Merck's Opinions on Australian Mammals

John Merck

Australian Mammalia

Mammals: occur in three major groups: Which comes to mind first when thinking about Australia?

Marsupials:

We know them well: In the form of the opossum Didelphis virginianus marsupials live in our gardens and turn up as road kill on our highways. What makes marsupials different from other mammals?

  • Reproduction: Therian mammals dispense with egg-laying. Marsupials give birth to young at very early stage of development. The young crawl into a pouch, where they grab onto a nipple and continue to develop. Note: When embryo is still in uterus, a tentative egg shell is deposited and reabsorbed. This system allows marsupials to pump the babies out, conveyor belt fashion. Consider the opossum: at any one time, one litter in uterus, one in pouch, and one riding on back. Further enhancing their fertility is the tendency of sperm cells to conjugate and swim in tandem, enhancing male potency.

    In another sense, this is very limiting. Marsupials display peramorphic predisplacement in the development of their forelimbs, which must be equipped with muscles and claws in order for the embryo to drag itself from the vagina to the pouch. As a result of this constraint, there are limits to the the degree to which the forelimbs can be modified, hence, there are marsupial "moles", "cats", and "gazelles," but no marsupial "bats" or "whales."


    Canis familiaris (left) and Didelphis virginianus (right) skull in palatal view from Chunnie's British Mammal Skulls and University of Texas at El Paso
    Dentition: Having trouble identifying a pouch in a fossil skeleton? Just check out the teeth. Marsupials have four sets of molars (right). Placentals (left) have only three.

  • Phylogeny: During the Mesozoic, marsupials arose in North America and their fossils are found as far away as eastern Eurasia, but soon became extinct in the northern hemisphere. Living groups diversified in South America where they occupied most of the predatory mammal niches such as the sabre-toothed Thylacosmilus (Skull). Today, South American marsupial diversity includes: A sad remnant of former glory.

    Note that Australidelphia - the Australian marsupials is monophyletic. Apparently a single radiation of marsupials into Australia occurred around 30 ma. (Notice anything odd?) Major Australian groups include:


    Monotremata

    Diversity:

    Ornithorhynchus anatinus the platypus:


    Tachyglossus aculeatus (Australia right) and three species of Zaglossus (New Guinea) The echidnas:

    Monotreme strangeness

    All in all, monotremes are reminiscent of the mammals of the Early Jurassic (the first major ecological radiation of mammals.)

    Biogeography: Both marsupials and monotremes show a distribution that connects South America with Australia, even though these continents are widely separate? WTF?

    The answer becomes apparent when we look at a paleogeographic map showing the distribution of continents at the end of the Mesozoic. At this point Australia and Antarctica are joined and Antarctica and South America are close together. It is easy to envision monotremes dispersing between these continents at this point. Indeed, go back to the Jurassic and all of the southern continents (including India and New Zealand) were part of a single land mass - Gondwana. Many land and fresh water creatures today (turtles, ray-finned fish families, plants, lungfish, etc.) display a gondwanan biogeographic distribution in which they are spread throughout the southern continents.

    But marsupials didn't reach Australia until the Oligocene Epoch (30 ma) At this point Australia was separate but still close to Antarctica. Remember that Astralidelphia has one South American member, the monito del monte. This creature apparently represents a back-propagation of Australian marsupials to South America.

    Do you think Antarctica was ever inhabited by marsupials?

    Pleistocene Australia: Why are there marsupials and echidnas in New Guinea? Consider Australia at the last glacial maximum (~20,000 years ago) when sea level was 80 m lower than today. Australia, New Guinea, and Tasmania are all one land mass.