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Re: Origin of feathers
My name has appeared on this list a number of times recently, and I'd like to
respond to all the comments just once, so here goes.
I gave a talk at Dinofest entitled "Dinosaur brooding behavior and the
origin of flight feathers" and my collaborator Mark Orsen presented a poster
entitled "Was brooding a selective pressure in the evolution of flight
feathers?" The poster elaborated on points that I was unable to do justice to
in a twenty minute lecture. The essential point of our presentation is that
brooding provides the selective pressure to explain the missing link in
Ostrom's cursorial theory of the evolution of bird flight. Stated another
way: in the absense of flight, why would feathers lengthen on the arms of a
theropod bird-ancestor? Answer: to protect eggs and chicks (our concept
assumes that short contour-type feathers already covered the body and arms of
Dr. Ostrom has encouraged us to elaborate this idea since we contacted him
about it last year. He's gone so far as to say it "provides a whole new
window" on the problem.
It is too bad I can't provide the readers of this list with photos, because
once Mark and I began a search for graphic proof of the key importance of
wing-brooding among modern birds, we were astonished at the compelling
evidence we found in the published literature. My talk included a photo of a
mother duck sheltering a dozen or so chicks, by holding her arm downward (in a
posture even the most primitive of theropods could have attained) and
spreading her primary and secondary feathers to cover a large area around her.
This photo is seen in the book "Wildfowl" by Eric Hosking and Janet Kear.
Another extremely compelling photo shows a mother perigrine falcon sheltering
two chicks from a rainstorm by "taking them under her wings." The babies are
nearly as large as her, and it takes every inch of feather she's got to cover
them (in "Australian birds of prey," by Penny Olsen). Finally, a Gray Gull
shields its chicks from the sun by spreading its wing feathers like an
oriental fan above them (TR Howell, U Cal Publ Zool 104, plate 13). These and
many other examples demonstrate that wing-feather brooding is probably an
ancient trait, basal to all modern birds (by parsimony).
But why assume that flight came first, then brooding? Brooding is so
critical that most chicks never get the chance to fly unless they are properly
brooded by their parents. As others before us have noted, Oviraptor needs
some sort of additional coverage to shelter its eggs (notably behind the
forearm and beside the hind leg - right where the mother duck was holding out
her feathers). We also noted that Oviraptor's arm is in an orientation
related to the recovery phase of the flight stroke, that is, parasagittal with
a slight supination. This, conveniently, would have directed a set of primary
and secondary wing-feathers right over the eggs. Many other postures were
possible for Oviraptor's arm, so we think that this is not a coincidence.
Rather, we propose that the need to manage large sets of arm feathers DROVE
the evolution of the modern bird's folded wing posture (and Oviraptor is about
half-way there). This is also a good way for a cursorial hunter (like the
Roadrunner or Archaeopteryx's ancestors) to manage a bulky set of brooding
feathers. Furthermore, even the SHAPE of wing feathers could have evolved
under selective pressure to improve this folding-shelter form of arm covering.
Why did maniraptors have such a birdlike arm when everyone knows they
didn't fly? - because the semilunate carpal evolved to help manage the
brooding feathers. This explains the seemingly prescient evolution of arm
architecture - as if the theropods somehow KNEW they would someday fly and
therefor evolved more and more bird-like arms. Of course this is impossible!
However, the evolution of arm bones and postures makes sense if the pressure
came from brooding (longer feathers are better, but what do you do with them
when you're hunting?) and not the other way around.
Finally, look at Sereno's fine stereo images of the wrist of Herrerasaurus
(JVP 13, 425 1993). This basal theropod could already cock its wrist to the
side, in the direction that modern birds do when folding up their nice long
wing feathers. Why did Herrerasurus need to bend its WING OOPS ARM in this
direction, all the way back in the Triassic? Hmmmm.
One other thing. I was a bit dismayed at several postings stating that our
ideas were nothing new. To me, scientific thought is in itself an
evolutionary process, building on past history. I believe that our ideas are
original ADDITIONS to this ongoing stream of understanding. Thanks for
reading a long e-mail.
Thomas P. Hopp Ph.D.