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Re: Big Craters
At 12:33 PM 11/24/99 -0500, Jerry D. Harris wrote:
>Message text written by INTERNET:Dinogeorge@aol.com
>>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.
I quite agree with this. As it looks now, it is even difficult to tell by
types of fossils if a bed is Late Jurassic or Early Cretaceous (look at the
controversy over the dating of the fossiliferous beds of China).
> 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.
Oh, say, like the splitting of the continents.
> 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.
The generic level is almost useless for this. Even ordinary stage
boundaries can show almost complete turnover at the generic level.
Howver, one oldish list I have of Mesozoic mass extinctions does list two
more or less around the Jurassic-Cretsceous boundary (though these are
probably mostly based on the marine record, not the terrestrial one). They
are a Kimmeridgian-Tithonian extinction (which is smack in the middle of
the Morrison beds). The list also mentions a Valangian-Hauterivian
extinction. But is annotated as being "minor", which probably puts it
down at close to the limit of statistical significance.
Neither of these events seem to have had much impact on the terrestrial
biota, and the second may be a statistical artifact.
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