Re: Why does Australia have so many native marsupials

Georgina:

In order for animals to live in a place, they have to get there. For a 
very long time, Australia has been very difficult for land animals to 
reach. Some time between 60 and 70 million years ago, Australia began to 
separate from Antarctica, and since that time it has been drifting slowly 
towards Eurasia. The last time that Antarctica and Australia were 
connected to other continents was about 140 million years ago, when both 
of those modern continents were part of the supercontinent of 
Gondwanaland. What this means is that for the past 140 million years, if 
you did not already live in Australia it has been virtually impossible to 
get there. Only animals that could swim, fly, or raft across the ocean on 
floating debris could make the journey. 

What does this mean for land mammals? The oldest mammals are much older 
than 140 million years. However, these creatures were vastly different 
from, and more primitive than, any modern mammals. Most modern mammals are 
placentals (dogs, whales, rhinoceri, bats, etc.). Other modern mammals are 
marsupial (most Australian mammals, opossums, many extinct South American 
mammals) or monotremes (platypus and echidna). If one just considers 
living mammals, all monotremes, most marsupials, and a few placentals live 
in Australia and nearby islands, whereas the rest of the world's mammalian 
fauna is almost totally dominated by placentals. 

Placental mammals have a number of evolutionary innovations that help them 
make their way in the world. Key among these are live birth of babies 
nearly ready to live on their own, and the production of milk 
(specifically designed to nourish babies) by the mothers. Marsupials are 
more primitive, because their babies are born in a "premature state" 
compared to placentals, and because their milk delivery system is less 
efficient. Monotremes are more primitive yet: their babies hatch from 
eggs. 

But what does more or less primitive really mean in terms of living in the 
world or going extinct? Well, we have a natural experiment that gives us 
an answer to this question. For a long time, South America had two things 
in common with modern Australia: the continent was isolated (because 
Central America was under water) and the mammal fauna was marsupial--
placentals lived in North America, but most had never made it to South 
America. There was always a trickle of travel between North and South 
America, but beginning about 7 or 8 million years ago, most of Central 
America became exposed above the sea, and the trickle became a flood. By 1 
or 2 million years ago, many of the "primitive" South American mammals 
were extinct. It seems that when placentals and marsupials interact, more 
often than not, the marsupials disappear.

The oldest known placental mammals are about 79 million years old, and the 
oldest known marsupials are about 76 million years old [according to my 
sources; there may be newer data]. An estimate for the latest common 
ancestor of the two groups places that unknown creature at about 116-133 
million years ago. Recalling that Australia has been more or less isolated 
for 140 million years, and almost completely isolated for at least 60 or 
70 million years, it seems that a great deal of luck was needed for either 
marsupial or placental mammals to colonize the continent at all. The 
Australian mammalian fossil record is scanty, but it indicates that 
marsupials were already widespread, abundant, and diverse around 30 
million years ago (which means they must have reached Australia long 
before then). By contrast, aside from one small group of rodents and a 
certain kind of bat, all the placentals there (e.g., dingos, pigs) are 
recent immigrants; either known or suspected to have been brought to 
Australia by humans. The bottom line appears to be that, somehow, a few 
marsupials made it to Australia or were there when the continent broke 
away from Antarctica. For millions of years they have evolved and 
diversified to occupy all kinds of niches that in other parts of the world 
are now occupied by placental mammals. Placental mammals wiped out most of 
the marsupials in the rest of the world (including South America only a 
few million years ago) but because only a few placentals ever reached 
Australia, and most of those very recently, they have not (yet) succeeded 
in replacing the marsupials there. The two monotremes, even more primitive 
than marsupials, survive in Australia but just barely. If they had to 
compete with a diverse community of placentals in addition to the 
marsupials they are used to, they probably would not survive.

I hope this very brief summary helps!

David Kopaska-Merkel
Geological Survey of Alabama
P.O. Box 869999
Tuscaloosa AL 35486-6999
USA
(205) 553-2284
FAX (205) 349-2861
www.gsa.state.al.us

References

Lillegraven, J. A., Kielan-Jaworowska, Zofia, and Clemens, W. A., eds., 
1979, Mesozoic mammals: The first two-thirds of mammalian history: 
Berkeley, University of California Press, 311 p.

Simpson, G. G., 1980, Splendid isolation: The curious history of South 
American mammals: New Haven, Yale University Press, 266 p.

Smith, A. G., and Briden, J. C., 1977, Mesozoic and Cenozoic 
paleocontinental maps: Cambridge, Cambridge University Press, 63 p.