Re: Bipedalism

["John V Jackson" <jjackson@interalpha.co.uk>]

-----Original Message-----From: Rob Meyerson Date: 06 April 1998 23:35


>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 don't think I've ever really noticed what gaits squirrels use.  (I've had
black squirrels run up and down my jeans in the Crown Prince Palace Gardens
in Vienna, don't you know!  I should have looked at the footprints they
left!)

Why did our hypothetical protodinos stride rather than hop?  Humans are the
only mammalian fully bipedal striders - but then our arboreal ancestors were
too big to leap about in the trees.  I suppose it's only the smaller types,
the leapers, who tend towards bounding/hopping.  (Kangaroos' ancestors would
need to be small to fit this idea perfectly.)   Of course, the protodinos
were probably carnivorous, and the thylacine, though with strong hopping
tendencies, reverted to proper quadrupedalism.  I don't think there have
been any carnivorous hoppers known (apart from that odd american mouse?),
and anyway, lizard-like things don't go a bundle on hopping, the odd croc
excepted.

>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.  <

  Good point.  As you say, in trees, so-called bipedal types actually use
their hands (if they've still got them); it's only on the ground that
they're bipedal.

>  Does the fossil record let us see a bipedal animal adapt to ground life?
>What happens next?<

Perching birds?  Ex-bipedal ornithiscians?  Maniraptorans? Modern humans?
Australopithecus africanus (who is thought by some to have gone back to the
trees somewhat)?  Kangaroos?  Tree kangaroos?  I suppose the best answer is
"Just about anything".

>. 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. <


  No problem to my mind - over 250 mill. yrs we'd surely expect at least as
much!

JJ