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Re: Big Craters



Message text written by INTERNET:Dinogeorge@aol.com
>We don't know anything about the J-K boundary in Africa, so we can't tell 
whether there was an associated extinction event on the continent where the

impact took place.<

        True.  However, the more direct point is that there is evidence for
an impact significantly larger than Chixilub with no _global_ extinction of
dinosaurs.

>But there certainly is quite a change in the nature of the 
dinosaurs worldwide fas one crosses the J-K boundary.<

        There is?  What is it?  I still see diplodocoid and brachiosauroid
sauropods, stegosaurs, ankylosaurs, small compsognathid and big allosauroid
and spinosauroid theropods milling about.  No sudden changes at anything
except the generic level.  The only moderate change is an increase in the
number of larger ornithopods, but we see the beginnings of this even in the
Late Jurassic, so it's not a huge surprise.  The most remarkable change in
the dinosaur faunas at the J-K boundary is similar to that of all the
invertebrates:  increased regionalization and endemism, but I think this
has more to do with the tectonic regime than an impact.

>Until we can improve 
sampling of fossil record at J-K boundary, it is perhaps premature to say 
that there was no mass extinction.<

        I guess we're back to semantics about what is a "mass" extinction. 
Certainly it doesn't look like there are any common dinosaur genera across
the boundary, but even at slightly higher taxonomic levels, no extinction
is easily visible.  

>I believe Bakker has published evidence 
from the Morrison about a possible J-K mass extinction.<

        This is based on rather flimsy evidence, IMHO.  It is based on the
lack (at the time of his publishing) of stegosaurs, torvosaurids, and large
populations of sauropods in the North American Early Cretaceous (which is
substantially removed, temporally, from the Kimmeridgian Morrison
Formation); however, as we've seen, there are plenty of sauropods still
milling about, and allosauroid theropods are still there.  There are also
primitive theropods around, as indicated by the Arundel and Cloverly forms.
 True, we don't have any North American Early Cretaceous stegosaurs (yet),
but since stegosaurs persisted in the Eurasian Early Cretaceous, this is a
localized extinction, at best.

                _,_
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--____-===(  _\/                         \\/ \-----_---__
           /\  '                        ^__/>/\____\--------
__________/__\_ ____________________________.//__.//_________

                     Jerry D. Harris
                 Fossil Preparation Lab
          New Mexico Museum of Natural History
                   1801 Mountain Rd NW
               Albuquerque  NM  87104-1375
                 Phone:  (505) 899-2809
                  Fax: ; (505) 841-2866
               102354.2222@compuserve.com