

Re: dinosaur->bird when?

>I have seen this claim on other occassions and have to ask; how good is the
>data on the great dinosaur fade? It would seem to me that even under the
>very best of conditions, large terrestial animals would still leave a very
>spotty and pathetic fossil record, not the kind of data that one would want
>to base population trends on. Surely marine invertebrates or the various
>microfossils would be a much better source of data for population trends in
>the late Cretaceous and, from what I understand, they show no signs of
>population fades. So just how good is this dinosaur data?

Paul et al -

        How good is it?  Same as always!  Just because there is an
extinction event -- even a catastrophic one -- doesn't mean that the
_fossil record_ is altered:  even if there are abnormally large numbers of
deaths, there still has to be mechanisms in place to turn them all into
fossils!  This can happen in small localized circumstances:  upwellings of
deoxygenated water can cause mass kills which, in the same deox water have
great potential for fossilization (and some instances of this kind of thing
are known in the fossil record); mass kills like the Albertan bone beds,
the Cleveland-Lloyd Quarry, and the Ghost Ranch quarries are other
examples.  But if some tremendous catastrophic event killed massive amounts
of organisms globally, globally the mechanisms for creating fossils still
haven't changed.  That is, the rivers and their floodplains (to take one
terrestrial example) haven't suddenly been overwhelmed with extra amounts
of water and sediment with which to flood the countryside to hitherto
unexplored extents!  There may be more dead animals around, but since the
fossilization mechanisms are the same, only the same number of organisms
will make it into the fossil record.

        There's a name for this:  the Signor-Lipps Effect (named after its
discoverer, Joe Effect).  I don't know a great deal about it, other than to
say that because of it, there is some not-insignificant amount of argument
about the clarity of the terrestrial fossil record at, for example, the
close of the Cretaceous.

        Another thing to keep in mind is that it's dangerous to extrapolate
from the record of dinosaurs in western North America what was occuring
with dinosaurs globally.  Western North America just happens to be one of
the only places on the planet which records a relatively steady number of
terrestrial vertebrate fossils (and plants, too!) both below and above the
K/T boundary.  So really, when we say that the dinosaurs went extinct at
this boundary, we're really saying that they went extinct at the boundary
_in North America_.  We don't, as yet, know this as certainly anywhere else
(although investigations are underway!).  Even if we work from the starting
point that the Chixilub impact is the ultimate cause for the extinctions of
the time, western North America is relatively close to the site, and is
probably going to experience the effects of the impact sooner, with greater
density, and probably faster than anywhere else.  So, even _if_ the impact
is the cause of the extinctions, it may have occured quicker in North
America than anywhere else, and dinosaurs really may have gone out a tad
more gradually elsewhere (of course, if we're dealing with a period of even
just a few years, such resolution is unlikely to be visible in the
terrestrial record!  But that's a whole 'nother argument...)

Jerry D. Harris
Denver Museum of Natural History
2001 Colorado Blvd.
Denver, CO  80205
(303) 370-6403
Internet:  jdharris@teal.csn.net
CompuServe:  73132,3372


"Oh greeeeeat, now I get to spend the
summer with my _braaaaain_...

                                                -- Bart Simpson
