Author: Chris Nedin (cnedin@geology.adelaide.edu.au) Title: The Natural History of Marsupi

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Author: Chris Nedin (cnedin@geology.adelaide.edu.au)
 Title: The Natural History of Marsupials
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> The question is not how the escaped from Australia etc, but how
> they got back there. But the evolutionist faces this problem as well.
> Would one of you care to volunteer your answer?

The marsupials and placentals diverged from a pantotheran stem stock
in the late Cretaceous. The first marsupials appeared in North America
approx. 80 Mya e.g. _Alphadon_ (marsupials can be distinguished from
placentals by their dentition - marsupials have 3 premolars and 4
molars whilst placentals have 3-5 premolars and 3 molars).Towards the
end of the Late Cretaceous marsupials start appearing in South America
(Peru and Bolivia).  In the Eocene marsupials radiated into Europe,
North Africa and reached Asia by the Oligocene. However these groups
rapidly became extinct.  South America parted company with North
America in the Eocene, effectively blocking the rapid radiation of
placentals in North America at this time from spreading to South
America. During the Eocene, marsupials reached Antarctica, which was
attached to South America and Australia at this time.  Marsupials
could follow a belt of _Northophagus_ vegetation all the way around
from southern South America, across Antartica into southern Australia.
The first marsupials appear in Australia in the Oligocene via this
route. Australia parted company from Antarctica in the Miocene
effectively isolating the marsupial fauna here.

Although there has been a marsupial fossil found in the Oligocene of
Asia, it closely resembles the European form _Peratherium_, which is a
didelphid, and has little affinities to Australian forms. Therefore
collonization of Australia from the north is not considered viable.
Australian marsupials (extant and extinct) share many affinities with
South American marsupials and extinct Antarctic forms, indicating a
southern migration route for marsupials and explaining the lack of
placental mammals in Australia. By the time South America redocked
with North America in the Plio-Pleistocene, South America was already
separated from Antarctica and Antarctica from Australia. So the
reintroduction of placentals into South America couldn't continue on
to Antarctica/Australia. Note: this does not preclude *any* placentals
from reaching Australia, just *significant numbers* of them.

Evidence:
Fossil 
- the close affinities between Australian and South American/Antarctic
forms compared with European/Asian forms.

- The occurrance of _Northophagus- vegetation in Australia, Antarctica
and Soth America at this time. (Important note: _Northophagus_ seeds
cannot tolerate prolonged immersion in sea water - they degrade
rapidly i.e. no transport by sea).

Radiometric:
- dating shows the chronological progression of marsupials out from
Noth America, through South Ameriaca and to Australia

Palaeomagnetic:
Apparent polar wander paths for Australia and Antartica are identical
prior to the Miocene - indicating that before the Miocene
Australia/Antarctica acted as a coherent unit i.e. were joined.


|Plio-
|  Pleistocene  | ----- Nth/Sth America redock
|-------------- |
| Miocene       | ----- Antartica/Australia split
|-------------- |
|               | ----- Marsupials extinct Asia/Nth Africa/Europe
| Oligocene     | ----- Marsupials in Australia/Asia
|-------------- |
|               | ----- Marsupials extinct Nth America
|               |
|               | ----- Marsupials reach Antarctica
| Eocene        | ----- Radiation of Placentals in Nth America
|               | ----- Sth/Nth America split, 
|-------------- |                       Marsupials in Europe
| Paleocene     |    
|--------------------
|               | ----- Sth American Marsupials
| Late          |
|   Cretaceous  |
|               | ----- First Marsupials (Nth America)
|               |