[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: DUMB METATHERIANS vs EVIL, SMART PLACENTALS



> 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.

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)!

> > 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.

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?

> 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.

> I am relying on the notion
> of alien invaders and a lack of defence in the natives--a common enough
> phenomenon.

A common enough phenomenon on small islands and/or when humans are involved.

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.)

> But what happens when an entirely new player comes
> in?

I'm not so sure whether placentals really were totally new to NA at Hell
Creek times. After all, NA and Asia were apparently connected over most or
all of the LK and probably even some time earlier. Does someone know more on
this?

> 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).

> > 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)?

No, there are small ones: high up in high trees (Burramyidae), the honey
possum (*Tarsipes*), and -- perhaps more relevant -- "marsupial mice" (a
comparison with shrews would fit better) which are included in Dasyuridae
with quolls, Tasmanian Devils and thylacines. There are also very small
marsupials in SA among Didelphidae, Caenolestidae and Microbiotheriidae.

The indigenous Australian murids inhabit dry areas and stay there, AFAIK
(they seem to have entered an empty niche, I'd say).

What actually is called a possum in Australia?

> 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..."

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

> Am I missing something?  I know there were multis
> and monotremes--but they are _not_ what I thought was being discussed.

Multis survived the spread of plesiadapiforms and rodents -- all possible
competitors, one might think -- in NA and died out, AFAIK, in the Oligocene
(with the primates?). Monotremes have, as far as is known, never reached NA.

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.

> > 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

What do you mean? When a phylogenetic branch becomes extinct, it becomes a
dead end. This is circular.

> are
> caused by either predation or competition.

Examples, please.

> 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.

> 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.)

> 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...

> 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).

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