Mock Test 6.1 | Academic Reading

READING PASSAGE 1

You should spend about 20 minutes on Questions 1-13, which are based on Reading Passage 1 below.

Extinction Mysterious of The Dinosaurs

A.       Everybody knows that the dinosaurs were killed by an asteroid. Something big hit the earth 65m years ago and, when the dust had fallen, so had the great reptiles. [Crack IELTS with Rob] There is thus a nice, if ironic, symmetry in the idea that a similar impact brought about the dinosaurs’ rise. That is the thesis proposed by Paul Olsen, of Columbia University, and his colleagues in this week’s Science.

B.       Dinosaurs first appear in the fossil record 230m years ago, during the Triassic period. But they were mostly small, and they shared the earth with lots of other sorts of reptile. It was in the subsequent Jurassic, which began 202m years ago, that they overran the planet and turned into the monsters depicted in the book and movie “Jurassic Park”. [Crack IELTS with Rob] Dr Olsen and his colleagues are not the first to suggest that the dinosaurs inherited the earth as the result of an asteroid strike. But they are the first to show that the takeover did, indeed, happen in a geological eyeblink.

C.       Dinosaur skeletons are rare. Dinosaur footprints are, however surprisingly abundant. And the sizes of the prints are as good an indication of the sizes of the beasts as are the skeletons themselves. Dr Olsen and his colleagues therefore concentrated on prints, not bones.

D.       The researchers looked at 18 so-called ichnotaxa. These are recognisable types of footprint that cannot be matched precisely with the species of animal that left them. [Crack IELTS with Rob] But they can be matched with a general sort of animal, and thus act as an indicator of the fate of that group, even when there are no bones to tell the story.

E.       Five of ichnotaxa disappear before the end of the Triassic, and four march confidently across the boundary into the Jurassic. Six, however, vanish at the boundary, or only just splutter across it; and three appear from nowhere, almost as soon as the Jurassic begins.

F.       That boundary itself is suggestive. The first geological indication of the impact that killed the dinosaurs was an unusually high level of iridium in rocks at the end of the Cretaceous, when the beats disappear from the fossil record. Iridium is normally rare at the earth’s surface, but it is more abundant in meteorites. [Crack IELTS with Rob] When people began to believe the impact theory, they started looking for other Cretaceous-end anomalies. One that turned up was a surprising abundance of fern spores in rocks just above the boundary layer-a phenomenon known as a “fern spike”.

G.       That matched the theory nicely. Many modern ferns are opportunists. They cannot compete against plants with leaves, but if a piece of land is clearly by, say, a volcanic eruption, they are often the first things to set up shop there. [Crack IELTS with Rob] An asteroid strike would have scoured much of the earth of its vegetable cover, and provided a paradise for ferns. A fern spike in the rocks is thus a good indication that something terrible has happened.

H.       Both an iridium anomaly and a fern spike appear in rocks at the end of the Triassic, too. That accounts for the disappearing ichnotaxa: the creatures that made them did not survive the holocaust. The surprise is how rapidly the new ichnotaxa appears. Eubrontes giganteus, for example, is there a mere 10,000 years after the iridium anomaly. [Crack IELTS with Rob] The Eubrontes prints were made by theropods- the dinosaur group that went on to produce such nightmares as Allosaurus and Tyrannosaurus- and Eubrontes is already 20% bigger than any theropod track recorded from the Triassic.

I.        Dr Olsen and his colleagues suggest that the explanation for this rapid increase in size may be a phenomenon called ecological release. This seen today when reptiles reach islands where they face no competitors. [Crack IELTS with Rob] The most spectacular example is on the Indonesian island of Komodo, where local lizards have grown so large that they are often referred to as dragons. The dinosaurs, in other words, could flourish only when the competition had been knocked out.

J.        That leaves the question of where the impact happened. No large hole in the earth’s crust seems to be 202m year old. It may, of course, have been overlooked. Old craters are eroded and buried, and not always easy to find. Alternatively, it may have vanished. [Crack IELTS with Rob] Although continental crust is more or less permanent, the ocean floor is constantly recycled by the tectonic processes that bring about continental drift. There is no ocean floor left that is more than 200m years old, so a crater that formed in the ocean would have been swallowed up by now.

K.       There is a third possibility, however. This is that the crater is known, but has been misdated. The Manicouagan “structure”, a crater in Quebec, is thought to be 214m years old. It is huge-some 100km across-and seems to be the largest of between three and five craters that formed within a few hours of each other as the lumps of a disintegrated comet hit the earth one by one. [Crack IELTS with Rob] Such an impact would surely have had a perceptible effect on the world, but the rocks from 214m years ago do not record one. It is possible, therefore, that Manicouagan has been misdated. That will be the next thing to check.