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K-T selection event



Random musings . . .

I've never entirely bought into the notion of a meteor strike causing
the terminal-Cretaceous extinctions.  So I keep thinking about other
things that might have done it.  My main question is: why did the
extinction occur the way it did?  Why were dinosaurs wiped out and not
mammals or birds?  Why mosasaurs and not sharks?  Why ammonites and not
nautiluses?  

In trying to figure out what happened, it seems to me that the most
important thing is that whatever happened was _selective_.  In other
words, it was a selection event.  There was some distinct, consistent
trait or combination of traits that spelled "survival."  What was it?  I
don't know, but I'd like to offer a thought for discussion:

On land at least, there was a definite pattern to the survival. 
Dinosaurs, invariably large, got wiped off the face of the map.  Mammals
were hard hit, but survived almost entirely in smaller forms.  Birds
were also hard hit, but survived.  Lizards and snakes survived.  I have
no information on the sizes of the birds that survived.  But basically,
everything over about ten kilograms mass was wiped out, while smaller
organisms took heavy losses but survived.  Does that mean size was the
determining factor?  I don't think so.

This is where my new thought (or at least, I've never seen it anywhere
else) comes in.  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?

-- JSW