In Your Face: Geology and Vertebrate Natural History of Australia
Geology BasicsRock types:
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.
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?
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:
- Globally in the shallow marine tropics
- Locally in places where topography favors shallow water clear of nasty sediment.
Incredible Vertebrate DiversityOn 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 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.
SquamataAustralia 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
- MOST iguanians use the tongue to pick up prey. (Obviously, secondarily derived plant eaters, such as the green iguana don't do this.)
Australian iguanians are represented by the taxon Agamidae, whose members are referred to as "dragons." Common examples include bearded dragons and water dragons, although I look forward to seeing Boyd's Forest Dragon , the throny devil, and the frilled lizard.
Yellow-spotted monitor Varanus panoptes
One synapomorphy is also the etymology of the name:
- A scaled tongue. In some this facilitates the division of the front of the tongue into a sensory organ.
Australian scleroglossans include a huge range of interesting critters including:
- Pygopodids - limbless geckos.
- Skinks - Burrowing lizards in North America, their Australian cousins occupy many "normal lizard" niches.
- Varanids (aka monitors or goannas) - Small to large lizards like the Yellow-spotted monitor (right) that have come to occupy top predator niches usually reserved for mammals. Living varanid lizards are able to elevate their metabolic rates by using a gular pumping of the hyoid skeleton to ventilate the lungs more efficiently. Today's varanids are a sad remnant of earlier glory. Consider Megalania prisca, the giant monitor of the Pleistocene. Today, the largest Australian monitor is the perentie.
Oxyuranus microlepidotus the fierce snake - world's most poisonous
Dreadful SnakesSnakes are scleroglossans. In Australia one group predominates: Elapidae. Familiar as coral snakes and cobras. Thoughts to consider:
- Of the world's ten most venomous snakes, nine are Australian elapids. Number one: The fierce snake Oxyuranus microlepidotus.
- Why all the scary snakes? Two thoughts come to mind:
- Desert ecology
- Prolonged evolutionary arms race with skinks, their primary food.
- In an odd reversal, Viperidae (the vipers) is completely absent form Australia. In fact one group of elapids - the death adders - has convergently evolved a viper-like morphology.
Biological ghostsSome 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:
- Rapid escape
- Nocturnal activity
- Cryptic coloration
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:
- Stabbing incisors & large shearing teeth
- Long legs and fingers with opposable thumbs
- Weight = 110-140 kg
- Why did Australia become a refuge for types of mammals that have suffered or become extinct elsewhere?
- Why are the faunas of Australia like those of Tasmania and New Guinea but unlike New Zealand or Eastern Indonesia?
- What happened to the missing predators and giant hebivorous marsupials