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THERIANS (marsupials and placentals)

     Therian mammals are those which give live birth to their young.  There are two groups of therian mammals: marsupials and placental mammals.

2) MARSUPIALS

OPOSSUM  

.  Marsupials differ from the placentals in their teeth and the possession of a unique marsupial bone in pelvis.  Their embryos are born alive but they are born at about the same developmental stage at which monotreme mammals hatch from eggs. Kangaroo infants weigh only about 1 gram at birth.  Marsupial newborns are ectothermic (cold blooded) when they are young due to their size.   Marsupials embryos possess a thin eggshell membrane and a vestigial egg tooth. This shell allows for diffusion but no contact between the mother and fetus which might trigger the mother’s immune system. (Lillegraven, ).  Marsupial development, which is completed during lactation in a pouch, takes longer and has a higher energy cost than placental development.  The malleus and incus (middle ear bones) are retained in a jaw joint when they are young; only after they leave the pouch to they become incorporated into the middle ear (Lillegraven, ).

     Why were marsupials so successful in South America and Australia?  North and South America were separated from the Upper Cretaceous until about 4 million years ago and therefore modern placental mammals reached South America only recently.  Australia, obviously, still lacks direct land contact with any continent where placental mammals dominate. 

     A diversity of fossil marsupials are known from South America and Australia.  They were carnivores that bore superficial similarities to medium sized placental carnivores.  Borhyaena resembled a wolf, Thylacosmilus a tiger sized saber tooth cat, and one was bear-sized.  Lycopsis was a coyote-sized predator.

LYCOPIS BORYHYENA
SKULLS MARSUPIALS

Diprotodon (above) measured 10 foot long and was the largest known marsupial.   Simostenurus was an unusual kangaroo which had only one toe on each foot.  The fossil kangaroo Procoptodon stood 2.6 m (8 ft.)

procoptodon  

 

     When North and South America joined about four million years ago, placental mammals from North America (cats, wolves, bears, deer, llamas, horses, elephants, and others) migrated to South America where they caused the extinctions of many primitive South American mammals, including the majority of marsupials.  There are still a number of marsupials which survive in South America, although they are limited in size from the that of rodents to that of opossums.

marsupial skins