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Quo vadis J-K extinctions? (was RE: Big Craters)



Message text written by "Thomas R. Holtz, Jr."
>The Morrison dinosaurs are Kimmeridigian, or maybe earliest Tithonian. 
The
Lakota Formation, Yellow Cat Member of Cedar Mountain Fm, and good Wealden
Group dinosaur assemblages are Barremian.  We are missing...over 20
million years of dinosaurs! This is like using dinosaurs from the late
Coniacian (85 Ma) and mammal assemblages from the earliest Paleocene (65
Ma) to argue for the asteroid
impact hypothesis of the K-T boundary.  You can fit the whole series of
changes from the Aquilian to the Judithian to the Edmontonian to the
Lancian
within this time frame.<

        Right!  Although I didn't spell this out in my posting, I tried to
imply that I acknowledged the huge time gap existed with the "temporal
difference" statement.  Sorry I didn't make that clearer!

        Nevertheless, it still seems to me that the decreasing abundances
of "typical" Late Jurassic clades (diplodocoids, stegosaurs, allosauroids,
etc.) and increasing abundances of rarer Jurassic forms (ankylosaurs) plus
radiation of novel forms (big ornithopods) in the Early Cretaceous is
better explained by things like niche competition and other, more standard
evolutionary turnover models than by a huge ecological disaster like an
impact.

        My god, did I just say "models?"  I've been hanging around
geophysicists too long...  8-P


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