Merck's Opinions on Australian Mammals
Australian MammaliaMammals: occur in three major groups:
- Monotremata: Platypuses and echidnas
- Marsupialia: Pouched mammals
- Placentlia: Placental mammals
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?
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
- Didelphomorpha (opossums)
- Paucituberculata (caenolestids)
- Microbiotheria (one living species: the monito del monte)
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:
- Dasyuromorphia (small predators - 71 species)
- Peramelemorphia (bandicoots and kin - generalists and insectivores - 24 species)
- Notoryctomorphia (marsupial moles - 2 species)
- Diprotodontia (herbivores - 137 species): These occupy a huge range of niches and include most of our favorites, kangaroos, wallabies, koalas, wombats, and some impressive fossils (e.g. Diprotodon. Note: for all their differences, these creatures share two distinct synapomorphies (evolutionary novelties):
Ornithorhynchus anatinus the platypus:
- Aquatic, specialized for hunting small invertebrates by means of an electroreceptive sense organ housed in its snout.
- Females dig deep burrows in river banks where young are nursed.
- Sexual dimorphism: Males equipped are with venomous spurs on hindlimbs. (Once deems a specialty of platypuses, but now known to have been common in mammals of the early Mesozoic - a plesiomorphy! E.G. Zhangotherium)
Tachyglossus aculeatus (Australia right) and three species of Zaglossus (New Guinea) The echidnas:
- Superficially convergent on ant-eaters. Specialized for ripping open termite mounds and eating occupants.
- Armored by hedgehog-like spines.
- Strong burrowers, capable of burrowing to escape predators.
- Actually rather smart, although their brains are organized differently from those of placentals (E.G. no corpus callosum).
- Ranges throughout mainland Australia and Tasmania, and part of New Guinea.
- Living monotremes are toothless as adults.
- Egg laying! Platypus incubates eggs inside burrow. Echidna carries eggs in a pouch.
Note: This a retained ancestral feature, not an evolutionary novelty!
- Limbs splay out to the side of the body like in non-mammalian therapsids.
- Today only three species on monotremes. There is a very small fossil record of "stem monotremes" going back to the Jurassic in Australia and South America. E.g. Steropodon: Fossil specimen.
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.