The Continuing Story of Gliders to Dinosaurs

["Matthew Bonnan" <mbonnan@hotmail.com>]

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.

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.  While squirrels, for instance, will sit up on their haunches 
from time to time, would anyone argue that they are habitually bipedal 
during locomotion?  The limbs of most of these gliders are not completely 
independent of one another, whereas the muscular electromyography studies 
done on birds during walking and flying clearly show a distinct separation 
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.

Further, George brought up kangaroos and how some live in trees, perhaps 
tying bipedality in this group to originally arboreal '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?  What of large 
cats which also climb trees -- would we say they were on a path to 
bipedality as well?

Let's forget systematic and cladistic arguments for a moment and look just 
at functional morphology.  The femoral head of most dinosaurs is 
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?  Birds do have femora 
which splay sideways, but this appears to be an after-effect of the 
horizontal position of the femur in these animals -- a splayed femur doesn't 
interfere with the body during locomotion, where a horizontal and straight 
femur would.  Perhaps this is an adaptation to arboreality in birds from a 
ground up dinosaur, since the femoral head of birds is still cylindrical -- 
ever tried moving the thigh of a turkey or chicken out from the body?  Takes 
quite a bit of effort! =)  Instead, as birds began to take to the trees from 
a ground-up dino, they modified their limbs in a different place than 
mammals.

The entire theropod skeleton from the hips down does not strike one as an 
animal body plan necessarily exapted for an arboreal life style.  The 
cylindrical femoral heads and hinge-like ankles would make climbing around 
in trees pretty difficult.  Look closely at primate, squirrel, and reptile 
limbs and ankles -- they are very different than theropods and allow a great 
range of hand and limb mobility not found in dinosaurs.

Many birds are undeniably arboreal, but the modifications they have made to 
accomodate this appear to me to be changes from a ground-dwelling archosaur 
(likely a dinosaur) -- they keep their cylindrical femoral heads and 
hinge-like ankles, and instead modify their knees and femoral angles.  These 
exaptations in fact may be due to tail loss and have only by accident helped 
birds become better at being arboreal.  Loss of the big caudofemoralis 
longus muscle that pulls the thigh back in most dinosaurs would have forced 
bird ancestors to use knee flexion and extension which would help reorient 
the center of gravity over the feet.  As a result of this, suddently 
arboreal niches open up to once terrestrial animals.

None of this is to say that George's tree-down hypothesis is not valid.  In 
fact, his point about bipedality as an exaptation to arboreality is quite 
interesting.  However, we should all be open to various scenarios of bird 
and dino evolution and look more closely, not just at systematics, but at 
how the animals actually go together.

Matt Bonnan

______________________________________________________
Get Your Private, Free Email at http://www.hotmail.com