[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: Flightless Birds (was Re: BCF AND PDW)
Jonathan Wagner wrote:
<It also seems to be a commonly made generalization that flightless
birds do not often last in competition with established terrestrial
fauna ("the age-old battle between flightless bird and mammalian
carnivore" [Romer?] comes to mind). Again, this may not always be so.
However, if I were to make a third generalization out of the first
two, I'd say that, in order to become flightless, a taxon would have
to have both the motive (open niches) and the opportunity (an
environment where a bird could start doing the terrestrial thing again
without worrying constantly about being eaten by its ground-bound
Best example, of course, are phorusrhacoids. While little
psilopterids were large, they could fly, and resembled nothing more
than small kori bustards or large seriemas (Cariamidae). Well, of
course. Like the secretary bird, they could defend themselves if
threatened, and fly when neccesary, but were limited by their size to
go after larger, more abundant prey (assumably) because then the
brontornithids and phorusrhacids (the BIG phorusrhacoid killer-birds)
come along and they've lost flight. But suddenly, they can stand up to
those marsupials and mammals that gave them such a hard time when they
were smaller, psilopterid-like.
Dinosaurs fight back. The mammals took over at the end of the
Cretaceous . . . now its personal. . . .
This meant those puny little furballs are outdone by those
not-so-puny-anymore feather-dusters (with arms, REAL arms, and claws,
and that two-foot skull like a carnivorous toucan).
Thus un-island production of flightlessness was arrived at, and
kept. This was a true case of secondarily flightlessness, but was it a
reversal, or a convergence?
Semantics, really. To get to the point, the optimum survival of the
group was achieved by losing flight. Period. Physiologically, it was a
paedomorphic change (oh, no, the "p" word. . . !) But evolutionarily?
It was a shift towards the optimum body form and fuction. If manual
claws helped in whatever was needed to perform a specific task, then
so be it. Massive legs are convergent, of course, with size. Beak
towards feeding. Reduction of sternum, carina (keel), and furcula all
relative to the new form: all that muscle and bone could best be used
elsewhere, again, to achieve optimum survival. It's that simple.
In regards to George, BCF seems to me a very good way of detailing
exactly what J. Wagner described: an animal, arboreal, develops this
really efficient structure in climbing: flexing wrists, loosely
appressed metacarpals with digits that spread when extended, and a few
other stuff. "Yahoo! [no pun intended] Look ma, I can climb better
than you!" That form has longer arms as a consequence. The skull
refers to diet, mostly, so that is unneccesary to explain.
But climbing, which most extant birds do today without use of the
arms, is suddenly an opening into a better form of arm as opposed to
those puny Triassic dinosauriforms running around nipping each other's
tails. To them, legs and jaws are better. Who cares?
Suddenly two lineages of dinosauriform emerge, lagosuchids, and
dinosaurs. Surprisingly, the short hindlimbs and relatively long
forelimbs of *Eoraptor* plus an assumed manual state as seen in
*Herrerasaurus* could be used for climbing. Two more lineages. The
more avian *Eoraptor* has a better climbing capacity than
*Marasuchus*, becuase the latter's long tail and really long hindlegs
are detrimental to climbing. Wa-la.
Let's not run into the Triassic birds situation for now. I have a
few things to say about *Protoavis*, but suffice it to say, I think it
may be at least an archosaur.
*Eoraptor* gives way to coelophysoids, though they were arguably
contiguous and diverged from a common ancestor; however they were
related, *Coelophysis* has even more avian-style characters than
*Eoraptor*. Pleurocoels, skeleton very well hollowed, very long
sigmoid neck, thin-boned skull, etc.
For a while, there don't seem to be any contenders for birdiness in
dinos. Then along comes the Tetanurae: even more hollowed-outness and
pneumatics, plus a tendency to be elegant and slender (though
*Coelophysis* is very graceful, IMO). Of course, there's your rogues,
and these do not apoint themselves birdy---funny, they're the
carnosaurs, and the spinosaurs. The contender comes when
*Ornitholestes* and *Compsognathus* come on the scene, forgetting for
one moment *Proceratosaurus*. These were even considered so avian, it
was shocking to see they were not. They were even feathered in early
1900's restorations, wot?
It went on: Coelurosauria. While tyrannosaurs totally forgot they
were more birdy than *Coelophysis*, with their huge-boned heads and
massive vertebral architecture, their lineage provided more room to
develop. Opsithopuby in the clades comprising Bullatosauria, Paraves
(of course) and Oviraptorosauria, we have the intense feeling of
avian-ness, so much so that it cannot be doubted except by a few
hold-outs, that birds and dinosaur have a very close relationship.
Not to criticize Feduccia and Martin and Ruben, of course, for I
will die before then. But we possess a unique set of ever-increasing
birdiness. It cannot be known for sure whether flight was possible as
early as the Triassic in the dinosaur lineage, but in a way,
*Eoraptor* was the first bird-like dinosaur, and succesive generations
of taxa produced a very efficient system, sometimes by exaptation,
others by paedomorphosis, and still others by what would seem sheer
ingenuity [all speculation of course, Uuvuu's still having his way
with my brain, what's left of it anyway]. So, in a way, all dinosaurs
are productive variations on an initial strategy that led to the power
of flight in birds. Features became flight-able, in succeeding taxa,
and we found birds on the way. My opinion is that climbing may indeed
have been an integral component to the evolution we see today in
hummingbirds, just not that flight popped up right then, so in all
plausibility, dinosaurs (and so by consequence, birds) are "trees-down".
I do not disagree with George, Feduccia, Troutman, or you, Wagner;
nor all the others who either oppose or support BCF, BADD, BAMM, or
BAND. My point here is that each theory is strong in one suit, and
this recent proposal has merit into the possibility above. None of us
is wrong. None of us are sure (or can be sure) we are right, just that
we can get damn close!
"I'm sure I forgot something. . . ."
DO YOU YAHOO!?
Get your free @yahoo.com address at http://mail.yahoo.com