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A long post about DUMB METATHERIANS vs EVIL, SMART PLACENTALS



> > > 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 [...]!
>
> 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?

So they are not outcompeted by placentals.

> > 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.

Only placentals have ever got into sea and air (hypotheses have been made
that because because marsupials need quite well-formed forelimbs at their
crazily early birth, marsupials with highly altered forelimbs may be
impossible; of course, one can easily say that this is based on negative
evidence). And land -- I'd say that placentals were just faster to
diversify.

> Or are you seriously suggesting that the
> opossums have only just begun their take-over?

Of course not. I'm seriously arguing that there is no take-over at all going
on in this case, even though the opossums don't enter a completely empty
niche, AFAIK.

> Or are you saying placentals are it thanks to a series
> of unfortunate bolide strikes over the Cenozoic.

This is exaggerating it, but chance does play a role in evolution.

> > > 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.

Oh! I didn't know either! :-o

> > Just once more on the biggest competition scenario, the Great American
> > Interchange -- quite something happened there,

means: quite something died out, though last time I read something about it
the dating issues (did they die out before the land bridge formed?) were
unresolved or debated or something.

> > 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.

Well, I'm trying to say that if humans had never entered SA, *Toxodon* and
*Macrauchenia* (and lots of other Pleistocene megafauna) might still not be
extinct. Of course, this is a speculation based on debated evidence.

> After all, our big brains are afforded by our reproductive
> mode.

I've read the contrary -- our brains are so big as to considerably impede
birth. In a marsupial the brain could grow much larger in the pouch because
it needn't pass the pelvis.

> And, it's not as if we are giving favoritism to our fellow
> placentals.

Of course not, why?

> > > 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. [...]
> All extinctions are likely have multiple causation.

OK, you're right that grinning or my following statement are not scientific
arguments (by themselves). Just IMHO your last sentence is on the same
level -- IMHO background extinction is terribly complex and would need to be
explained for every single species anew, but mass extinctions are something
totally different.

> 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.

You call this a catastrophe when you compare it to the K-T??? Mass
extinctions affect the whole globe and practically all habitats and clades,
and they do not grade into background extinction.

> 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.

I hereby claim that "ecological forces" become practically irrelevant when a
big meteorite strikes. They may affect survivorship and surely affect why,
in the case of the K-T, the mammals and birds rather than the crocs or frogs
took over the world, but not the extinction itself.

> > 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.

Just as I feared :-( . So they don't say much that's testable.

> 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.

May explain why they were faster to diversify after the K-T, but not why
more marsupials than placentals died out at the K-T, if I'm correctly
informed.

> > 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?

That those eutherians didn't survive in SA? No, AFAIK, but I haven't read
the whole paper and don't have it here. (I couldn't think of an explanation,
neither of one why the NA and European metatherians died out.)

> > > Most extinctions which involve dead ends of phylogenetic branches
> [...]

thanks for your clarification

> or are you suggesting the ultra-catastrophist position of Niles Etheredge
> (sp?) who says extinctions ONLY occur during catastrophies?

Eldredge and Gould's Punctuated Equilibrium? I'm not suggesting that. BTW,
this is not as extreme as it sounds, it just consists of the hypothesis that
speciation is a rather sudden event that involves genetic bottlenecks.

> 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.

But totally uncomparable to a mass extinction like Eocene-Oligocene, K-T,
Tr-J, P-Tr...

> > > 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.

Means that the first paragraph of my previous post (IIRC) stands
unchallenged. I didn't mention any terrestrial invertebrates (because I know
of none that died out then).

> There is no fossil record of birds across the boundary.

There is one, at least for footprints. 60 cm above the boundary,
"transitional shorebirds" were alive and well, just like some meters below
it.

> It is not known whether enantornithines
> disappeared before or at the boundary, nor whether this
> [my interpretation: the extinction of enantiornithines] was a gradual or
> catastrophic extinction.

True.

> 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_).

Pterosaurs? Lizards? Zalambdalestids? Freshwater animals? I don't buy it...
_Pseudoextinction is accounted for in the numbers_ because they are from
(what I remember of) the 1996 book by J. David Archibald (ref later if
needed).

How do you explain the marine extinctions, BTW?

> > > 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.

But different ones than after the boundary, AFAIK...
You know the SVP meeting abstract from 2000 that states that, based on
postcranial evidence, zhelestids are not related to (cet)artiodactyls but to
zalambdalestids and therefore have become extinct at the K-T?

> 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.

Ah! (Could you give me the ref? :-9 )

> > > 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 can say this in different words -- it allows me to have just one cause for
lots of complex consequences.

> I am trying to
> decouple extinctions from any such _event_ and suggest they were
> _processes_.

The suddenness with which mass extinction happen in the fossil record, apart
from other things, argues _strongly_ against this.

> After all, what you are suggesting is something we have
> _never_ seen: extinction without first being endangered.

True.

> In every case
> that we are aware of, species first become stressed, then they blink
> out.

Equally true. Well, we can never observe historical processes. As I quoted,
the present is not necessarily the key to the past. The past is the key to
the present, and to the future.

> (check archives).

I haven't gotten farther than August 1994 so far... :-(

> > 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.

Such as?
Well, I'll reread that chapter in his Vertebrate Palaeontology, 2nd Edition.

> 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.

"Any known catastrophy" -- hm. I wouldn't say that. Being rather uninformed,
I must leave this to others to answer.

> 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.

Parental care is suspected for "therapsids" rather than for early
archosaurs, and things that don't fossilize are negative evidence -- relying
on this is largely untestable IMHO.

> 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.

Fashions can mislead terribly, but they can also lead to the discovery of
realities that were unknown when other fashions dominated. Atomism was a
fashion in chemistry for a long time, and now it has become an observable
fact. I could list lots of other examples.

> > 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.

Just that we're not talking about islands when discussing the K-T or just
western NA during the K-T, we're talking about the whole planet respectively
an entire continent. The Pleistocene megafaunal extinctions, whatever their
cause(s), did not happen overnight.

> 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.

You don't suggest that dinosaurs evolved in Tibet -- the only isolated
continent in the Triassic, drifting in the middle of the Tethys -- and
miraculously escaped over hundreds of km over the ocean tens of Ma before
Tibet crashed into Pangea? :->

"Macroevolution" doesn't happen in places where it leads to competition (if
I read what I know about the fossil record correctly). Why should it?

*****************************************************
Did you want to answer to the following and clicked "Send" too early?

> > 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).