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Re: Big Craters



Message text written by INTERNET:Dinogeorge@aol.com
>>No sudden changes at anything
 except the generic level. >>

>This is just what I was referring to. Large diplodocoids and
brachiosaurids 
pretty much vanish across the J-K boundary, replaced by much smaller 
sauropods;<

...but there are still diplodocoids (if not diplodocids) and
brachiosaurids.  Let me see if I can clarify what I'm thinking here:  what
evidence we have of taxa in the Kimmeridgian sediments on one hand and the
late Neocomian on the other show differing genera in the same higher
taxonomic groupings ("families" and such).  To me, this speaks of just
standard, run-of-the-mill evolution (cladogenesis, etc.) that one would
expect to see over the 20 million year time span involved.  We don't see
the kinds of apparent sudden demise of entire clades and sudden emergence
or diversification of entirely different clades across the J-K boundary
like we do at, say, the P/T boundary or the K/T boundary.  The regular,
run-of-the-mill evolution produces just the kind of generic turnover that
we see on both sides of the J/K boundary.  Correct me if I'm wrong, but it
seems to me that you're saying that an ecological perturbation sizeable
enough to cause one genus to go extinct and be replaced by another genus of
the same clade counts as a mass extinction...?  Why can evolutionary
effects such as niche competition not explain the intraclade turnovers we
see?

>abundant stegosaurs are gone from North America (only rare Chinese 
and European forms survive into Early Cretaceous) and are replaced by 
ankylosaurs.<

        Well, the commonality of stegosaurs compared to ankylosaurs seems
to have switched in the Early Cretaceous from the positions in the Late
Jurassic, but they're still present.

>Don't recall any spinosaurs from the Jurassic; they all seem to 
have arisen in the Early Cretaceous, maybe to fill a niche vacated by 
extinction of allosaurids.<

        Well, I was using "spinosauroid" in the sense that it has been
thought in the past to include torvosaurids (a tenuous position, IMHO, at
present) -- if the two aren't as closely related as has been assumed, then
of course you're correct and it doesn't say anything about the possibility
of extinction.

>Theropod systematics is still too unsettled to be 
certain which groups successfully crossed the J-K boundary. The extinction
is 
not as well defined as the K-T extinction, but I think there's something 
there worth looking into.<

        Most certainly!  As Tom pointed out, we lack the necessary
terrestrial sediments from the earliest Early Cretaceous to make the best
possible assessment of the situation.  However, the presence of
allosauroids on both sides of the boundary still indicates that they
couldn't have gone extinct.  


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                     Jerry D. Harris
                 Fossil Preparation Lab
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