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Re: DUMB METATHERIANS vs EVIL, SMART PLACENTALS



On Fri, 18 May 2001, David Marjanovic wrote:
Bois wrote:

> > Brains are energy and time hogs
> > in development--but the luxury of being able to spend more time on this
> > may or may not be an advantage depending on the environment.

> So what? Opossums, with their rather small brains, have successfully entered
> NA, and now plunder the city dumps together with the much larger-brained
> raccoons (I have never been to the USA, I'm citing this and have forgot the
> ref, as usual)!

How is this relevant to paleoecology?  Opossums in the wild likely have
different enough niche requirements so that they can coexist with
racoons.  For now, anyway.  So?
 
> Very simple -- why should placentals have outcompeted those LK NA marsupials
> that died out at the K-T if this didn't work anywhere and anytime else?

Good grief!  Placentals are the dominant mammal forms across most of the
planet--on land, sea, and air.  Or are you seriously suggesting that the
opossums have only just begun their take-over?  Or are you saying
placentals are it thanks to a series of unfortunate bolide strikes over
the Cenozoic.

> > In noting the possibility of placental
> > predation and/or competition of K/T marsupials,
> 
> Never predation. There were badger-sized Tasmanian Devil-analogs in LK NA
> (e. g. *Didelphodon vorax*), but the placentals were all much smaller.

_Cimolestes magnus_ was comparable in size.  And, anyway, _D. vorax_ is
irrelevant because it survived the K/T.  

> Just once more on the biggest competition scenario, the Great American
> Interchange -- quite something happened there, but phorusracids, ground
> sloths, glyptodonts, and (alone among Notoungulata and Litopterna) *Toxodon*
> and *Macrauchenia* survived it long enough to have encountered humans. (Ref:
> a big book from 1983 with mammalian paleofaunal lists, I can find it at a
> library.)

Humans are placentals.  I'm not sure that a fair minded
alien ecologist would exempt us from contributing to placental
domination.  After all, our big brains are afforded by our reproductive
mode.  And, it's not as if we are giving favoritism to our fellow
placentals ("Now, son.  Go out and kill all those little marsupial
bastards so our little rat friends can have their niche space.")

> > Perhaps part of my unwillingness to accept the bolide is due to a
> > desire to have these ecological concepts be relevant at the K/T.
> 
> <broad, open grin for which there is no smiley> Ecologican concepts are very
> relevant in normal times, but not at mass extinctions. They just can never
> explain it (as long as they are less extreme than, say, Snowball Earth).

Again, this is what we are arguing.  Gainsaying is not the same as
arguing.  All extinctions are likely have multiple causation.  Even
catastrophies are played out within an ecological context.  For
example (though not proven), local extinctions of Panama birds were caused
by explosion of small mammals populations, which was due to large cats
swimming off because humans flooded once continuous range making
mountain-top islands.  This is what I find so horrendous about the
dominion--self-proclaimed in Night Comes To the Cretaceous--of physical
sciences: a near total disregard of the complexities of ecological forces.

> What do Sergeant & Currie include in "condylarths"? Normally this is a term
> for early herbivorous placentals, and no LK NA metatherian was herbivorous
> AFAIK.

They don't specify.  However, there was also an increasing
dietary diversity in endemic placentals--and an increase in
size!  _C. magnus_ is the biggest placental ever seen around those parts.
 
> BTW, a new JVP paper (March 2001) says that all early SA marsupial and
> placental sites (Tiupampa in Bolivia, Itaboraí in Brazil, something in
> Peru...) are all early Paleocene. They contain possible fragments of the LK
> NA insecti-/carnivorous eutherian *Cimolestes* and apparently several
> "condylarths" that didn't survive for longer in SA.

Do they hazard a hypothesis for how come?

> > Most extinctions which involve dead ends of phylogenetic branches
> 
> What do you mean? When a phylogenetic branch becomes extinct, it becomes a
> dead end. This is circular.

I meant to say "all extinctions other than pseudoextinctions", i.e.,
fossils which look different but who spawned surviving species.  For
example, every creature from whom you descend is not really
extinct--but, something like a cretaceous hadrosaur is extinct because it
did not contribute DNA to any living creature.

> > are
> > caused by either predation or competition.
> 
> Examples, please.

Of course we can't know what the circumstances were for _any_ extinction
event--except those we observe, today.  But we infer because of an
absence of alternate hypotheses, that is, in non-catastrophic times--or
are you suggesting the ultra-catastrophist position of Niles Etheredge
(sp?) who says extinctions ONLY occur during catastrophies?  And then, how
do you define "catastrophy"?  Foxes released onto goose nesting islands
recently caused the near extinction of some species.  This was
catastrophic for the geese, but a simple predation event for the
foxes.

> > There are
> > periods when such extinctions are more intensive than other periods.  The
> > K/T would appear to be one of those times.
> 
> No, mass extinctions are not just times of increased background extinction,
> they are a totally different phenomenon where whole large clades die out and
> practically everything is affected nearly regardless of its habitat.

Let me get it right this time: except for dinosaurs, nothing much happened
in terrestrial _vertebrate_ communities.  There is no fossil record of
birds across the boundary.  It is not known whether enantornithines
disappeared before or at the boundary, nor whether this was a gradual or
catastrophic extinction.  All other vertebrate extinctions--except
dinosaurs--can be explained (I should say, can be hypothesized) by
invasions, replacements, pseudoextinctions, and sympatric evolution (e.g.,
_D. vorax_, and _C. magnus_).

> > I'm not saying that I know
> > that it happened, just that there were many new evolutionary
> > prototypes--carnivores, primates (?), neornithines, etc., that were
> > diversifying just at this time,
> 
> No. Wrong. After it, and because of it (an empty world -- lots of empty
> ecological niches -- every silly mutant that survives at all can found a new
> species). (And only if you include creodonts in carnivores, AFAIK.)

Of course there was big-time radiation into empty niches after dinosaurs
became extinct.  But, in terms of niche diversity and morphology, changes
were already underway before.  A small carnivore has been
found at the earliest Paleocene; this is at least suggestive of a pre-K/T
carnivore--sorry, I should have made that clear.

> > and that, therefore, large scale
> > competition and predation replacement was more likely at this time.
> 
> What on Earth can compete with a *T. rex*? Or a *Triceratops*? Or...

We are arguing about placental/marsupial and neornithine/enantiornithine
predation and competition.  The catastrophist position requires that you
bundle everything under the same extinction force.  I am trying to
decouple extinctions from any such _event_ and suggest they were
_processes_.  After all, what you are suggesting is something we have
_never_ seen: extinction without first being endangered.  In every case
that we are aware of, species first become stressed, then they blink
out.  If there is doubt that _all_ the K/T extinctions were part of the
event and that they may have been processes, then the mass extinction is
not so massive.  At that point we can talk about dinosaurs in their own
right.  Which I have (check archives).

> > Which
> > researchers argued against this?
> 
> Lots. For the sake of name-dropping, Michael J. Benton who explained what
> happened during the Triassic (earlier assumption: dinosaurs outcompeting
> other archosaurs outcompeting "therapsids" -- all wrong).

Well, Benton bases his conclusions on admitted ignorance of any specific
critical adaptations that would enable an archosauran take-over.  This is
strictly negative evidence; and very poor negative evidence at
that!  Saying that dinosaurs radiated thanks to luck--in this case, even
without any known catastrophy--is just a wild guess.  Benton expects to
see "suites of adaptations" and finds none.  But, for example, behavioral
traits (which don't fossilize) may be critical for all we know.  An
important parental care trait may appear and have critical value.  Again,
I'm not saying that we know this, only that others also don't know.  Even
Michael Benton admits a "bias" to catastrophic explanations here.  As
such, he is a child of 20th. Century biology--a time when adaptationist
ideas were out of favor and chance was in favor.  This has more to do with
fashion than reality.  We must really admit we just don't know how this or
that thing happened--and leave ourselves open to alternate hypotheses.

> One would expect that a replacement by competition lasts long and is
> gradual.

This is so contingent.  I don't see why replacement by competition and/or
predation must run at any particular pace.  We've seen many island
extinctions happen overnight--predation and competition being the known
cause here.  Yes, niche dimensions are broader, extinction rates slower on
continents.  But a brand new evolutionary product--one with a critical
innovation--may have a rapid effect on naive populations when introduced
to a new continent. Dinosaur radiations may well be an example of
that--for all we know.




 This is never seen during mass extinctions (off the top of my head,
> procolophonids were doing perfectly fine half a million years before the
> Tr-J and were perfectly dead above it, and, according to interpretations of
> footprints, the maximum size of theropods increased by a third within 20,000
> years after the Tr-J because the rauisuchians were gone).
>