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Re: DUMB METATHERIANS vs EVIL, SMART PLACENTALS




On Thu, 17 May 2001 darren.naish@port.ac.uk wrote:

> On metatherians vs placentals and so on....
> 
> I haven't been following this thread, so apologies if I'm adding 
> comments that don't add up to much BUT - - the concept that 
> metatherians are somehow intellectually (??) or 'adaptively' (??) 
> inferior to placentals is surely very very flawed. I don't know if John 
> Bois is actually stating this in his communications but the inference is 
> certainly there.

I made no such inference (intentionally, anyway).  Adaptations owe their
success to specific environments.  I am a strict relativist.  However,
there are absolutes in terms of specific abilities and their effect on
ecological communities.  In many if not all marsupials, brain development
is delayed in favor of developing the facio-cranial muscles and bone
structures necessary for early suckling.  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.  One area in 
which I would like to see more research is how this translates in actual
foraging--particularly in locating prey.  But, for a koala, this is
totally irrelevant, right.  She has a digestive system which is far
superior to any other eucalyptivore.

> Marsupials successfully invaded Europe, eastern Asia and northern 
> Africa during the Palaeocene-Eocene and stuck it out in Europe until 
> the Miocene. In historical times,  _Didelphis_ has moved north 
> through N. America and now occurs as far north as Canada. This all in 
> the proverbial face of established placental competition.

Yes.  Not sure what your point is.  In noting the possibility of placental
predation and/or competition of K/T marsupials, I am relying on the notion
of alien invaders and a lack of defence in the natives--a common enough
phenomenon.  I grant that this may be reversed--marsupials taking over
naive climes.  Indeed, this is a fascinating subject.  Wilson's
island biogeography studies infer a relatively fixed number of
niches.  But what happens when an entirely new player comes
in?  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.

> Also worth 
> noting is that Australasia is far from a 'marsupial haven': murids have 
> (probably) been in Australia since the Eocene yet there has never been a 
> mass die-off of small Aussie marsupials at the hands of ruthless clever-
> witted rats and mice. 

Is it relevant that in my first 26 years in Melbourne I saw lots of rats
and mice--but no small marsupials (i.e., below possum-size)?

> > Then how exactly did marsupials get snuffed when placentals
> > didn't?  Marsupial bolide homing device? 
> 
> The [Cretaceous] animals being discussed are not marsupials: they are 
> metatherians.

I quote Sergeant and Currie 2001 The "Great Extinction" that never
happened: the demise of the dinosaurs considered. _Can. J. Earth Sci 38
239-247.
"...there was a quite rapid expansion in the number of condylarths in
North America and a corresponding reduction of the marsupials with which
they were competing..."  Am I missing something?  I know there were multis
and monotremes--but they are _not_ what I thought was being discussed.

> John's theories seem to rely on simple notions of 
> competitive replacement, an idea that is too simple when we're talking 
> about ecologically diverse groups consisting of hundreds of species. 
> Other researchers recognised this several decades ago.

Most extinctions which involve dead ends of phylogenetic branches are
caused by either predation or competition.  Species don't just give up the
ghost and say: "I've got to become extinct now, goodbye."  There are
periods when such extinctions are more intensive than other periods.  The
K/T would appear to be one of those times.  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, and that, therefore, large scale
competition and predation replacement was more likely at this time.  Which
researchers argued against this?