move to the ground 
and evolve bipedalism; then, they move back to the trees to evolve flight.

I know, I know, the above scenario involves more than one drastic change in 
habitat, which is a detriment to the idea.

Comments?


Shalom,

Rob Meyerson

***
"Listen to the song, not the words."
                -Kosh