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Re: Theories on the extinction of dinosaurs



In a message dated 11/21/99 3:02:55 PM EST, jbois@umd5.umd.edu writes:

<< Does this refer to the Lillegraven and Eberle paper?  If so, I disagree
 with your conclusion.  So many coincidences seemed to have occurred at
 this time, I think you have biased one over the others.  For example, as
 noted in the L and E paper, there was an important immigration of mammals
 "just at the time" non-avian dinosaurs became extinct.  On the other hand,
 L and E report no evidence of iridium (I don't know if they tested).  So,
 evidence of new mammals (admitedly, only conylarths), no evidence of
 impact.  Additionally, there is zero evidence that _any_ impact _ever_
 caused _any_ extinctions, direct evidence, I mean.  Mammals are proven
 enemies of large flightless birds.  Surely they have earned the right to
 be considered a prime suspect. >>

No, this is new work. A survey of the Hell Creek Formation up to the K-T 
boundary. All fossils, not just dinosaurs, were counted. There is no evidence 
for any kind of faunal change up to the boundary, except that at 1.8 meters 
below the boundary (representing about 10,000 years of deposition) no fossil 
animals of any kind were found--taken as evidence of a local change in fossil 
preservation conditions, not as evidence of pre-boundary extinction, since 
some genera survive the impact and the 1.8-meter gap and reappear above the 
boundary (dinosaurs do not).

>Direct< evidence of a past asteroid impact causing mass extinction can only 
be acquired by time machine. >All< evidence for any causes of >mass< 
extinction is entirely indirect and based on coincidences in the fossil 
record, since, e.g., humanity has not yet had the opportunity to observe and 
document the effects of a large asteroid impact. So by your definition there 
is zero evidence for >any< kind of cause for any mass extinction.

For that matter, we still have no proof of causes of >local< extinctions, 
just a bunch of speculation, except in the case of direct human intervention. 
Whole subject of extinction needs to be carefully rethought and redefined, 
including such concepts as cause and effect as inferred from coincidences in 
fossil record. Bad things happen all the time to animals, so you can expect 
lots of coincidences among such bad things everywhere in the fossil record. 
Most of the time these yield no mass extinctions; why not? On the other hand, 
when a mass extinction occurs at the same time as a documented asteroid 
impact that scatters itself all around the globe, why should we infer no 
connection?