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Re: Dinosaurs to birds
At 02:45 PM 1/24/99 -0500, John Bois wrote:
>> 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_.
A mass extinction is best defined in terms of extinction *rates. A period
of time in which extinction rates are about 1.8sigma above average would be
considered a mass extinction. (I believe that 1.8sigma is the right
This does mean the boundaries may be a bit fuzzy, but the same is true for
the boundaries of a species, or for almost all other biological processes.
> 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?
Well, I have mentioned the selection against larger size that seems
restricted to mass extinctions. Indeed, at other times the selection seems
to go the other way, and *favors* larger sizes.
Archibald's book *suggests* several others, that are either not as
universal or not as well supported.
>> 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
I too maintain it was not the primary cause. But I do suspect it was a
major contributor, especially as parts of the marine extinctions do seem to
be fairly closely associated with it.
>> 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 to the 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.
The point is that *whatever* the cause, one species of Alphadon survived,
and another died. Why? It seems likely that it was essentially luck. So,
maybe the particular species of immigrating eutherians were more able to
compete with A. wilsoni than with a. marshi. But that in itself would be
essentially just a contingent piece of luck.
>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.
Nonetheless, it is there. *All* terrestrial species above a certain size
died, not only in the K-T extinctions, but also in the P-Tr extinctions.
And a similar pattern is developing in the current Pliestocene extinctions
(even if they are mostly anthropogenic).
>> 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
Those are broad clades, not individual species. All the ao\bove fact means
is that for some reason the two clades were differentially susceptible to
mass extinction processes.
Also, we really lack sufficient data on *either* group to characterize the
extinction levels at the end of the Cretaceous.
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