Re: Waimanu

[john bois <jbois@verizon.net>]

Michael Habib said in response to my argument that because pterosaurs were
limited to large-size-only niches they were doomed:

>Would you call elephants, artiodactyls, whales, ursids, etc all doomed?
...Because global phenomenon are rare, large-bodied supersoarers should be
robust to >extinction, not "doomed" to an early demise.

Yes...this is a good point.  But let's consider the large pterosaur/large
mammal analogy a little further.  Imagine a world in which the _only_
mammals were elephants, big bears, giraffes, and whales...and the
medium/small mammals were all replaced by some newly-evolved taxon.  Fair
questions to ask might be: why has the new taxon moved into all of the other
niches...and what is the delay in taking over the large-size niche?  Are the
large sized species immune to the forces that eliminated the globally
distributed smaller species?  A good starting hypothesis might be that there
was something in the biology of the entire taxon that predisposed it to
global extinction.  To propose a biological mechanism is not teleological.
Indeed, we apply such knowledge of predisposition-to-extinction today: e.g.,
island species are predisposed to at least local extinction due to no
defenses against continental predators!

> Personally, I think there are many misconceptions floating around about
> speciation and extinction, and their connections to clade traits.  I
> find that these misconceptions spawn most often from a feeling that
> there is some kind of ongoing 'war-like' battle for niche spaces...

I know you recognize that most community structure is the result of
competition/predation past...a war-like battle to be sure!

>...such
> that no clade could reduce in character diversity unless under duress.
> In reality, there are plenty of reasons why a clade might be under
> directed selection for large body size (or other traits), and they do
> not (on their own) imply that the clade is doing 'poorly'.

I absolutely agree with this _except_ in the case where an entire
clade--pterosaurs--were selected in one direction, for one trait: bigness.
This suggests (to me) a selection _against_ other traits.  I mean, to argue
for niche abandonment rather than niche replacement is romantic: species
don't simply give up niche space.  I would have thought that a prime engine
of morphological diversity was the tendency to divide up similar size class
niche among more species.  For sure, the number of actual niches has
_increased_ over evolutionary time.  So, when we see increase in niche
diversity enjoyed by one clade relative to another, we must ask questions.
I'm not saying there is _always_ a biological answer--just usually.  And so,
the questions, at least, are valuable.

JRC asked:

>Why are (soaring-bird niches) particularly specialized or demanding?

Today's marine soarers care for non-flying baby for several months.  This
requires a predator-free island with close-by resources.  I mean, parental
investment is very high in these species--this is "demanding" by definition!

>Falconiformes are mostly inland (a
>planform related difference).  Consequently, on the whole, the
falconiformes
>would have trouble accessing the soarers.

But there are several coastal species...and who knows what was around then.
I realize this is guesswork...but, with the likely pre K/T split, the
question of predatory birds influencing global community structure is now
askable.

>I was of the opinion that warm-bloodedness in pterosaurs was pretty well
>demonstrated.  Am I missing something?

Sorry about that. Are feathers relevant here?  Do you think the pterosaur
range extended to cold waters?

>Since the largest pterosaurs would be moving at more than
>steady-state stall speed when their feet leave the ground, they would also
>be moving faster by that point than any predator of the time would likely
be
>able to run.  For the biggest pterosaurs, time to launch would be on the
>order of a half second, too short a time for a surprise predator to move
>very far.  I'd say that in their normal habitat, the big pterosaurs would
be
>pretty much uncatchable unless ill, injured, or captured at sea by a marine
>predator.

I find this hard to believe.  Are you talking from a standing start?  Half a
second?  For an animal with a ten meter wing span.  Something like this is
shown on Walking with Dinosaurs when Q quickly takes off.  My students are
quick to cry "fake"at this representation of a quick take-off.  Looking to
be informed.

Finally, on the issue of whether or not large soaring pterosaurs were likely
to be replaced by birds with or without a global catastrophe: I am of the
impression that (absent man, at least) albatrosses are limited more by
predation/competition for predator-free nest sites close to food than by
food availabitlity _per se_.  As such, the survival of a particular soarer
is likely dependent upon either its ability to drive a competitor off an
island or its ability to nest at more inaccessible sites.  I would be
interested on your view about pterosaur latitudinal distribution and fish
abundance in tropical vs. arctic regions.