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Bumpy extinction rates.



Allan Edels said (a while ago):

> The type of PURELY competitive supplanting of the non-avian dinosaurs as
> proposed by HP Bois has never been documented before or since.

Not sure what you're saying I'm saying.  I believe it is a generally
accepted belief (if nonverifiable in ecological time) that current niche
delineations are "the ghosts of competition past" (to quote Connell of
Balanus clam fame).  And perhaps there is no such beast as "pure"
competition.  For example, pigs and skunks on an island off California may
have limited each other due to competition.  An eagle discovers the island
and eats all the little piglets.  Skunks win.  In a sense this is
competition because the skunks can handle the predatory environment better.
Or, the possible example of Panama canal turning continuous range into
hilltop islands.  Jaguars swim off.  Small mammals pops. explode.
Ground-nesting birds become extinct.  Tree-nesting relatives are better
competitiors due to accident of their nesting in trees.
In any case, in this argument we were talking about pterosaurs, not
non-avian
dinos.

> The statistical anomaly of so many clades
> disappearing within a short time frame (again, geological time) requires
> some additional methods above and beyond the normal background local
> species extinction reasons - i.e. competition, disease, environmental
> changes, etc.

Background extinction?  This is a concept borrowed (perhaps?) from
background radiation.  A _constant_ stream of radiation from space
punctuated by cosmic events.  This is _not_ the case with extinctions.  The
slope of increasing diversity on this planet over geological time is
incredibly bumpy.  Nothing like a regular background is seen!  Indeed, each
extinction is its own tragedy, has its own unique cause, etc., etc.
_Average_ rate of extinction is a different concept than "background"
extinction.  Averages smooth out the bumps; background has no bumps.  The
record of extinction is bumpy.

> I agree that the survivors of the K-T end-time out-competed the other
> animals.   I merely suggest that the rules of competition were
> drastically changed by the bolide, thereby enabling the successful
> out-competition of the dinosaurs and other non-survivors. All of the
> normal pressures still existed - but ALL were intensified by the new
> constraints due to the changes in the environment - brought about by the
> bolide.

I think this is a very even-handed and sensible statement.  And it
approaches the idea that the cause of a particular extinction is often just
a matter of taste.

> For the edification of younger list members, it is important to remember
> the theories of the K-T extinctions PRIOR to the bolide idea.  All of
> the ideas that HP Bois has presented (at least in a gross, overview
> version) had been presented and debated for decades, with very little
> agreement.

Can you show me where it was stated that dinosaurs were hit because they
couldn't hide their eggs; that this strategy is practically absent today;
that pre K/T pred/prey size ratios approached those of today's predators of
eggs and hatchlings.  I'm not laying claim to the idea (nor is it a very hot
commodity, unfortunately), I'm just curious if it really had been presented
in this form.

> Today I see the bolide as the major causal agent, triggering or enabling
> several other processes to take effect, creating the extinctions that we
> see.

Very likely true.  And this leaves open the perhaps more intriguing question
of why the body plan didn't reclaim former dominance (in the open-field
niche, at least).  I mean, ostrich, rhea, and emu adults are virtually
untouchable!  It is an amazing body plan that suffers only from
susceptibility of its reproductive mode, i.e., having to stay stationary for
months at a time, and having hatchlings that a very small fraction of the
parent size.

> Bottom line, IMHO: Is the extinction of the non-avian dinosaurs (and
> several other clades) the action of the usual competition that somehow
> all culminated at the same time? Or did the bolide strike change enough
> of the environment so that features of the various survivors that were
> once merely of minor importance, became essential to survival, enabling
> them to out-compete the others who didn't have those features?  I think
> you know my answer.

What features are you referring too (if any)?