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Re: Bumpy extinction rates.



----- Original Message -----
From: "John Bois" <jbois@umd5.umd.edu>
Sent: Saturday, July 27, 2002 8:26 PM

> 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.

It is, see below. Just that small punctuations are pretty frequent.

> The slope of increasing diversity on this planet
> over geological time is incredibly bumpy.

Not sure if the slope is real rather than an artifact that shows that the
fossil record gets better the younger it is.

>  Nothing like a regular background is seen!

Oh yes there something like a regular background, if you don't look at the
curve of diversity but that of extinction rate. I've stared at such a curve
(Sepkoski's) for hours and hours in the lecture Catastrophic Geology held by
Rampino. There are spikes upward (mass extinctions) all the time, but no
spikes downward.

> _Average_ rate of extinction is a different concept than "background"
> extinction.

True, and average extinction rates are not what this thread is about.

> > 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).

What do you mean?

> I mean, ostrich, rhea, and emu adults are virtually
> untouchable!  It is an amazing body plan [..]

You can't bring a mammal into that shape, if you mean that. Closest you can
get are kangaroos.
Or do you mean why there are so many mammals around today, instead of a
world filled with birds? Maybe because a lot more mammals than birds
survived and evolved into the empty niches just as fast as birds. Not to
mention that flightless birds were among the terrestrial top predators
throughout the Paleogene of many and the Neogene of some continents.

> > Bottom line, IMHO: [...] 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)?

Probably very small size, independence from green food chains, and suchlike.