Re: Extinction

[Tommy Tyrberg <tommy.tyrberg@norrkoping.mail.telia.com>]

At 15:11 2000-03-24 -0500, you wrote:
>
>
>On Fri, 24 Mar 2000 Dinogeorge@aol.com wrote:
>
>> Not exactly right. A mass extinction will have a >single< global cause,
not 
>> >multiple< causes. One such single cause can be asteroid impact. The
problem 
>> with multiple causation is to get all the causes to coincide in their 
>> effects; the more causes you throw into the mix, the less chance there is 
>> that your hypothesis will stand. The asteroid impact at the K-T boundary
was 
>> gigantic, with devastating worldwide physical effects whose traces have
been 
>> well documented in the geological record.
>
>It is not accurate to say traces of iridium are traces of a "devastating
>world-wide" event--unless you're claiming dinosaurs died from iridium
>poisoning.  And other such effects have not been shown globally.  Even
>fullarenes cannot kill dinosaurs as far as I can tell.
>
This is just quibbling. An impact that is large enough form a 150 km crater
and to emplace a centimeter-thick deposit world-wide would certainly be
extremely devastating (shockwave, heat radiation, ejecta blanket, tsunamis,
dust blanking out sunlight, injecting vast quantities of rock vapour, water
vapour, carbon dioxide, sulfur dioxide and nitrogen oxides into the
atmosphere just to mention the most obvious effects). To what extent these
effects these effects are visible in the geologic record is really
immaterial since there is no way a Chicxulub-type impact could happen
*without* having these effects.
>
>> If you say that this event and the 
>> K-T mass extinction had nothing to do with one another and are purely 
>> coincidental, you have to show positively why you are denying the obvious 
>> conclusion.
>
>Many things that are bagged into the mass extinction "event" occurred
>before: pterosaurs, mosasaurs, ammonites experienced a cretaceous-long
>decline, rudist clams.
>Some/many extinctions are pseudo extinctions, the result of rapid
>speciation (mammals, for example).
>Specifically, among terrestrial vertebrates the ONLY group that
>experienced true extinctions were the dinosaurs!  And, by the way, the
>"boundary" at sea has not been exactly matched to the terrestrial
>boundary.
>And temporal clusters of extinction may occur as a purely statistical
>phenomenon (shoot blindfold at the side of a barn and you will make
>clusters).
>
How about enantiornithine birds? And do You imply that the iridium anomaly
at sea and on land aren't the same age? And that mass extinctions are just
random fluctuations, even the Permo/Triassic one?

>> It is not sufficient to merely list possible alternative causes 
>> of mass extinctions and assert that they might have caused the K-T
extinction 
>> instead.
>
>But these "alternatives" have solid evidenciary basis. Volcanism,
>sea-level regression, mountain building, climate change, novel
>speciation--these things really did happen!!
> 
Yes, they happen all the time, and usually with no drastic effect on the
biosphere. There has been about 8 regressions of approximately the same
order of magnitude as the Maastrichtian (but probably a great deal faster)
just in the last 800,000 years with no discernable effect on either marine
or land biota. Climate changes have also been extreme in the last several
million years, also without causing any large-scale extinctions.

>> We have measured the motion of the moon...
>
>But many of the arguments revolve around the _difficulty_ of measuring
>things that happened 65mya.  In your analogy, we cannot accurately measure
>the motion of the moon; we can't tell which way it moves.
>
>> Similarly, anyone who denies 
>> the connection between the K-T mass extinction and the K-T asteroid impact 
>> must now >show<, not merely >assert<, that these events are disconnected
and 
>> coincidental.
>
>But this assumption that the burden of proof is on this side is just
>that--an assumption.  I assert the following: asteroids happen all the
>time; evolution of mammaloids may be a once-in-a-universe event.  Timing
>of dinosaur extinction occurs (where it is observed, at least) just at the
>time of the evolution of Carnivora.  This clade, and others--I
>hypothesize--have restricted the distribution of large egg layers ever
>since.  In addition, predation is a well-known cause of extinction.  We
>known nothing of any asteroid caused extinction.
>Other hypotheses are respectable.  Habitat fragmentation recorded at the
>K/T is another well documented agent of extinction.  So why this
>favoritism to your favorite hypothesis?
>
As far as I know Carnivora do not turn up until well into the Paleocene.
Also You are making a common mistake in constructing an explanation which
might work in North America, but not elsewhere. Carnivores can hardly have
exterminated dinosaurs in South America which they did not reach until the
Miocene, in Australia where they only arrived 4,000 years ago (the dingo),
or New Zealand (nineteenth century). Also I have never been able to
understand how a regression is supposed to cause habitat fragmentation,
normally it's the other way around, *rising* sea levels fragment habitats
and isolate populations.
>