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Re: _Night Comes to the Cretaceous_ Ad Infintium




On Sun, 9 Aug 1998, David Lessin wrote:

> The problem here is one of resolution. Catastrpohic extinctions WILL look
> like a gradual extinction if too "coarse" a scope is taken. In order to
> regress the dangers of pseudo-extinction from the question at hand, simply
> do not look at species level extinctions. Look at the number of familial and
> genera represented before and after the extinction event. Individual species
> evolve much more quickly than families and genera. It is unlikely that all
> members of a family or genus would be caught in the middle of a speciation
> event during anytime, never mind a mass extinction event.

And when you do that and other adjustments the kill rate goes down,
according to Archibald: "In comparison to other intervals in the late
Cretaceous and early Paleocene, the K/T vertebrate turnover which is only
10% higher at the generic level."
( source for this is at
http://cass.jsc.nasa.gov/meetings/programs/kteventwa.txt)

> Yes it is true that many taxa were in a decline during the late Cretaceous.
> This decline, however, was in all likelihood, not significant enough to wipe
> out whole families GLOBALLY!

But universality is a problem for all explanations (see below).  

>  51% of all families (55% of the genera) in
> N.A. that have been associated with terrestial food chains were wiped out at
> the K-T, whereas only 8% of families (26% of genera)from  freshwater aquatic
> foodchains. Bony fishes, lissamphibia, turtles, and crocodylians show far
> greater survivabilty than lizards, snakes, dinosaurs, and MAMMALS.

>From J. David Archibald's: Dinosaur extinction and the end of an era: what
the
fossils say (pub. Columbia univ. press)
% species survival in N.E. Montana
Sharks =0
frogs and salamanders =100%
multituberculates = 50%
PLACENTALS = 100%%%%%%%%
marsupials = < 15%
lizards = < 30%
champsosaurs = 100%
crocs and alligators = > 75%

So it very much depends on which mammals you are talking about.  And I
really can't think why a bolide winter would favor placentals over
marsupials.  A more plausible explanation for this differential is
competition, i.e., placentals were outcompeting multis and marsupials!
And if this is so you have another major element missing from the
so called mass extinction.

> In modern
> ecology, it also well-understood, that freshwater aquatic biomes can recover
> much more quickly than other ecosystems.

But there are easier explanations than the bolide.  For example,
regressing seas create more stream systems.  This was occurring just at
this time.  (This also explains the disappearance of sharks and rays from
this particular strata).

> Further....., there is DIRECT evidence of PRIMARY SUCCESSION across much of
> this continent (from New Mexico to Saskachewan)- strangely enough, these
> sediments are deposited directly on top of the layer containing the iridium
> spike!

And yet at other locations (Europe, for example) searches for such a spike
have been unfruitful-even in places with a continuous sequence across the
K/T.  So universality is an issue here as well.  I know it is difficult
for any theory, but the great failing of the bolide-as-sufficient idea is
that it does not do a good job in the particulars.  How long a period of
photosynthesis reducing conditions is long enough to kill all
non-dinosaurs but leave all placentals unharmed?  How hot is hot enough to
kill all non-avians but enable mass survival of avians?  How cold is cold
enough...  How many firestorms?  How much acid rain is enough to kill all
non-avian dinosaurs and allow frogs and salamanders to live.  The devil
really is in the details.

> Given that all animal life is dependent upon plant life, I would pay more
> attention to the plant fossil record to determine what really happened to
> different animal groups at the K-T.  If anyone is interested, I have
> several references on this subject and will be glad to send them
> off-list.

I am sure there are enough of us interested for you to post these
publicly.

Recapitulating, the real challenge to this idea is to explain why
placentals, birds, frogs, crocs, turtles, and lizards survived an event
that killed _all_ large and small (remembering that there must have been
millions of small juveniles present at the time) non-avian dinosaurs
whether they were in hills, deserts, valleys, river terraces, forests,
plains, at all locations around the globe.