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astroblemes and evolution



Astroblemes and Evolution
-------------------------
About 65 million years ago, a large comet or asteroid slammed
into what is now the Yucatan peninsula/Gulf of Mexico. The resulting
crater has been estimated at over 200 km and possibly as much as
300 km in diameter. Measurements from around the world show a
spike in siderophile elements such as Iridium in deposits from
that time. The first discovery of the Iridium-rich layer was made by
the son-father team of Walter and Luis Alvarez. 

The body causing the havoc was around 10-20 km in diameter. Nobody
knows exactly what happens when one of these nasties hits the earth. 
Current models suggest a dust cloud engulfs the earth and curtails 
photosynthesis for about 3-6 months. The effect of this on living things 
is hard to model, but the fossil record indicates:

       - No vertebrates larger than a breadbox proliferated from the
         late Cretaceous through to the Tertiary. In other words, 
         "the meek inherited the earth". 

       - Animals that spend at least part of their lives underground, in
         hibernation, or in stasis/cocoon stages, etc. seem to have
         made it through in fairly good shape.

The dinosaurs being rather large, terrestrial animals were decimated.

Dr. Bakker's theory involves the formation of land bridges, which allow
the migration and mingling of animals that had been previously segregated.
I think this is a good model for the Jurassic/Cretaceous faunal turnover
within the Dinosauria. The "Breakfast Bench" site at Como Bluff, Wyoming
preserves a record of the Jurassic/Cretaceous turnover, and has no doubt 
influenced his thinking alot. 

When it comes to really big turnovers in the fossil record, i.e
(Cretaceous/Tertiary, Permian/Triassic, Ordovician/Silurian) there
is at least some evidence for large astroblemes at all of these crossroads
in evolution. I'm a strong believer in the astrobleme theory. I would even
go so far as to postulate that you can't hide a really big impact in the
fossil record. Furthermore, I believe there is a periodic nature to the
phenomenon.

Below are some references from the bibliography of my educational wallchart 
entitled: "A Correlated History of Earth" (Pan Terra, Inc. 1994). This 
wallchart includes maps of plate motions, orogenies, major volcanics, 
glacial epochs, geostratigraphy, major asteroid impacts, classic fossil
localities, evolutionary history of the major Kingdoms, Phyla, Classes.
All subjects are correlated in time.  It became evident to me that as I 
graphed out all of the data for this project, that the strongest
correlations were between very large astroblemes and major extinction
events in the fossil record. Large volcanic events such as the Siberian
traps and Deccan traps also correlate with two of the largest extinction
events in the fossil record, so we're still a long way from being able
to say exactly what happens when the big one hits. However, if one were to
hit next week, it would certainly be the end of civilization as we know it.
Godspeed to those who search for and chart apollo objects and comets!
I can't think of a better use for some of the SDI research than to divert
a large comet(or asteroid) on a collision course with mother Earth.

References:
------------
Terrestrial Impact Structures, R. A. F. Grieve, Ann. Rev. Earth Planet Sci.,
vol 15: 245-270, 1987.

Asteroid and Comet bombardment of the Earth, E. F. Shoemaker, Ann. Rev.
Earth Planet Sci., vol 11: 464-494, 1983.

Earth's Near Death Experience, J. Alper, Earth Magazine, Jan. 1994.

Geological and Biological Consequences of Giant Impacts, D. J. McLaren,
W. D. Goodfellow, Ann. Rev. Earth Planet. Sci., vol 18: 123-171, 1990.

Astrogeological Events in China, Xu Dao-Yi et al, Geological Publishing
House, Beijing, China, 1989.

Extinction, S. Stanley, Scientific American Books, Inc. 1987.

Terrestrial  mass extinctions, cometary impacts, and the sun's motion
perpendicular to the galactic plane, M. Rampino and R. Strothers,
Nature, v308, p 709-720, 4-19-1984.

Extinction of Species by periodic comet showers, M. Davis and Piet Hut,
Nature v308, p 709-720, 4-19-1984.
NOTE: This volume of Nature has a collection of pertinent articles.

Periodicity of Extinctions in the geologic past, D. Raup and J. Sepkoski,
Proc. Nat'l. Acad. Sci. USA, v81, p 801-805, 1984.

Imapct theory of mass extinctions and the invertebrate fossil record,
W. Alvarez et al., Science, vol233, 3-16-1984.