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Astroblems and extinctions
I've been enjoying the discourse and might as well add a comment or two.
1. "astroblem(es)" are structures caused by impacts, not events. Literally,
an astrobleme causes nothing but local metamorphism. Impacts may be
effectors of widespread events.
2. I read with interest several lists comparing extinction events of
numerous geological intervals and astrobleme sizes. And that makes
an often-missed point: impacts are common, ordinary events, as are
smaller extinction events. Something phenomenal, like the P/Tr and
K/T ought to be marked by equivalently extraordinary astroblemes
if those are the causative factors. Now, Chicxulub would seem to
be such, except:
a. there is still doubt that it is an astrobleme (see Meyerhoff, et al,
1994, _Geology_ v. 22(1), p.3)
b. there is no suitable crater for the P/Tr event, and:
c. there are numerous craters without suitable mass extinction
3. Remember in all this discussion, especially "periodicity" of events,
all geological periods, eras, eons, and most ages were _defined_
by extinction events. I.e., the Cambro-Ordovician boundary is
marked by a change of trilobite families, not by rocks. "Change"
in the fossil record means extinction and rediversification. Its
the norm, not the exception;
4. Finally, all extinctions are certainly not equal and therein lies the heart
f the problem: what taxonomic level of extinction is significant?
For example, the question about the # of species extinguihsed in modern time
Prehistory begs the Q. of significance. Loss of 25 species
of Hawaiian parakeets (a reality) is hardly equivalent to loss of a keystone
species like, say the timberwolf or bison. Loss of families are not
equivalent to, say, losses of ways of life such as, say, volant (flying)
reptiles at the K/T boundary. This is what makes analysis of mass extinction
such a terribly subjective matter.
For the record, the K/T impact hypothesis still leaves me unconvinced
as a means of dinosaur extinction. I believe Mike Williams correctly
argues against models showing terrestrial dinosaur ecosystems were
intact at the latest Maastrichtian. My observations are the same.
David Schwimmer, Columbus GA