Re: Catastrophic Extinction?

[Tom Holtz <tholtz@geochange.er.usgs.gov>]

>        Mr. Jaroff, however, has stated that "the catastrophe is pretty
>well established," and that "opposition to the asteroid (or, more likely
>the comet)

Why more likely?  Comets are better for the 26-Myr repeater Nemesis
hypothesis, but most planetologists would agree that, to generate the
iridium layer, you'd need something a LOT more metallic than a dirty
snowball...

>theory is now largely confined to a dwindling number of
>paleontologists."  He has cited support for the catastrophic theory from
>geologist Eugene Shoemaker, of the U.S. Geological Survey, and has used
>the examples of paleontologists Steven Stanley and Clemens to show that
>paleontologists are gradually (and somewhat grudgingly) coming to favor
>the theory.
>
>        Is Mr. Jaroff's position accurate?  If so, it seems to run
>counter to the writings of almost every paleontologist I have read, and
>the evidence discussed in those writings.  I was hoping that, if it
>wasn't too much trouble, someone out there more knowledgeable than I
>could briefly summarize the current state of meteor impact theory.  If I
>am mistaken, or if I have been left behind by new revelations on the
>extinctions, I'd like to know, as I think would Mr. Jaroff.

Well, here's my (professional dinosaurian paleontologist's) take on this:

The evidence for a gigantic impact at the K-T boundary is very secure,
although some impacts previously attributed to the boundary are not the
correct age (e.g., the Manson Crater is demonstrably too old by ~12 million
years!).

This crater coincides (so far as we can tell with the admitedly-patchy
fossil record) the extinction of the latest Maastrichtian dinosaurian
faunas (the Lancian faunas of the western U.S., Lametan faunas of India,
Sinpetran faunas of Transylvania, etc.).

As shown by various studies, there is no apparent decline (at least at the
family level) within dinosaurian populations during the last ~3 million
years in western North America - i.e., the Lancian fauna appears to be
stable over its duration.

So, it is not unreasonable to associate the impact (or impact-generated
conditions) with the extinction of these, the last of the nonavian
dinosaurs.

HOWEVER,

Over a longer interval of time (the Campanian and Maastrichtian, or the
last 16-18 million years of the Cretaceous), there is a decline in the
diversity of the dinosaurian faunas.  Some groups represented by many
fossils of many species (esp. centrosaurine ceratopsids, hadrosaurin and
maiasaurin hadrosaurines, helmet-crested lambeosaurines) in the Campanian
are very rare (1 or 2 species) during the early Maastrichtian and absent
during the late Maastrichtian.  The absence in the late Maastrichtian
cannot be easily explained by alledged depositional-environmental bias,
since the late Maastrichtian is represented by nearly as many localities
and more diverse (not less diverse) paleoenvironments than the late
Campanian.

Therefore, we can interpret these groups as being extinct in the same
fashion that we interpret all nonavian dinosaurs are extinct in the
Cenozoic (i.e., we look for 'em, but we don't find 'em).

This same time period, the Campanian and Maastrichtian, is an interval of
much environmental change.  Substantial mountain building, intense
vulcanism (esp. the Deccan Traps of India), and the draining of the major
epicontinental seaways (e.g., the Western Interior Seaway, the Turgai
Seaway) all happen during this time (and may be related to each other).
Based on more recent analogs, this may have caused considerable climatic
change globally.  This potential climatic change may be reflected in the
rise (at least in late Maastrichtian western North America) of flowering
plants to ecological dominance, and in the changes among the assemblages of
dinosaurs.

So, my opinion is yes, the impact may be the ultimate cause of the final
extinction of the last assemblages of dinosaurs.  However, fossil evidence
shows changes occuring within the dinosaurian faunas for several millions
of years prior to the K-T boundary which cannot be associated with the
impact.

I'll leave off the matter of other organisms for other posters...

Hope this helps.

                                
Thomas R. Holtz, Jr.                                   
tholtz@geochange.er.usgs.gov
Vertebrate Paleontologist in Exile                  Phone:      703-648-5280
U.S. Geological Survey                                FAX:      703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092
U.S.A.