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Re: ?Latest paper on the extinction of dinosaurs in North America

> Dean A. Pearson, Terry Schaefer, Kirk R. Johnson, Douglas J. 
> Nichols:
> Palynologically calibrated vertebrate record from North Dakota 
> consistent
> with abrupt dinosaur extinction at the Cretaceous-Tertiary boundary, 
> Geology
> 29(1), 39 -- 42 (January 2001)

Yes, that appears to be the latest publication regarding  N.A.

> Abstract: 
> When calibrated to the K-T boundary, occurrences of 
> vertebrate
> fossils indicate an uppermost Cretaceous barren zone (Fig. 2). Where 
> the
> basal Fort Union is Cretaceous, this gap is 1.76 m. At Mud Buttes, 
> where the
> K-T boundary and the formational contact are coincident, the gap is 
> 2.37 m.

First observed in the Hell Creek Formation by J. David Archibald back in
1982.  He was also the first to label it the "barren zone".
Has a similar barren zone been found at the top of the Lance Formation?

> These results differ from Sheehan et al. (2000), who listed a 
> minimum of 10
> dinosaur specimens in the uppermost 5 ft (1.52 m) of the formation. 
> We
> checked their unpublished data (Sheehan, 2000, written commun.) and
> ascertained that all their vertebrate sites in this interval were 
> near Mud
> Buttes, implying that their survey recorded dinosaur fossils where 
> we found
> none."

Possibly due to having been "picked over" by the previous research team? 
How fast does that particular section of outcrop erode?  I note that
their respective publication dates are only separated by one year!  When
did each team conduct their field work?

> "Our data show that there is a gap below the K-T boundary in our 
> study area,
> but it is an interval in which we found no vertebrate fossils of any 
> kind,
> even those of vertebrate groups known to have survived the K-T 
> boundary
> event (e.g., fish, amphibians, turtles, crocodilians).

Phosphate leaching via acid rain?

> It is 
> possible that
> this gap is a function of depositional conditions associated with 
> the
> formational transition rather than a reflection of true terminal 
> Cretaceous
> faunal diversity.

Also a good possibility.  Deposition rates are vastly different between
the Hell Creek formation (high, steady deposition rate) and the Fort
Union formation (lower, variable deposition rate).

>  In contrast, Sheehan et al. (2000) reported 
> numerous
> dinosaurs within 1.52 m of the K-T boundary from the southern part 
> of our
> study area.

Close, but no seegar.

>         Our vertebrate data and those of Lillegraven and Eberle 
> (1999) both
> show a diverse, dinosaur-rich fauna in the highest Cretaceous 
> vertebrate
> sites,


> a pattern that is consistent with abrupt extinction of 
> dinosaurs.

Ooooo.  I don't follow.

It's hard to know how long that "barren zone" lasted, and its hard to
know *how* the extinctions proceeded within that barren zone.  I guess
the word "abrupt" can be interpreted more than one way in this case.

> Combined with high levels of plant and insect extinction (Johnson et 
> al.,
> 2000), these data support the model of a terminal Cretaceous biotic
> catastrophe."
> Have I already mentioned the youngest hadrosaur tracks, those 
> dinosaur
> fossils that are not going to be dissolved by acid rain, in this 
> thread?
> They are 37 cm below the boundary, writes Lockley.

Close, but no seegar.

> Long live the youngest ammonite, found 10 (ten) cm below the 
> boundary.

Close, but no se....well, now I'm just being repetitive  ;-)

Dont' get me wrong.  I believe that the mass extinction (particularly the
terrestrial portion) was abrupt and that it was probably caused by an
impact.  I just think that it is hard to prove scientifically.


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