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Re: K-T selection event



On Mon, 26 May 1997 22:34:28 -0700 Jonathon Woolf <jwoolf@erinet.com>
writes:
>jamolnar@juno.com wrote:
>
><snip>
>> 
>> You're not the only one who feels this way!  The problem is, the
>> extinction was very selective, and yet there doesn't seem to be an 
>array
>> of consistent traits in the survivors.  Plus, people are still 
>tallying
>> the survivors.
>
>Which is a big reason why I think there's still a missing element.  
>For
>a hundred years people have been trying to find a single coherent
>explanation for the K-T extinction, land, sea, and air, and no one has
>ever managed it.  To me, that indicates a strong possibility that 
>we're
>asking the wrong question -- that we can't find one cause for _all_ 
>the
>extinctions because they didn't all have the same cause.  Maybe
>dinosaurs got the chop because of some massive upheaval in land
>ecosystems, while marine groups got taken out for different reasons. 
>Ammonites pose the least mystery of all -- they'd gone through mass
>extinctions several times in the Mesozoic, and in at least one of 
>those
>cases they were down to just two or three genera.
>
I agree with the multiple cause scenario, since many of the extinctions
seem to happen in a stepwise fashion as you get to and cross over the
K-T.

>> One idea for the land critters might be that there were climactic
>> extremes that would take out large animals of intermediate 
>metabolism.
>> Aridity was increasing, there was intense global volcanism, global 
>sea
>> levels were falling, and the inland seaway in North America 
>disappeared.
>> This could have made for more seasonal swings in temperatures and
>> rainfall.  The low metabolism animals could hibernate through the 
>crisis,
>> the high metabolism animals could migrate away quickly to refuges.  
>Maybe
>> the "in betweeners" couldn't do either of these effectively.  
>
>I've not heard of high-metabolism mammals like shrews and mice being
>able to hibernate or migrate real well.  

Were mice alive at the time?  I know shrews or close relatives were.  At
the very least, they may have been small enough to live in underground
refuges.
>
>> Another idea I've seen is that small animals, birds and mammals 
>could
>> find refuges from the acid rain from the volcanism, and dinosaurs 
>may
>> have had sensitive skin.  But that wouldn't explain why many 
>amphibians
>> survived (most of which are acid sensitive and dependent on 
>freshwater).
>
>Maybe whatever was raining was rendered harmless by freshwater?  Or
>maybe the rain wasn't acidic but alkaline?
>
How can volcanism cause alkaline rain?  Too much sulfur.

>> But my problem with all of these musings is that we can't just 
>ignore the
>> extinctions in the air and the sea.  There was so many extinctions 
>going
>> on all over, we can't just focus on one set of ecosystems and ignore 
>the
>> rest, because they are all linked.  
>
>Are they?  I'm not so sure of that.  It seems like an obvious
>conclusion: all the extinctions happened at about the same time,
>therefore they have a mutual cause.  It's almost too obvious, in fact. 
>

They are not necessarily linked in that they would all go extinct
together, but they are linked in the sense that dominoes are linked when
you push them over.  The law of unintended consequences works even
without humans.

>In keeping with Murphy's ninth law (in any collection of data and/or
>ideas, the one that seems most obviously right is the one most likely 
>to
>be wrong <g>), I keep thinking that throwing out that conclusion could
>lead to some new and productive approaches to the problem.
>
Thomas Kuhn would be proud!  Yes, if we throw out *assumptions* we may be
able to get to new and creative approaches to the problem, and different
questions to ask.  But I don't think we can throw out the idea of linkage
totally.

There are some ideas we can come up with, but are untestable because of
the way fossils preserve.  Bakker's idea about diseases being a cause is
a valid one, but many diseases show no traces on bones and teeth, so if
such a "dino plague" happened, we would never find out, and couldn't
prove or disprove it.  I'd like to see more ideas that not only are
creative and new, but also predict the kinds of fossils we should find,
and their proportions in the record, etc.  Extinction is too dicey a
thing.  Heck, has anyone really answered all the questions about the
great Permian extinction yet?

Judy Molnar
Education Associate, Virginia Living Museum
vlmed@juno.com
jamolnar@juno.com
All questions are valid; all answers are tentative.