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Re: Theories on the extinction of dinosaurs



[This is slightly modified from my personal response to take into account
some objections that were raised].

At 11:40 AM 11/18/99 -0500, Dinogeorge@aol.com wrote:
>In a message dated 11/18/99 11:27:50 AM EST, sarima@ix.netcom.com writes:
>
><< Yep.  This is why the impact is included as *one* of the causes in all
> multicausal models. >>
>
>Multicausal extinction theories suffer from the problem of having all the 
>causes come together simultaneously all over the world to produce the 
>extinction. As the number of causes goes up, the probability of this 
>happening drops >way< down.

Except that they generally involve causes that are *observed* to have
co-occurred at the time of the extinctions.  We *know* that the Deccan Trap
emplacement, sea-level drop, cooling of the climate and the Yucatan impact
*were* all in effect simultaneously.  So a priori probability is irrelevant
here.  The coincidence *did* happen, regardless of its significance to the
extinctions.  (And if there was oceanic anoxia at the K-T boundary, then
that too was in effect at the same time as the other potential causes)

Also, some of these factors are unusually powerful at this time.  The
Deccan Traps are one of the larger flood basalt domains, surpassed in
magnitude only by flood basalts that are more or less contemporaneous with
other, *larger* mass extinctions.  For instance the larger Siberian Traps
correspond in time to the P-T extinctions, which were the worst in the
Phanerozoic.

As for the oceanic anoxia (if present), this factor is frequently
associated with lesser mass extinctions throughout the Phanerozoic, such as
the Cenomanian-Turonian extinctions.

Thus there are independent reasons for associating these particular causes
with mass extinctions.  There were few other times in history when so much
disruptive activity was taking place in the same span of time, and as far
as I know all of *those* times are also mass extinctions.  Under these
circumstances their *observed* coincidence at this time takes on added
significance.

> On the other hand, even the asteroid impact 
>theory is a multicausal theory, in the sense that something else going on in 
>the world must have made the dinosaurs particularly vulnerable to an 
>asteroid-impact extinction at the time that the impact happened. After all, 
>there were other impacts in dinosaur history that did not have the 
>devastating effect of the K-T impact (though they may have been responsible 
>for some of the abrupt changes seen in the dinosaur fossil record).

This is not the classical version of the impact scenario.  As Alvaerez and
company put it forth, the impact *alone* was the cause of the extinctions.
The statement has been made that any impact of that size would cause mass
extinctions.  The other impacts have been treated as irrelevant on the
grounds they were all much smaller.

Because this is the most widely accepted form of the hypothesis, I prefer
to term any model in which the impact is only the final of a series of
causes as a multicausal model.  This makes for less confusion, as it avoids
the tendency to see an implication that the impact itself was fully sufficient.

--------------
May the peace of God be with you.         sarima@ix.netcom.com