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Re: _Night Comes to the Cretaceous_




On Sat, 8 Aug 1998 Dinogeorge@aol.com wrote:

> On the other hand, it could also be true that some paleontologists are
> resistant to having a physicist step in and tell them the way things happened
> at the K-T boundary, so they reach for ever more far-fetched alternatives to
> the impact, when the correct explanation has been sitting under their noses
> for the past two decades. This blade cuts both ways, I'm afraid, and doesn't
> bring us closer to the truth.

In my experience paleontologists are very wary of accepting explanations.
This is reasonable since it is hard to test historical hypotheses.  Most
seem content to speak the truth, which is in this case: "we may never
know".

> What's a "pseudo-extinction"? Are all the organisms dead, or are they not?

When one species evolves into another, the ancestor species ceases to
exist.  This is a pseudo extinction.  
For example, according to David Archibald: "During the Latest Cretaceous
and the Paleocene in Western North America, disappearance rates for
mammalian genera track appearance rates, both reaching their peak in the
early Paleocene following the extinction of non-avian dinosaurs.  Some of
the disappearances during this time were pseudo extinctions that resulted
when ancestral species disappeared during speciation."

In this way a creative bout of adaptive radiation can look like a mass
extinction! 
 
> I suggest we begin at the other end: (1) There was a gigantic asteroid impact
> exactly at the end of the Cretaceous; of this we are now certain. (2) A host
> of Cretaceous organisms vanished at exactly the same time, or very shortly
> after, all around the world. Duh!!

But I've already said that Alvarez is talking about "stepped
extinctions".  A host of organisms did _not_ vanish at exactly the same
time.  By starting at the other end you've got ot backwards.  You have an
"event" and you are trying to shoe-horn all the data into an instant of
time.  The best evidence we have suggests this is not what happened.
Merely repeating it over and again doesn't make it true.   

> Most modern bird orders evolved during the early Cenozoic, quite likely in
> adaptive radiations following the K-T event from a few randomly surviving
> neornithan species. 

This is unsupported by anything but negative evidence.  It's  simply too
early to tell if whether molecular clock data is right when it says
most orders evolved _before_ the K/T!  if birds exhibited mass-survival
over the K/T, wouldn't this falsify the bolide-as-sufficient idea?  

> Enantiornithan birds didn't make it across the K-T
> boundary, but fossils of these are still so abysmally rare that we really
> can't conclude that they were wiped out by the impact.

But even Feduccia recognizes a gradual decline in enantiornithan birds
over the end of the Cretaceous.

> The major issue of the K-T extinction is not the cause (asteroid impact) or
> whether there >was< a mass extinction (there was); the major issues are which
> organisms survived it, and why, and what happened to their descendands
> afterward.

Unless you can explain survival as well as extinction, you don't have an
explanation for extinction!  The bolide-as-sufficient idea does not
explain either the timing or the pattern of extinction and survival.  As
such, it was an event, that's it.