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



----- Original Message -----
From: "John Bois" <jbois@umd5.umd.edu>
Sent: Monday, July 15, 2002 4:00 AM

> >         So at present it looks like the small pterosaurs outside
> > Azhdarchidae died out something like 50 Ma before the big ones. Can
hardly
> > have had the same cause IMHO.
>
> Outside of catastrophies (which you're not invoking)

Not sure I'm not. Maybe it went more stepwise than gradual, the fossil
record is probably not good enough at the moment to tell. There was the J-K
mass extinction (relatively mild, but apparently with noticeable effects on
pterosaurs; 3 impact craters around that time), the Albian-Aptian (a little
one, with an Oceanic Anoxic Event), the Cenomanian-Turonian (a slightly
bigger one, with an impressive OAE and hints at an impact)

> To say these forces [competition & predation]
> caused both early and late K extinctions is not a problem.

Of course it is. Because not just single species but whole clades died out
completely.

> You
> seem to believe...but I don't know what you believe.  What do you think
> could have driven these species into extinction?

I don't know :-)

Wait a minute. There is that scrap of jaw that the museum here is so
excessively proud of... it's thought to come from a rather small pterosaur
and is called *Ornithocheirus bunzeli*. The Gosau Fm is Maastrichtian, isn't
it? There are 2 K-T sections in Austria (complete with Ir anomaly), and IIRC
both involve the Gosau Fm...

> I mean, it is not as if the birds eat all the insects, right?

I don't know.

> I think what we are seeing is a kind of niche partitioning:
> birds are better competitors at some times, and bats at others.

This _is_ competitive exclusion: each has its niche and keeps the other out
of evolving into that niche.

> > Predation? There is a bat species
> > (is it the false vampire?) that more often than not eats sleeping birds.
>
> Relevance?  The bat does this at night, right?

Yes. I'm just not so sure that this bat is afraid of flying during the day.
:-)

> > Not
> > to mention the owls at night. And I repeat that bats are more
> > maneuverable,
> > so they should escape birds of prey _easier_ than birds.
>
> What kind of maneuvers?  Can they take off vertically?

Of course... both upwards and downwards. :-)

> What are the relevant maneuvers?

?

> > Another explanation, from Pat Shipman: Taking Wing: Bats get sunburn
much
> > too easily, and bats with heavy sunburn on their wings (a specific
example
> > is mentioned) can't fly. Don't know if that holds for all bats (after
all,
> > bat wings are frequently dark brown to black).
>
> Relevance?  Melanin.

Classic example for the theory of exaptation... melanin must be present
before the bat can fly into sunlight.

> Rise early before sunset, after sunburn time.

That's what bats famously do. They don't go further.

> This is reaching.

I don't understand that.

> > But... creatures that grow fast and extend in the periphery before they
> > start to fly are totally freed from this problem. No remotely
ball-shaped
> > pterosaur is known, despite lots of juvenile specimens.
>
> Are you really saying allometry is irrelevant to airworthiness?

I am really saying that not all amniotes show the same allometry. Again: No
remotely ball-shaped pterosaur specimen is known, even the smallest
"*Pterodactylus*" specimens known look as aerodynamic as those 3 times their
size.

> If not, are
> you really saying that juvenile frames are just as airworthy as adult
> frames.

I am -- for those pterosaurs with more or less known ontogeny. Not for
everything in general.

> > I can cite literature (later if needed) about Eocene rheas of apparently
> > modern size, whatever else "modern aspect" means.
>
> I would like that ref.  And thank you.

Sorry, can't find it right now. It's about an Eocene site in Antarctica;
preserves the last known gondwanathere, several SA marsupials and SA
ungulates except Notoungulata, a rhea and a phorusracoid. I'll look for it.

> I would be very surprised if lurking cat predators were not
> among the reasons why rhea avoid treed areas. But the
> puma was the cat I was looking for.

I see.

> > > for late K pterosaurs?  Remote nesting was probably the only available
> > > strategy for them. So...
> >
> > So what? Maybe that was just what they did. There always were islands,
if
> > you want such extreme protection.
>
> But predator access to those islands has changed over time--with the
> evolution of new/different predators.

There have always been rather inaccessible islands, I think, and there are
strategies like colony nesting.

> > Assuming I've understood what you wrote so far:
> > [...] Because we know that *B.* didn't do it, while in the
> > current form of your speculation every bird of prey should be
> > able to do it, you must do what I wrote above: show why *B.*
> > couldn't have done it while falconiforms could have done it,
> > and that K falconiforms existed (fossils).
>
> We have no idea what *B* could or couldn't do.

We know that *Boluochia* did not kill all pterosaurs because there were
still pterosaurs 60 Ma after *B.* appeared.

> Ditto for falconiforms.  How can I possibly do that?

See above: make a good case that K falconiforms existed, and find a crucial
difference between them and *B.* that can show why they, unlike *B.*, could
have killed off the pterosaurs.

> You know this is pure speculation!

Then why do you present it like a hypothesis? Then why do you build heaps
and heaps of more speculation on this fundament? ~:-|

> However, there are some facts to work with.  And they favor
> such speculation:
> 1) Most pterosaurs became extinct from causes other than the bolide.

Probable (not totally certain if we separate extinction from
pseudoextinction).

> 2) No pterosaurs handed over their niche as a gift to rising new species.

But maybe they died out for other reasons, their niches became empty, and
birds filled them faster than pterosaurs could. Good idea, and even easy to
test -- with a fossil record much, much better than what's known at the
moment :-)

> 3) Something forced them out of their niche.

Not necessary.

> From these facts and inferences,

One probable fact and a series of 4 weak inferences. To continue the series
with another inference... you get the point. The impact sounds better to me
:-)

> It might even be that
> enantiornithines outcompeted pterosaurs early, and neornithines, late.

The only possibly fish-eating enanti described so far is *Halimornis*. The
only Mesozoic birds for which a swift-like lifestyle (like anurognathids)
has been proposed are the Confuciusornithidae (in the Ostrom Symposium
volume), and while their wings suggest this, their beaks don't.