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Australian Mammals: Evolutionary Development as a Result of Geographic Isolation
Australia’s Mammals Mammals are animals that all produce milk to suckle their young. Mammals have fur or hair.
There are three types of mammals: Monotremes are mammals that lay soft-shelled eggs. They are only found in Australia and New Guinea. The Platypus and the Echidna are the only monotremes left today. Marsupials are mammals that give birth to young which are in an immature state. The females often have pouches in which to carry the young. Only 30% of the world's marsupials are found outside Australia. The main groups of the 180 species of Australian marsupials are: o Macropods --Kangaroo and Wallabies o Carnivorous marsupials -- Numbats, Moles, Dasyurids o Possums, Gliders, and Cuscus o Bandicootes and Bilbies – Tazmanian Devils o Koalas and Wombats Placentals are mammals that produce a well developed placenta which allows the young to be born in a more developed state. Most species are found in other parts of the world. Only the following few are found in Australia: o Native mice and rats o Bats o Dingoes o Seals o Whales, Dolphins, and Dugongs
The native mammals of Australia are derived from two sources. The oldest mammals in Australia (marsupials and monotremes) are descendants of animals that were on the continent when it was separated from Antarctica. The others, the bats and rodents, moved westward from Southeast Asia as Australia drifted northwards.
Continental Drift (Theory of Plate Techtonics) The unique development of Australian mammals is due mainly to the evolutionary effects of geographic isolation.
The land masses of the world have not always been where they are today. At one time all the continents were joined together in one land mass called Pangaea. Geologists working on theories of continental drift (more correctly called the "Theory of Plate Tectonics") agree that the continents, as we now know them, have been constantly on the move, merging with each other, breaking into different groupings and joining again.
The rigid crust of the Earth is considered to have broken into a number of "plates" that are moving independently in different directions. The continents themselves are lighter rocks sitting on top of these plates. Where two plates collide the softer, upper layers of rock are compressed and folded to become mountain ranges. Erosion by ice, rain and wind then produces the mountain shapes familiar to all of us. The Himalayas and the Andes are examples of mountain ranges recently formed in this manner.
Australia was one of the oldest of the current continents to break off of an original division of Pangea called Gondwana which included Antarctica, Australia, India, Africa and South America. This means that Australia has been in geographic isolation as a continent longer than almost any other landmass. As Australia has drifted across the globe, the land and its climate has changed. For example, during past ages there have been many variations in sea levels and at times, much of what is now dry land was covered by shallow seas in which sediments and fossils collected. In fact, in the past, Australia has been so dissected by shallow seaways that it was reduced to several large islands. At other times the land mass has been high above the surrounding oceans with land connecting Tasmania, Kangaroo Island and New Guinea to the Australian mainland.
Monotremes No fossil monotremes (platypus and echidna) have been found outside of Australia and New Guinea and only a few fossils are known. Although a piece of jaw, about 100 million years old, from an ancestral form of the platypus was recently discovered, very little is known about the evolution of these animals.
Marsupials The oldest known fossils of marsupials date from about 100 million years ago. They have been found in North America. From the fossil record it appears that marsupials then moved east into Europe as well as southward into South America. They died out in Europe and North America, possibly through competition with placental mammals, but were successful in South America. One species later re-invaded North America (opossums) and it is believed that others traveled to the Australian continent by way of Antarctica, before the three continents broke up. Evidence in support of this theory comes from the discovery of a fossil marsupial in Antarctica in 1982. Generally, it is accepted that the opossums, or a similar group, were ancestral to the Australian marsupials. From this opossum-like stock many different kinds of marsupials developed, including native cats, bandicootes, kangaroos, wallabies, possums and wombats.
Mega-Fauna Besides the 120 different species of marsupials living in Australia today, there were many more that are now extinct, including the so-called "giants." Many of these giants, or Mega-fauna, were larger varieties of animals that are alive today. For example, there were giant wombats, koalas, and kangaroos. Some of these giants were still in existence about 10,000 years ago. The reason for their decline is not known; perhaps it was the change of climate that occurred at the end of the last Ice Age or perhaps they were hunted by the Aborigines. We may never know for sure.
Introduced (Non-Native) Species As Australia drifted eastward to Southeast Asia, it became possible for other animals to cross the marine barriers and become established here. The first bats flew across no later that 15 million years ago, followed by rodents some 5-10 million years ago. The rodents would have floated across on storm debris such as mats of vegetation.
At present time, some of the most widespread and successful mammal species are those that have been introduced by humans. The dingo may have been brought to Australia as recently as 4,000-5,000 years ago by the Aborigines. The feral cat, house mouse, black rat, rabbit, fox, horse, donkey, camel, goat, pig, feral cattle, and others were introduced by Europeans and are so well established that they must now be regarded as a part of Australia's mammal fauna. Questions Answer the following questions based on the above reading and our class discussion.
1. Identify the following animal illustrations. Beside each label write a “P” for placental, a “MO” for monotreme, or a “MA” for marsupial. Example: rats– P
Word Bank: platypus, bat, wombat, dingo, Tasmanian Devil, kangaroo, echidna, koala
2. What is the definition of a mammal?
3. What are the two sources of Australia’s native mammals? a. b.
4. Compare and contrast placental mammal, monotreme, and marsupials. Give two examples of each type of mammal (must not be an example given in the pictures above) a. Compare:
b. Contrast
c. 2 Examples of Placentals: a.______b.______
d. 2 Examples of Marsupials: a.______b.______
e. 2 Examples of Monotremes: a.______b.______
5. What is evolution?
6. What is geographic isolation in regards to speciation? 7. What is the theory of plate tectonics?
8. Describe the pattern on continental drift from Pangea to the current location of the continents today.
9. How has the continental drift of Australia contributed to the unique development of its mammals?
10. What is a possible explanation marsupials for why marsupials died out in North America and Europe?
11. What is a possible explanation for Mega-fauna dying out in Australia?
12. What were the first two placental mammals introduced into Australia and how did they get there?
13. Introduced species can have a devastating effect on an ecosystem. Name five introduced (non-native) species: a. b. c. d. e.