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Re: Iridium near fossils?

I have seen several good talks on this subject by paleobiologists from
the Smithsonian, most recently one by Dr. Kay Behrensmayer.  It is true that 
vertebrate fossils have not been found in association with Ir-rich boundary 
layers.  But, this is probably not due to global "immolation" of the
corpses.  
Dinos must have been dying and falling into swamps and rivers right up to the 
time of the terminal Cretaceous impact, so there ARE probably a "normal" 
number of fossils from this time in sedimentary rocks somewhere.  The problem 
is that large vertebrate fossils are very rare, and the bone-beds needed 
to ensure a good date are even rarer.  Given the restricted areas in 
which we can look for fossils from the late Cretaceous, and the poor odds of 
finding animals of any age, no less a particular age, it is not at all
surprising 
that we haven't found any right at the boundary layer.  You have to
remember that
dinos were big animals living mostly on land, and they didn't give rise to
extensive fossil deposits as did, e.g., many small marine species.
Similarly, you
can't find the Ir layer everywhere (although you can find it in many places), 
and you'd have to find your fossils in one of the places where the boundary
layer 
is well exposed. Moreover, with any fossil that you do find, there is always 
the problem of trying to decide if it was transported, or if the sediments
were reworked at a later time.  Single fossils are always going to present you
with this problem, making interpretation of any find from right near the
boundary 
difficult.

The bottom line here is that we are limited by very poor statistics.  If 
bone-beds of dino fossils DO exist right at the boundary, there are
good reasons why we have not found them.

jeff

p.s.  This whole discussion reminds me of questions people asked in 1980,
after
the initial discovery: why don't you find dino fossils sheared off at the
knees
by the initial shock waves?

Dr. Jeffrey N. Grossman       phone: (703) 648-6184
US Geological Survey          fax:   (703) 648-6383
954 National Center
Reston, VA 20192, USA


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