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Meta- vs. eutherian K-T extinctions (was Re: K/T impact)



----- Original Message -----
From: "John Bois" <jbois@umd5.umd.edu>
To: "David Marjamovic" <David.Marjanovic@gmx.at>
Cc: "The Dinosaur Mailing List" <dinosaur@usc.edu>
Sent: Wednesday, May 16, 2001 3:07 AM
Subject: Re: K/T impact (was Re: [for some reason] Tim's theory)


> On Tue, 15 May 2001, David Marjamovic wrote:
>
> > If eutherians (include placentals) are thought to originate from Asia
> > (this is not completely sure, AFAIK) and were superior competitors to
> > metatherians (include marsupials), then we'd expect that there were no
> > marsupials in LK Asia.
>
> But there _was_ an invasion of placentals moving, apparently, from North
> to South and timed just at the K/T.

The earliest South American metatherians are either from just below it or
just above it, dating seems to be controversial.

> If the new animals invaded and
> were successful enough to leave a pretty good fossil record, niche
> space had to be surrendered by the marsupials.  This has nothing at all to
> do with Asia. On the contrary, the success of different body plans is
> utterly dependent on the context of specific niches.

At least some NA metatherians (Marsupialia is the crown clade) had niches
that no eutherian (Placentalia is also a crown clade) occupied. If I look at
http://www.dinosauria.com/jdp/misc/hellcreek.html, "The Mammalia from the
Hell Creek Formation", I can't see metatherians becoming rare in comparison
to the eutherians... there are more of the former.

> For argument's sake,
> the placentals evolving in Asia may have been involved arms races with
> marsupials, such that marsupials were more competitive there.

So why the [vertical gene transfer] didn't the Asian metatherians come with
the Asian eutherians to NA and remained there happily ever after?
Is it actually sure that placentals came from Asia to NA? I mean, various
sister groups are strewn all across the northern hemisphere...

> > In short, marsupial extinctions at the K-T in NA aren't consistent at
all
> > with placental replacement. Makes the choice rather easy.
>
> Then how exactly did marsupials get snuffed when placentals
> didn't?  Marsupial bolide homing device?

Differences in physiology... different susceptibility to pyrotoxins... what
do I know?

from another post, also by HP John Bois:

> The claim that the bolide killed
> marsupials and left placentals untouched strikes me as completely
> fanciful.

It is. Marsupials were just hit much harder than placentals. In western
North America.

from yet another post:
> No, I'm going to tell you that science progresses by rigorous examination
> of alternate hypotheses;

Indeed. The impact can explain all K-T extinctions at once, and it
happened...

> that the bolide hypothesis for marsupial
> extinction is weaker on its face than competition with placentals because
> 1) replacement is a ubiquitous fact of the fossil record;

Replacement yes. After a group has died out, another one slowly evolves to
fill that niche.

> 2) competition
> and predation are phenomena which are observed and
> demonstrable;

Competition and predation are not the same in the least, and also not
related. And competition is rare, much rarer than thought for a long time.
Even things like the Great American Interchange have much less to do with
competition than initially thought, AFAIK.

> 3) bolide-caused extinction of any organism has not been
> observed and may be untestable (this doesn't mean it didn't happen--just
> that obserable phenomena that can explain things are preferable to
> unobservable phenomena than can explain things)

As HP Tim Williams noted, "Our judgements on the relative likelihood of
competing scenarios should not be limited to what we, as humans stuck in a
particular time and place, can see and touch directly." The present is not
always the key to the past. The past is the key to the present, and to the
future.*

*(c) Night Comes to the Cretaceous, IIRC

> --while lack of food has
> been proposed for dino extinction (on account of their greater size)  no
> one to my knowledge has proposed _any_ proximal mechanism
> for marsupial extinction due to bolide.

Not just lack of food, as I just wrote. An impact that size is so
catastrophic that not extinction but survival must be explained, and this is
greatly affected by untestable chance. Survival of the luckiest.
30 % of lizard species died off, IIRC.

> Then, if you subtract marsupials, and birds from the "mass" extinction,

Why birds?

> you do not have such a massive extinction

Still enough remains. Everything that could fossilize, at least, was
affected.

> and you lose the kind of
> umbrella explanation-for-everything provided by uncritical examination
> of bolide theories.

Uncritical! Ha!

Sometimes the causes of complex changings _are_ simple. It _is_ a
catastrophe when a mountain falls down from the sky.