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Dinosaur Extinction (Again)
1) What we define as an "entire group" is largely a matter of semantics; a
species is an "entire group" of individuals, and plenty of them were wiped
out at the end of the Cretaceous. And birds _are_ part of the dinosaurs as
a group. Restricting your arguments to "non-avian dinosaurs" is _pure_
2) You _can not_ restrict your arguments to terrestrial forms for the sake
of convenience. The fact of the matter is that several "entire groups" of
aquatic life became extinct at the same time. This is highly suggestive of
an extinction mechanism that affected both terrestrial _and_ aquatic forms.
3) There is also evidence that most - if not all - of the taxa that became
extinct at the end of the Cretaceous were experiencing reduced diversity
immediately before they became extinct - as well as several other taxa that
survived. Once again, this is suggestive of an extinction mechanism that
affected all of them.
So what was that mechanism? To answer this question, we must first stop
asking why so many taxa became extinct, and start asking why so many taxa
did not recover from this event. Extinction is a natural, normal thing. The
dinosaurs went through several phases of it during their reign, usually
accompanied by several other taxa. But of course, prior to the K-T, they
always recovered - many "entire groups" of dinosaurs became extinct, but
were replaced by other, newer groups.
These extinctions appear to have been due to the sorts of causes put
forward by the 'gradualist' school of extinction theories and, indeed, they
also appear to have been responsible for the extinctions at the K-T: only
the gradualist theories can explain the apparent reductions in diversity of
so many taxa. They _do not_ explain, however, why no group of dinosaurs (or
pterosaurs, or plesiosaurs, or ichthyosurs, or mosasaurs etc.) recovered on
this occasion. That's where the 'catastrophist' theories come in.
There is strong evidence that there was a meteorite or comet impact at the
K-T; there is also strong evidence of considerable volcanic activity (the
Deccan Traps). However, there is also strong evidence for several of these
types of events occurring throughout the reign of the dinosaurs. So the
fact that such events occurred at the K-T shouldn't matter, right?
Wrong. The earlier catastrophes had minimal effect because they occurred at
times of great diversity; what's unique about the K-T is that these events
occurred at _the same time_ as a natural extinction event. At full
diversity, such a catastrophe might have wiped out a multitude of
individuals, but because there was such an abundance and diversity of them
to start of with, few "entire groups" would have become extinct. However,
when there was already a natural extinction event going on, you wouldn't
have as much to start off with; hence wiping out the same number of
individuals would very likely result in the extinction of "entire groups"
of them. Furthermore, at the K-T we appear to have had _two_ great
catastrophes occurring at the same time as a natural extintion event. No
wonder so many taxa became extinct!
So the question then becomes: why did certain taxa survive the end of the
Cretaceous while others didn't? It would take one hell of a catastrophe to
wipe out _all_ life on earth - not even full-scale nuclear war could do
this. But while some taxa will always survive, the pattern of extinction
and survival will not be random. The first animals to go in any extinction
event are those at the top of the food chain, and that's exactly what the
dinosurs, and the other taxa that became extinct in their various biomes
[is this the right term?], were.
The above scenario takes into account _all_ that is known about the K-T
extinction event (well at least all that I know is known) and, I believe,
explains everything. There may be some points I am missing, however. If so,
please put them to me or, preferably, to the list.