Re: Extinction

[John Bois <jbois@umd5.umd.edu>]

On Fri, 24 Mar 2000, Tommy Tyrberg wrote:

> 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.

In my life time, science has determined that there is and that there isn't
enough matter to contract the universe. Suuch pronouncements are usually
made with a parenthetic: "...but we're not exactly certain."  I am sorry
to say that many in the astro field lack such humility and state as fact
that which they cannot know, viz, the effect of an asteroid on planet
Earth's ecology.
Now, you can _see_ the universe expanding, you can see the moon move from
one side of the sky to the other and so on.  We have never seen that which
you claim.  Indeed, the variables are so massively complex it must be
guesswork.

> >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?

There is not enough evidence to distinguish between the two main
hypotheses: enantiornithines were diverse and numerous right up to the
K/T; and, enantiornithines were gradually outcompeted by neornithines
culminating at the K/T.  I will say this though.  I think it is incredibly
far-fetched to say (as does Feduccia) that all neornithines are the
ancestors of a few lucky shore-bird survivors.  Such a claim suggests,
a) enantiornithines could not compete in the shorebird niche (but we know
they were a diverse taxon); or, b) the asteroid had smart-bomb
capability.

> 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?

Just parroting what I read in a recent Lillegraven article (ref. if
needed).  I'll check it.

> >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.

Archibald has argued persuasively that there was a great confluence of all
these forces just at this time.

> As far as I know Carnivora do not turn up until well into the Paleocene.

Earliest Paleocene (Fox and Th... 1994) ref. if needed)

> 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.

The Western Interior seaway drained thus taking away a huge area of
coastal plain.

The biogeography of these events is a challenge to any hypothesis since we
only have a record of the transition in North America--hence my 
invoking Carnivora.  However, if it is true that today's predatory guilds
limit the distribution of large egg layers to grasslands, wetlands, and
places with low predator density (e.g., New Zealand), it is worth looking
toward the point where these guilds first came into existence.  I believe
birds are a very important limiting factor in this--ostriches and rhea are
both suffer tremendous mortality of chicks from birds.  Plus, marsupials
were probably effective, too.  If this is the answer to the question
of: why no reconvergence to the dinosaur-size form? then it's worth
looking more closely for the time when this first started being true.