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flight and feathers
The question of why a number of birds have become secondarily flightless,
while other flying tetrapods have not, is an interesting one, and the
answers that have been suggested are excellent food for thought.
I would like to point out that although flightlessness has evolved
independently a number of times in birds, remarkably few modern birds are
flightless. There may well have been secondarily flightless pterosaurs,
flightless forms presumably are less likely to get into
fossilization-friendly habitats, but there may have been fewer species and
thus fewer opportunities.
When flight was first evolving in the avian line, there may well have been
frequent reversals. When flight was poorly refined, and forelimbs were not
highly modified, secondary flightlessness may have occurred many times.
Notice that modern birds that have become secondarily flightless cannot use
their forelimbs for either scansorial locomotion or prey acquisition; they
are too highly modified. But this was not true of early dino-birds. I am
by no means the first person to note these things.
The evidence suggests that flight in the dino-bird line did evolve in some
sense more slowly than than that in pterosaurs or bats (although this is
somewhat subjective). This may be because the flight surfaces of bats and
pterosaurs can seemingly serve no other function than those aerodynamic.
Feathers are much more versatile and evolutionarily plastic. Variations in
the form of these structures produce dramatic changes in aerodynamic
characteristics. Long before dino-birds refined their skeletons for the
high-performance flight that we have come to take for granted in modern
birds, they were conducting aerodynamic evolutionary experiments, and I
predict that the mosaic evolution suggested by Greg will continue to be
borne out by the discovery of more dino-birds. The upshot of all of this
is that there was more time, and therefore more opportunity, for secondary
flightlessness to occur in early dino-birds than in bats and pterosaurs.
I for one am perplexed as to why there should be a controversy over this
business of whether gliding must precede flying. Here we have an animal,
Archaeopteryx, that is clearly either an ancestor of living birds or close
relative of same. Could it glide? Of course. Could it fly? Yes. Could
its ancestors glide before they could fly? Presumably. Why should we
doubt it? Gliding requires very little. Paradise snakes do it with
neither feathers nor limbs. My point is that we don't have to speculate in
a vacuum about it. I must confess to a similar impatience with this
"ground-up," "trees-up," "trees-down" controversy. Is Archaeopteryx poorly
adapted for climbing? No. Yet presumably we are supposed to believe that
if it wanted to get from point A from point B by air, it would simply
refuse to climb a tree, or a bush, or a fern, until its flight performance
would enable it to launch directly off the ground. I ain't gonna climb, no
sir, not me.
On a somewhat unrelated issue, I have noted a few posts that focus on the
tail feathers of Caudipteryx, which is fine, and but the rather ratite-like
body feathers are also quite interesting and informative. The feathers of
this animal strongly suggest the two purposes of insulation and display
that have long been theorized to be important in the evolution of feathers.
The structure of the body feathers, similar to that previously seen in
Sinosauropteryx, could explain a great deal. Such feathers presumably do
not carry far on the breeze the way flight and contour feathers do. This,
combined with the fact that individually they could be rather easily missed
or misidentified as fossils, could explain their absence generally in the
fossil record. Question: How well are ratite feathers represented in the