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Re: NO SECONDARILY FLIGHTLESS THEROPODS
><< I do not see why a more " primitive " species cannot evolve
> flightlessness in the same way that modern species do. >>
<<They may, or they may not; they're certainly not >obliged< to. By the
way, I find your posts on paedomorphosis in the evolution of
flightlessness quite interesting. Please keep them coming; you have a
good command of the material.>>
Thanks for your interest and the compliment.
But, there is a possibilty that they _are_ obliged to. Now among
vertebrates that fly, as far as is known, only birds have become
flightless. What makes them special in this regard? The answer is rather
obvious : birds are an extremely adaptable group of vertebrates that are
able of equal locomotion on ground, sea, or air. But, even some birds
that live on ground or in the sea do not show progressive trends toward
flightlessness. The factor which turns a bird flightless is
paedomorphosis which makes the forelimbs vestigilize, feathers
degenerate, and the bird to stay a chick the rest of its life.
Paedomorphosis is simply the factor which makes a bird flightless and
which may be the only factor which makes a bird flightless. Roadrunners
are cuckoos ( Cuculiformes ), they were once arboreal as shown by their
zygodactyl foot, now they are teresstrial cursors. They fly little
compared to other birds but they are not flightless, they show hardly
any trends toward flightlessness. Chickens are another good example,
though slightly different. Chickens do not fly that much, hence their
white breast muscles, however they are showing a few trends toward
flightlessness that can be linked to paedomorphosis. The shortening of
the forelimbs, increased massiveness and robustness of the vertebrae,
and unfused tarsometatarsus ( in the Dorking fowl ), show that
paedomorphosis is taking place. Chickens are a good example of how
paedomorphosis and isolation can effect a bird to where it can become
Now, it seems that paedomorphosis is the only way that birds become
flightless. Why they become flightless by this way is somewhat easy.
It's the easiest way to become flightless and possibly the only way.
What other factors do we have? Not using the forelimbs is one way. The
fault of this hypothesis is:
1) There are no other selective factors that can explain the extreme
vestigilization of the forelimbs.
Other than that factor there are few other factors which can adequately
explain how a volant animal will go flightless.
Before somebody brings up the "penguins didn't go flightless through
paedomorphosis" issue, let me adress this issue. As outlined by
Feduccia, the evolution of penguins can be explored through a
pseudo-phylogeny. The first stage would be be a petrel-like creature
that would fly around water. Soon the animal would start to dive into
water and use its wings for underwater and submarine flight as shown by
diving petrels. Soon the animal would use its wings only for submarine
flight. Essentially the animal would be an underwater flier, not an
aerial flier, but essentially the same thing. ( The actual idea of the
pseudo-phylogeny was made by Storer but I haven't read that paper yet).
Hope this will settle this issue ( unless any other questions or
rebuttals are in line ),
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