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Re: K-T selection event



On Sun, 25 May 1997 07:12:41 -0700 Jonathon Woolf <jwoolf@erinet.com>
writes:
>{...}  AFAIK, the land tetrapods that survived were either
>_heavily_ coldblooded (ectothermic, poikilothermic, bradymetabolic) or
>_heavily_ warmblooded (endothermic, homeothermic, tachymetabolic).  
>The
>"hotblooded dinosaurs" debate in my mind hasd more or less resolved
>itself in favor of the idea that dinosaurs showed a range of metabolic
>rates, from sauropods which were probably mass homeotherms to small
>theropods which were probably a lot like slowed-down birds in their
>metabolism.  In simpler terms, if you put lizards at one end of a
>metabolic spectrum, and mammals and birds at the other, the dinosaurs
>fill much of the space in-between.
>
>There are only two or three basic types of selection events, and one 
>of
>them is a massive removal of intermediates along a particular range. 
>Only the two ends are left.  On a small scale, this can produce a
>speciation event.  On a large scale, it can produce an extinction.  On 
>a
>very large scale, it could produce the effect we see in land tetrapods
>at the end of the Cretaceous: extremes at either end survive, and a 
>huge
>number of in-betweeners get the chop.  So, I wonder: what sort of 
>event,
>or circumstance, or occurrence, would select intermediate metabolisms
>for death and extreme metabolisms for survival?
>
>Comments, thoughts, ideas, anyone?
>

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.  
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.  But there
were climactic extremes in previous Mesozoic extinction events, and yet
the dinosaurs managed to make it through those.

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). 
Perhaps an element, which would get more concentrated in the body of a
large animal (whether they are high on the food chain or not) was
suddenly in abundance and disrupted the reproductive capacity of
dinosaurs preferentially.  Minor elements such as selenium can make a big
difference in mammal reproduction, so I wonder about dinosaurs.  Or it
could have gone the other way, an abundant mineral became scarce and
disrupted their reproduction.

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.  It wasn't until years after all the
deforestation went on during the Vietnam war that people realized the
connection between the deforestation and the changes in the
monsoon/flooding cycles and the offshore fishing.  It's hard enough
discovering and understanding the linkages between systems today -- to do
so for the Mesozoic will take tons of detective work.

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