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)
|               |


Chris

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cnedin@geology.adelaide.edu.au   |  "How can Nedin be trusted"      |
Dept. of Geology & Geophysics    |   C Wieland  Director,           |
University of Adelaide           |   Creation Research Foundation,  |
South Australia  5005            |   Queensland Australia           |
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