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astroblemes and extinctions



>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". 

Not entirely true, probably for the reasons you describe below.  Among the
"larger than a breadbox" sized animals which persisted were several genera
of crocodillians, river turtles, and champsosaurs (crocodile-like diapsids
of uncertain affinity).

>
>       - 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.

Actually, medium-to-large (troodontids, dromaeosaurids, hypsilophodontids,
and several other groups around in the Late Cretaceous were smaller than
humans).

>
>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. 

The evidence is strong for Maastrichtian migrations due to a major
down-drop in sea level during the last 9 million years of the Cretaceous. 
Bakker's Breakfast Bench site may not have the J/K transition: recent,
mostly unpublished stratigraphic and pollen work shows that all of the
alleged-Cretaceous Morrison site are actually Jurassic.  Also, there isn't
much to the J/K "turnover", as almost all families (with the exception of
diplodocid sauropods) which are common in the Late Jurassic are found in
the Early Cretaceous.

>
>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. 

Actually, there has been no supported evidence of an astrobleme at the
Permian/Triassic extinction, which is by far the most severe in the
post-Cambrian world.

[passages deleted - discussed the wall chart]

>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. 

It is important to note that the Deccan Traps begin millions of years prior
to any iridium spike, so anyone who wants them to be generated by the K/T
impact has to believe in time-travel as well...

>However, if one were to
>hit next week, it would certainly be the end of civilization as we know it.

I certainly agree with that.

>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.

[referrences deleted - see original posting]
                                     


Thomas R. HOLTZ
Vertebrate Paleontologist, Dept. of Geology
Email:Thomas_R_HOLTZ@umail.umd.edu (th81)
Phone:301-405-4084