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Re: (extinction)



----- Original Message -----
From: "John Bois" <jbois@umd5.umd.edu>
Sent: Wednesday, July 03, 2002 2:55 PM

> > > Marsupials suffered only local extinctions--
> >
> > Metatheria as a whole suffered greatly.
>
> Is it worth making the point that they survived and all the non-avian
> dinosaurs didn't?

My point is that they _barely_ survived. That you are wrong in writing that
"only local extinctions" took place and that non-neornithean dinos were the
only really affected tetrapods.

> > > Crocs live.
> >
> > Not all.
>
> Is it worth making the point...?

At least in part, the pattern of croc extinctions and survival is compatible
with the impact. If is _better_ compatible with something else, then I'd
like to see evidence for that. If the latter isn't the case, then this is of
course _no_ evidence per se that the impact did it, but then the impact
remains a viable explanation, and to date the only one thought up so far.

> >         It is expected that generalized crocs survive such an event.
They
> > aren't dependent on "green food chains", they can eat carrion and fast
for
> > months.

That's the ecological part. :-) Animals that ecologically resemble today's
crocs are probably good competitors in post-impact conditions (thanks, HP
Michael Habib!), when food is scarce (and competitors are likewise rare). --
Throughout the Paleocene AFAIK, crocodiles and sharks were the absolute top
predators and the biggest vertebrates in the world. One more reason why
crocs grin ;-)

> Emus also fast.  I would be surprised if fasting was not a part of many
> non-avian dinosaurs' routine.

I mean month-long fasting (as I wrote). Like an ectotherm. Emus don't do
that, do they?

> The pattern of extinctions on land is either targeted or diffuse--or, some
> combo.  Inasmuch as an entire series of niches were vacated by one
> clade--and probably no other--

If you don't accept my other examples for clades that were wiped out, then
please write why.

> the phenomenon is interesting to me and
> appears "targeted" (teleologically speaking).  As such, it requires
> explanation.

Well, as long as the pattern of extinctions is compatible with the observed
fact of an impact, I can accept the latter as explanation for the former,
especially considering that something must happen when a 10-km-rock impacts.

> The current fav. hypo demands no such explanation: whatever
> happened, happened because it happened.

Not quite so simply. See above.

> This might be OK if there were
> not reasonable ecological explanations for the phenomena (e.g.,
> competition from placentals, competition/predation from neornithines,
> etc.).  Until these have been ruled out,

They have been, as far as possible with today's knowledge of the fossil
records. If you don't think so then please reply to the rest of my last
post. I'm serious, I am interested in alternative explanations.
        Neither have they been substantiated at any time AFAIK. Where are
the great similarities between cimolestans on one hand and stagodontids and
pediomyids on the other? (Or, for that matter, those between stagodontids
and pediomyids?) Where are the Asian cimolestans that killed off the local
stagodontids (OK, I only know of one, and that's the unnamed [still?]
"Gurlin Tsav Skull", but still)? What finished Asia's endemic eutherian
clade, and Asia's Djadochtatheria? What reduced the diversity of lizards?
        In earlier decades, such competition scenarios (especially ones
according to which mammals helped decimate the dinosaurs) have often been
built on the supposed occurrence of *Protungulatum* (an arctocyonian,
closest living relatives: artiodactyls and whales), and by implications lots
of other crown-group placentals, in the end-Maastrichtian. Wrongly dated.
*Protungulatum* is known from the beginning of the Paleocene, but not from
earlier strata. (Refs, if needed, later.)

I have not seen any hypothesis (and only one speculation) that involves
Neornithes as a killing agent in the K-T. All I've seen so far that has been
used as evidence are molecular clock estimates that flatly contradict the
fossil record, see the two websites I mentioned yesterday.

Sorry: You are correct with "local extinctions" if you regard Asiamerica as
one spot. LK metatherians from elsewhere haven't been found yet, except for
the tooth from Madagascar that might (I guess by paleobiogeography) be
related to Australia's living marsupials.

Oh, er, and there's no need to send me everything twice (once directly, once
via the list). I read everything that comes from the list, unless Norton
Antivirus prevents me from it. :-)