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Re: ...bats by day, and bats, birds, by night.




John Conway wrote:

> > Outside of catastrophies (which you're not invoking) competition and
> > predation are the prime causes (most likely) of most extinctions.
>
> What about changes in the environment,

Yes.  For soaring animals, there are several distinct techniques and methods
for extracting energy from different types of atmospheric dynamics.  Access to
these specific energy sources is dependent upon planform, aspect ratio, and
wingloading among other things.  Should atmospheric dynamics change faster
than a species is capable of morphing to accomodate, that species will
disappear.

> There are
> many, many things that can cause extinction at the species level - few
> that can cause the extinction of very large clades.

Well said.

> I don't think the facts you present are really facts, but a series of
> hypothesis, which may or may not be correct. And as each one depends on
> the one before it, they hardly add up to an unshakeable case.

ditto.

> Why did this process take so long? If birds were better competitors, why
> did they take tens of millions of years to out-compete pterosaurs? One
> would expect such direct competition to remove pterosaurs much faster,
> probably just a few million years. Maybe less.

Soaring ability is enhanced by a large available dynamic range in lift
coefficients. I find it interesting that the highest CLmax known among birds
is roughly 1.63 for albatrosses and Frigate birds, while the CLmax for some
pterosaurs appears to be on the loose order of 2.1-2.2, about 30% greater than
birds can achieve.  I wonder if this difference might have affected the
evolution of soaring birds?  Note that both pterosaurs and birds spend most of
their time at much lower lift coefficients.  The need to reach the limit is
reserved for special occasions.

> Most pterosaurs went extinct well before the K/T, but many didn't. Birds
> do not seem to have been able to compete directly with Azhdarchids, and
> their extinction coincides with the bolide.

Some birds may have been able to compete with Azhdarchids, but not at soaring,
not at that time. Only a few birds today can soar as well as they did then,
and these few modern birds may not have been capable of soaring with them with
late K atmospheric dynamics operating.  On the other hand, apparently the late
K azhdarchids would still be capable of high performance flight today.