Table of Contents
BEST IELTS Academic Reading Test 495
IELTS ACADEMIC READING TEST 495 – PASSAGE – 1
IELTS ACADEMIC READING TEST – 495
READING PASSAGE – 1
Marsupials and Monotremes
Why are they only in Australia?
There are three types of mammals. The placentals, which nourish their young before birth with an advanced placenta, make up most of the mammals on the planet, including humans. Australia is the world capital of the other two, very much smaller groups of mammals: marsupials and monotremes. Marsupials, like the koala and kangaroo, nourish their undeveloped young with milk from nipples in their pouches; and monotremes, examples of which are spiny anteaters (or echidnas) and platypuses, lay eggs, and once they hatch, nourish them with milk that comes from pores in their skin.
How is it that Australia has mainly pouched and egg-laying mammals and very few of the kind of mammals that populate the rest of the world? In 1992 palaeontologists came up with an explanation to this question and a new way of looking at what the evolutionary and migratory path of these animals might have been. They arrived at this by observing two single fossil teeth, unearthed on opposite sides of the globe. One of the teeth in the 1992 discovery was uncovered in Argentina from 60-million-year-old deposits.
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The tooth belonged to an extinct species of monotreme known as the Patagonian platypus. The second tooth, dug out of Queensland deposits, was at least 55 million years old and belonged to a primitive placental known as a condylarth. The key to this is the way the continents in the southern hemisphere formed.
Australia, South America and Antarctica were once part of a huge, long landmass, a later development of the continent which geologists call Gondwana and had previously included Africa. In terms of the age of the planet, this was relatively recent – somewhere between 65 and 100 million years ago – towards the end of the Cretaceous period. The question is though, why did placental mammals eventually dominate South America, and marsupials and monotremes only really populate Australia?
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The earliest known marsupial fossils have in fact been found in North America from around 125 million years ago when they split from placental mammals. They then moved down into South America where they flourished, despite later disappearing from North America. Recent genetic studies have shown that much of Australia’s marsupial population has descended from an order of early South American marsupials of which only one is in existence today, a small mouse-like animal in Chile called monito del monte, or ‘monkey of the mountains’.
The earliest marsupial fossil in Australia is 55 million years old, from a site in Queensland. However, between the findings on that site and those of other marsupial fossils in Australia, there is nothing in the fossil record for another 25 million years. At around 30 million years ago, Australia was heavily populated with marsupials, including the ancestors of most of the living species today, but there is little to tell us what was happening in the intervening period.
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Around 40 million years ago, the continent of Australia severed from Gondwana and began a long, slow drift north into tropical waters, which is still going on. Twenty million years ago, Australia entered a warm and wet phase, and marsupial diversity exploded, giving rise to numerous species of marsupial megafauna, including large kangaroos. Then 15 million years ago, rainfall declined and temperatures began to drop, a phenomenon which continues to the present day. Australia dried out significantly, and the continent’s great northern and central forests, which were the dominant landscape, were transformed into dry grasslands and deserts.
Over that 40-million-year period, Australia experienced rising temperatures. Due to its northern drift, it was above the latitude where glaciers form, and no new mountain ranges were formed during that time. Due to such geological inactivity, Australia has been left with the flattest and poorest quality of earth and the thinnest top soils of any of the continents. The survival of Australian mammals is likely due to their body types.
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Monotremes and marsupials have much slower metabolisms and energy needs than placentals that need to live in richly vegetated environments. It seems that this was not an advantage in other parts of the world, where placentals were the great survivors. South America was a continent of lush rainforests at this time, so its mammals were not in need of the survival skills that the monotremes and marsupials had. This is the probable reason behind the eventual dominance of the placental mammals on that continent.
Due to the poor soils, marsupials had to evolve the ability to survive the harshest climate on the planet but could manage with low rates of reproduction due to a lack of predators. There were very few carnivores in Australia. There was cat-like Thylacoleo, the marsupial lion; Thylacine, the Tasmanian tiger; and one carnivorous kangaroo. Sarcophilus harrisii, the Tasmanian devil, is the only large marsupial carnivore still in existence, though a number of marsupial rodents and the aquatic platypus can be defined as carnivores.
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Although there are still no fewer than a hundred marsupial species in South America today (and one opossum in North America), the 250 species in Australia are in such abundance that the small population in South America can barely be compared with them. There is very little evidence of placental mammals in the Australian fossil record before 5 million years ago, and this is most likely because the rise of the placental mammal in South America did not occur until after the continent had broken away from the great southern landmass.
Questions 1-4
Choose NO MORE THAN THREE WORDS from the passage for each answer.
1. What type of mammals are the most common?
2. What species was the monotreme fossil that was discovered outside of Australia?
3. Apart from Australia, modern continents were included in the later form of Gondwana?
4. In which geological period are monotremes thought to have developed?
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Questions 5-13
Do the following statements agree with the information given in Reading Passage 1?
In boxes 5-13 on your answer sheet write
TRUE – if the statement agrees with the information
FALSE – if the statement contradicts the information
NOT GIVEN – if there is no information on this
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5. The first marsupials came from South America.
6. All of Australia’s marsupials have directly descended from the monito del monte.
7. There is a gap in the fossil record for marsupials in Australia between 30 and 55 million years ago.
8. South America severed from Gondwana 40 million years ago.
9. Until 15 million years ago, Australia was mainly covered in forests.
10. Monotremes and marsupials were better able to survive a hostile environment than placentals.
11. Marsupials needed to reproduce quickly because so many animals would prey on them
12. The Tasmanian devil is the only large marsupial that eats meat.
13. Today there are hardly any marsupial species in South America.
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ANSWERS
1. THE PLACENTALS
2. THE PATAGONIAN PLATYPUS
3. SOUTH AMERICA; ANTARCTICA
4. CRETACEOUS
5. FALSE
6. FALSE
7. TRUE
8. NOT GIVEN
9. TRUE
10. TRUE
11. FALSE
12. TRUE
13. FALSE
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