Re: DUMB METATHERIANS vs EVIL, SMART PLACENTALS

["Tim Williams" <twilliams_alpha@hotmail.com>]

Darren Naish wrote:

> 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. 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.


To add to that, a tooth (named _Tingamarra porterorum_) from the early 
Eocene of Murgon in southeastern Queensland, Australia, has been regarded as 
pertaining to a "condylarth".  This is a paraphyletic group of placentals 
associated with the origin of subungulates, ungulates and whales.  This 
lineage of placentals obviously did not burgeon on the Australian landmass 
during the early Cenozoic, in stark contrast to the diprotodont marsupials.

I haven't heard anything recently on the affinities of _Tingamarra_, 
especially in light of the "break-up" of the Condylarthra.  However, some 
skeletal elements (including an ankle bone) from Murgon may be referrable to 
the genus and may clarify its status.

By the way, marsupials have a very neat way of reproducing.  Compared to 
placentals, a mother marsupial invests less nutrients in the young during 
the brief gestation period - the most physically rigorous period for the 
mother and the most precarious period for her unborn young.  The birthing 
process is a lot less taxing on the mother as well compared to placentals - 
and if the young is sickly or deformed, it has less chance of making the 
journey from the uterus to the pouch.  Hence, unfit offspring are "weeded 
out" at an early stage of development, before the mother has invested the 
foetus with too much of her valuable resources.

In some marsupials (including kangaroos) a "spare" embryo is available in 
the uterus, and is held for several months in a sort of stasis (called 
diapause).  This embryo (a blastocyst - a ball of 70-100 cells) is "ready to 
roll" if the first litter dies prematurely - or if the litter survives and 
has left the pouch.


Tim

------------------------------------------------------------

Timothy J. Williams

USDA/ARS Researcher
Agronomy Hall
Iowa State University
Ames IA 50014

Phone: 515 294 9233
Fax:   515 294 3163

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