Re: Waimanu

[dinoboygraphics@aol.com]

I think Michael's points are well taken; The "survivability" of a clade 
depends on many things, and larger animals are not necessarily less 
"adaptable".  The origination of a favorable mutation depends on many 
things, many of them tied to the intrinsic genomic history of a clade.  
Location on a chromosome, electrochemical stability with neighbords, 
stabilizing selective pressures, etc are just some of the factors that 
effect mutation.

Also, smaller populations are often thought of as more suceptible to 
extinction (and to a certain extent this is true), but genetic 
diversity within a population must be taken into account.  Also, 
reproduction style, since some reproductive strategies may more easily 
allow favorable mutations to spread through a population.

It's impractical to apply most of these to pterosaurs, but unless there 
is a preservational bias (and there may be) there was an apparent trend 
from the Late Jurassic into the Late Cretaceous that seems to show 
pterosaurs getting restricted to larger niches, while birds (presumably 
via competative replacement) radiate into smaller flying niches.  How 
long pterosaurs could have hung on to these larger niches is an 
experiment that we will sadly never get to run.

Scott Hartman
Science Director
Wyoming Dinosaur Center
110 Carter Ranch Rd.
Thermopolis, WY 82443
(800) 455-3466 ext. 230
Cell: (307) 921-8333

www.skeletaldrawing.com

-----Original Message-----
From: john bois <jbois@verizon.net>
To: dinosaur <dinosaur@usc.edu>
Sent: Sun, 28 May 2006 15:16:53 -0400
Subject: Re: Waimanu

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.