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ARE METATHERIANS INHERENTLY INFERIOR TO EUTHERIANS?
The current diversity of metatherians (marsupials) is much less than that of eutherian (placental) mammals. Is this due to inherent competitive inferiority in the former?
Are metatherians inherently inferior to eutherians?
Yes, because... Yes - Biogeography supports outcompetition
Metatherian fossils have been found throughout the world, but only remained diverse in South America and Australia until recently. Once the isthmus of Panamá formed and the Great American Biotic Interchange occurred, much of what had been a diverse marsupial metatherian assemblage - including hypercarnivores such as borhyaenids - in South America was replaced by placentals. Australia remained relatively isolated from eutherians, and this is why marsupials dominate the fauna today
Though Eutherians have caused the extinction of at least one notable marsupial - the Thylacine - in modern times and restricted the range of several others to Tasmania also.
Only the borhyaenids really went extinct with regards the South American metatherian radiation - caenolestids (shrew opossums), the monito del monte (Dromiciops - an Australodelphian), and didelphimorph opossums are still with us today. Borhyaenids probably went extinct just prior to GABI, due to climate change not outcompetition. Opossums even made in northward and now the range of Didelphis reaches as far as Canada.
Australia is not entirely isolated - placental species have made it across, but have not done as well as in other areas. Metatherian tree kangaroos also thrive in New Guinea, which has yet more placentals.
Vote on this point: Yes - Biogeography supports outcompetition
See history of changes to this point
Vote on this debate: Are metatherians inherently inferior to eutherians?
Modified:
06 Apr 2010
Point 1. Yes - Biogeography supports outcompetition
Metatherian fossils have been found throughout the world, but only remained diverse in South America and Australia until recently. Once the isthmus of Panamá formed and the Great American Biotic Interchange occurred, much of what had been a diverse marsupial metatherian assemblage - including hypercarnivores such as borhyaenids - in South America was replaced by placentals. Australia remained relatively isolated from eutherians, and this is why marsupials dominate the fauna today
Only the borhyaenids really went extinct with regards the South American metatherian radiation - caenolestids (shrew opossums), the monito del monte (Dromiciops - an Australodelphian), and didelphimorph opossums are still with us today. Borhyaenids probably went extinct just prior to GABI, due to climate change not outcompetition. Opossums even made in northward and now the range of Didelphis reaches as far as Canada.
Australia is not entirely isolated - placental species have made it across, but have not done as well as in other areas. Metatherian tree kangaroos also thrive in New Guinea, which has yet more placentals.