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Re: Dinosaurs to birds




On Sun, 24 Jan 1999, Stanley Friesen wrote:

> At 01:16 AM 1/24/99 -0500, John Bois wrote:
> >OK.  But this is absolutely hypothetical, right?  Whereas I can use the
> >fossil record to show trends in extinction and possible adaptations that
> >influenced those trends, can you show any evidence to support your
> >hypothesis of luck?
> 
> I never actually said it was my hypothesis, I was only pointing out that it
> is a viable hypothesis.  (Actually, it is Gould's idea, not mine anyhow).

Quite right.  And I agree it is a viable hypothesis.  I am arguing against
the idea that it is a _better_ hypothesis than, for example, predation,
inter-specific interactions, replacement of ancestor species by bouts of
sympatric speciation and/or immigrations.

> But at least *some* component of luck seems inescapable in mass
> extinctions, since there *is* evidence that selective processes act
> differently during such times.

I think the concept of mass extinction is very fuzzy.  For example, how
far apart may any given species' extinction be to be considered in the
same event.  This is _undefined_.  If you can't define something it
might not exist at all.  And yet we go in as if we all know what
it means.  Could you cite evidence of an actual selective process acting
at a time of mass extinction?

> >OK to Mt. St. Helens.  And OK to the idea.  But there is no evidence of
> >any kind connecting any event to any extinctions at the K/T!
 
> Hmm, so you deny the evidence for a bolide impact being involved?
> I chose Mt. St. Helen's as a small scale model of the sort of things that
> might happen after a bolide impact.

I don't deny there was a big hit at the time.  I do deny that we can
reliably tag extinctions to it.  Archibald argues quite well that habitat
fragmentation (a tried and true killer) explains patterns better than any
event.  And, after all, we need a _process_ to explain things, not an
event!

> Why did
> one species of animal (say Alphadon marshi) survive and *close* relative
> (say Alphadon wilsoni) die out?  (Note, both of these species are from the
> Lancian fauna of North America, so not only were they close relatives, they
> were sympatric - and this is not an isolated example).

I think you illustrate my point well in this example.  Several workers
have attributed the elevated extinctions of marsupials tothe immigrations
of eutherians we know occurred at the time.  Here is a mechanism which we
have witnessed time and again.  I don't see how it can be viewed as a
_strange_ phenomenon more likely attributed to luck associated with a big
bang sweepstake event.  Not that this didn't happen, only that we have
more parsiminious explanations at hand which we should perhaps deal with
before we get to the fantastic.
 
> Thus it is certainly true that larger sized species were more prone to
> extinction, and species capable of extended aestivation seem to have been
> particularly resistant to extinction.

These explanations are like the Dutch Boy with his finger in the dyke. 
Non-avian dinosaurs were also probably capable of aestivation.
The emu, for example, does not eat or drink for months on the nest (is
that torpor--not sure about the terminoloy here).  And many dinos
apparently nested in arid regions making such a strategy likely.  But, at
the very least it cannot be ruled out _a priori_. And there is no evidence
that proto-ungulates might have hibernated, nor proto primates, nor
indeed, _any_ cretaceous creatures.

The size issue is difficult since no one has been able to model a certain
set of circumstances that would generate such a cut-off point. 

> But within each of these broad
> categories, the fate of each individual species seems to be largely random
> with respect to all known factors.

Which is why I thought neornithines vs. enantiornithines was a good
challenge to this idea.  the extinctions were _not_ random.  They were
absolutely _selective_.