In Your Face: Geology and Vertebrate Natural History of Australia

John Merck

Geology Basics

Rock types:

  • Igneous: Rocks that form from the cooling and solidification of magma. Igneous rocks are generally have interlocking crystals that show no preferred orientation. E. G.:

  • Metamorphic: Rocks that form from the recrystallization of preexisting rocks under extreme heat and/or pressure. E.g.:

  • Sedimentary: Rocks that form from transported fragments of preexisting rocks. E.g.:

    In Australia, we will mostly be concerned with sedimentary and igneous rocks. Some highlights:

    Northern Territory: The Arnhemland Escarpment dominates the landscape. It is made entirely of one rock unit - the sedimentary Kombolgie Formation, representing ancient continental river and alluvial fan deposits. Why is it special? 1.7 billion years old and an extremely hard topography dominator.

    Queensland: The mainland of northern Queensland is igneous, representing:

    Ironically, Australia is the only continent with no active volcanoes today.

    But the real geologic standout is the Great Barrier Reef - the worlds primer factory for carbonate rocks. But why is this region such a great site for reef building?

    Carbonate chemistry:

    Equilibrium constants An equilibrium constant is equal to the concentration of products over reactants. In the case of carbonate system we see the following equilibrium constants:

    CO2 + H2O -> H2CO3 (K3 = 1*10-1.43)
    H2CO3 -> H+ + HCO3- (K3 = 1*10-6.40)
    >HCO3- -> H+ + CO32- (K3 = 1*10-10.33)
    Ca2+ + CO32- -> CaCO3 (K3 = 1*10-8.33 for aragonite and 1*10-8.48 for calcite)

    Looking at this reaction series, you would think that adding more CO2 would drive the reactions toward the right and increase CaCO3 precipitation, but this is not the case:

    Because they proceed in the same environment, reactants can "leak" from one reaction to the other. K3 is much greater for reaction 2 than reaction 3. Thus, H+ from 2 is able to leak into reaction 3, driving it to the left. paradoxically, precipitation of CaCO3 is facilitated by the reduction of CO2.

    Thus, CaCO3 is deposited where there is the least CO2 in the water:

    Incredible Vertebrate Diversity

    On the right is a cladogram of all of the major vertebrate groups. The wonder and glory of the Australia travel study is that it is possible that we could see representatives of each one. That could possibly even include the Australian lungfish (albeit unlikely.)

    That's far more vertebrate diversity than I can address here. For this half hour, I want to consider the critters in red that include some of the most conspicuous animals we will see.

    Australian Mammalia

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


    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:



    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.


    Australia has been called "the land of lizards," both because it is home to an interesting endemic radiation of squamates and because they are fantastically diverse, with over three times as many species as live in North America.

    There are hundreds of living squamate species. All of their diversity falls into two major groups:

    These groups appear in the Early Jurassic, with essentially modern looking forms occurring in the Cretaceous.

    Gilbert's dragon Amphibolurus gilberti
    Iguania: Contains iguanas (duh) plus a great diversity of smaller groups such as anoles, fence lizards, horned lizards, agamids, and chameleons, and others, for you herp enthusiasts. Has many synapomorphies. We'll be content with one - The use of the tongue in prey capture.
    Scleroglossa: Contains geckos, skinks, anguids, snakes, mosasaurs, Gila monsters, and varanids (monitors).
    One synapomorphy is also the etymology of the name:

    Australian scleroglossans include a huge range of interesting critters including:

    Oxyuranus microlepidotus the fierce snake - world's most poisonous

    Dreadful Snakes

    Snakes are scleroglossans. In Australia one group predominates: Elapidae. Familiar as coral snakes and cobras. Thoughts to consider:

    Biological ghosts

    Some extant communities only make sense if we consider their extinct members. Consider: Many of the large herbivorous mammals of Australia show evidence of adaptations for escaping large predators including:

    But in the modern Australian fauna, there is no large native predator from which they might be escaping. On the mainland, the largest is the quoll (Dasyurus). This is in contrast to nearby New Zealand, similarly bereft of predators, but home to many large and spectacularly helpless land animals. The Neogene fossil record, however, contains several predators that might well have preyed on them, including:

  • The "Tasmanian wolf" (AKA Thylacinus cynocephalus), extinguished in Australia roughly 3000 years ago but survived in Tasmania into the 1930s.

  • The "marsupial lion" Thylacoleo carnifex. This creature and its relatives had:

  • The giant rat kangaroo Propleopus.

  • Mekosuchinae: A radiation of semi-terrestrial crocodiles, such as Quinkana fortirostrum

  • The giant monitor lizard Megalania prisca

    Parting thoughst: