|MadSci Network: Evolution|
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.
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