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Birds of a Feather....



A question:  At what point in the phylogeny of birds did the Foramen
Triosseum (F.T.) evolve, along with the associated changes in musculature
and, most likely, the evolution of the avian flight stroke?  (For those who
are unfamiliar with avian skeletal morphology, the Foramen Triosseum is
basically a hole formed where the coracoid, clavicle (or furcula), and
scapula meet.  Through this hole passes the tendon for one of the major
flight muscles, the supracoracoideus, which controls the upswing of the wing)

Aside from just being a darn intersting thing to know, the answer to this
question would seem to have implications in both the evolution of feathers
and the evolution of birds themselves.

Regarding the evolution of feathers:  If the F.T. evolved after feathers,
then it would follow that feathers mos tlikely evolved for some purpose other
than flight, and were later co-opted into that role.  If the F.T. evolved
before feathers, or roughly at the same time (though I know this is difficult
to determine), then it is possible (probable, even?) that feathers evolved
for flight, and were later modifed for their other uses, such as insulation,
among others.

Regarding the evolution of birds:  Now, I have another question here: what
diagnoses a bird?  Is it the presence of feathers?  Is it the possession of
the Foramen Triosseum and associated musculature?  Or something else
entirely?  I'm no ornithologist, so please forgive the following suppositions
if they are way off :o)
Let's suppose, first, that true birds are those critters with the F.T. suite
of adaptations. 
Now, if birds descended from dinos, then we wouldn't really expect any of the
non-avian dinosaurs (shudder...) to have this feature (otherwise, they'd be
birds :o) ), but we might see some of the closely related dinos with the
beginnings of such a change (critters like the dromaeosaurs).  Do we see such
a thing?

Now, suppose that dinos are true bird descendants.  Not likely, in my
opinion, because dinos would then all share the F.T. suite and I know we
don't see this in the fossils.  It is possible, I suppose, for the earliest
(albeit avian) dino ancestor to go through some reverse-evolution, and change
the shoulder girdle back to the pre-bird version.  Possible, but I don't
think it too likely.

And then we have a third option: that dinos and birds, rather than sharing an
ancestor-descendant relationship, are sister taxa, both descended from a
common ancestor.  The development of the F.T., then would likely have occured
somewhere on the branch leading to the true birds, while the dinos kept the
typical "reptilian" shoulder girdle.  

So, we have three possible scenarios (there may, of course, be others, but
it's almost 1:00am, and my brain is starting to shut down) for the evolution
of birds.  Scenario number 2 I see as the most improbable, if only for the
importance I place on the shoulder girdle as a prime character.
Scenarios 1 and 3, I would think, would produce similar results with respect
to dinosaur shoulders.  Honestly, right now, I can't tell how we might be
able to differentiate them.  As I said, my brain is shutting down about now.
 

Anyway, all of this is in the realm of supposition.  What do we really see
regarding avian shoulder girdle evolution?

Dsmith.