Re: The Continuing Story of Gliders to Dinosaurs

[Stanley Friesen <sarima@ix.netcom.com>]

At 02:29 PM 9/25/99 -0700, Matthew Bonnan wrote:
>Okay.  Some more thoughts/questions on this trees down hypothesis.  The 
>argument has been forwarded by George and others on this list that 
>archosaurian bipedality is perhaps tied with an adaptation to arboreal 
>habits in various ancestral forms.

Or scansorial.
>
>One of the first things I noticed when thinking of arboreal mammals and 
>reptiles is that most of them are either obligate quadrupeds or facultative 
>quadrupeds.

Most are quadrupedal *in* *trees*.  Many (but not all) are bipedal on the
ground.  In primates the key determining factor is the extent of forelimb
dominance in arboreal locomotions.  The brachiating forms, the indri (a
lemur), the spider monkeys, and the apes (lesser and great), are more or
less bipedal (or that oddity, knuckle-walkers) on the ground.  The gibbons
are particularly interesting since, while in the trees, they combine
brachiating, running bipedally on top of branches, and quadrupedal *climbing*.

I suspect that a "vertical leaper" like the indri may be closer to the
ancestral form for at least the theropod dinosaurs.  When in trees this
form spends most of its time on more or less vertical branches, and it
often moves by leaping great distances between such branches.  This leaves
it with a substantial uncoupling between its fore- and hind-limbs, with the
hind being specialized for support and leaping, and the fore specialized
for clinging and climbing.  Transfer this to the ground and you get a
bipedal animal with longish forelimbs and grasping hands!

>of the two limb sets into very separate locomotor groups.  Does arboreality 
>really encourage evolution toward bipedality, then, or are there other 
>factors to be considered?  I think the answer to this question is yes.

The other factor is the *sort* of arboreal activity engaged in.  Squirrels
just run around on branches like they do on the ground.
>
>Further, George brought up kangaroos and how some live in trees, perhaps 
>tying bipedality in this group to originally arboreal 'roos.

Actually, that was *me*, not George.  And I was just speculating, lacking
detailed knowledge of the phylogeny of 'roos.

>  I know next to 
>nothing about kangaroo evolution, but it does not stand to reason that just 
>because some smaller 'roos are now exploiting an arboreal habitat that this 
>was the ancestral condition.  What do we know of the fossils?

More importantly, how does facultative arboreality fall out in a cladistic
analysis of 'roos.  Given the prevalence of arboreality in the nearest
outgroups, this seems like a good possibility, but it needs specialized
knowledge neither of us have to really find the answer.

>  What of large 
>cats which also climb trees -- would we say they were on a path to 
>bipedality as well?

Maybe, if they shift to emphasize vertically oriented climbing/leaping or
use of the front limbs to swing around in the trees.

>cylindrical, not spherical like the head in most mammals.  A cylindrical 
>head is great for anterior and posterior swings of the femur, but it greatly 
>limits the ability of the femur to move laterally and medially.  Look at the 
>femoral heads of Herrerasaurus or Coelophysis or any of the early theropods 
>-- all have this stiff-action femur.
>
>How does an animal with a cylindrical head adapt itself to scaling trees, 
>which would require more of a spherical femoral head? 

I am not entirely convinced of this.  I would like to see a more careful
analysis before I accept this analysis.  [Besides, I consider _Coelophysis_
a rather highly derived ceratosaur, and not a good model for basal
theropods or basal dinosaurs].  Even _Herrerasaurus_ may be to specialized,
due to its large size.  What is the situation with _Marasuchus_ and it kin?
 These may be closer to the ancestral state in dinosaurs.  Certainly it has
been suggested that _Marasuchus_ was a leaper, which may mean an indri-like
vertical leaper.

>The entire theropod skeleton from the hips down does not strike one as an 
>animal body plan necessarily exapted for an arboreal life style.

Scansorial might be sufficient.

--------------
May the peace of God be with you.         sarima@ix.netcom.com