Re: Nature of the fossil record

[Peter Sheehan <sheehan@xxxxxxxxxxxxxxxx>]

On Mon, 9 Jan 1995, N. MacLeod wrote:
......  Perhaps the best way
> to approach this problem is to turn the Signor-Lipps Effect on it's head
> and ask how far above the last dinosaur fossil occurrence you have to go
> before you are sure (in a statistical sense) that dinos are indeed missing.
> If that exercise is carried out and given the "thousands of fossils" that
> have been collected from that (and nearby) localities, I suspect that
> you'll be 95% sure you've seen the last of the dinos before you get very
> far into that barren interval.  Moreover, if we have hiatuses in that
> section (as we most certainly do) that fact complicates the situation and
> those complications must be taken into account.
> 
Since I do not believe the "barren interval" exists I may not be the best 
person to address the history of this unit.  

The "barren interval" is a supposed 1 m interval at the top of the Hell 
Creek.  It is very poorly documented.  It believe it was first mentioned 
before the iridium anomaly was being considered.  The top of the Hell 
Creek at that time was taken as a widespread coal layer.  The iridium 
layer now represents (at least to those on one side of the argument) the 
K-T boundary, not the coal bed.

The iridium containing layer was deposited over the Hell Creek 
landscape.  This landscape was dominantly flood plain, with local "coal 
swamps" and streams.  I believe that the iridium layer was eroded by rain 
where it was deposited on the flood plain, or by running water where it 
was deposited in streams.  In swampy areas it was more likely to be 
preserved.  In any event, the iridium layer is now difficult to find.  It 
has been located with certainty at only one place in MT and one in ND.  
There are a few other areas where slight iridium anomalies may represent 
the layer.

In the two sections where it is know it has little lateral continuity.  
Remember the 1 meter barren interval is defined as below the base of a 
coal bed, not below the iridium layer.  The coal layers are diachronous.  
The flood plains dominated the surface of the land at the time
of the impact.  

Most dinosaur bones are found in stream channel deposits, where the 
iridium layer was removed.

Some thousands of dinosaur bones have been found in the Hell Creek, but 
this does not mean their position in the H.C. has been determined to within
a meter.  From what I have seen of dinosaur abundance in flood plain 
deposits it is not surprising to me that no dinosaur bones have been found 
to be in any particular interval of flood plain deposit.  They are very
rare.  Couple that with the fact that the iridium layer has been found in 
very few places, and the entire idea of a barren interval below the 
iridium layer becomes very suspect.  I believe the highest dinosaur bone 
we found in our survey was about 60 cm below the probable iridium layer 
(based on a low iridium content).

The Signor-Lipps Effect can simply not be overcome in dinosaur 
sequences--the fossils are far too rare.  I still believe that for 
dinosaurs it is best to abandon attempts to identify the event using 
stratigraphic ranges of dinosaurs.  It would take armies of field workers 
to approach large enough sample sized.

As we have noted in our papers, a more reasonable approach is to look at 
communities of dinosaurs, as we have done.   If there were ecosystem 
changes sufficient to cause the extinction of dinosaurs during the last 
two million years of the K (Hell Creek time), the communities should have 
been changing like communities do that are in stress such a modern 
pollution.  We should have found changing dominants in the system, with 
dominant taxa being replaced by other taxa and ecologic diversity 
declining.  Instead we found the communities were quite stable through time.
We saw no evidence for stress.

Peter Sheehan