The Dinosaurs Footprints and Extinction

The dinosaurs, famously, went out with a bang. Geological record discovered in the 1980s suggests that an asteroid impact event may have caused the extinction of the dinosaurs. Something big hit the Earth 65 million years ago and, when the dust had fallen, so had the great reptiles. 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.

The first dinosaurs, in the late Triassic period (nearly 230 million years ago), apparently were not major components of the fauna; they were mostly small, and they shared the Earth with a host of other reptiles. However, by the Early Jurassic (about 30 million years later), after many other Triassic vertebrates had gone extinct, dinosaurs were diversifying rapidly. By then, they had become dominant occupants of many major terrestrial adaptive zones and turned into the monsters depicted in the book and movie Jurassic Park. 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.

The earliest known dinosaur footprints in North America came from the Eastern Seaboard, which was then full of rift valleys, similar to East Africa today. Interestingly, thousands of footprints have been found in this large stretch of area, but bones and teeth are rare and frag-mentary. And the sizes of prints tell as much about the sizes of the beasts as the skeletons themselves, so Dr Olsen and his colleagues focused on prints, not skeletons.

Those American rift valleys in the Triassic or Jurassic period contained lakes, which grew and shrank at regular intervals due to climate change caused by periodic shifts in the Earth’s orbit. This regularity, coupled with reversals in the Earth’s magnetic field (detectable in the tiny fields of certain magnetic minerals), means that rocks from this place and period can be dated to within a few thousand years. In addition, the muddy lakeshore sediments are perfect for recording the footprints of passing animals. By dividing the work, the ten authors of the paper were able to study such footprints in 80 locations.

The researchers looked at 18 so-called ichnotaxa. These are recognisable types of footprints that cannot be matched precisely with the species of animals that left them. 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 or teeth to tell the story. Five of the ichnotaxa disappear before the end of the Triassic, and four march confidently across the boundary into the Jurassic. Six, however, vanish at the boundary; and three appear from nowhere, almost as soon as the Jurassic begins.

That boundary itself is instructive. The first geological sign of the impact that killed the dinosaurs was an unusually high level of iridium in rocks at the end of the Cretaceous period when the beasts disappear from the fossil record. Iridium is normally rare at the Earth’s surface, but it is more abundant in meteorites. When people began to believe in the impact theory, they started looking for other Cretaceous-end anomalies. One finding was that there were an alarming number of fern spores in the rocks above the boundary layer – a phenomenon known as ‘fern spike’.

An iridium anomaly and a fern spike took place simultaneously at the end of the Triassic. That explains the disappearing ichnotaxa: the creatures that made them did not survive the holocaust. The surprise is how rapidly the new ichnotaxa appear. For instance, Eubrontes giganteus was there a mere 10,000 years after the iridium anomaly, and its footprints were made by theropods, but were already 20 percent larger than those of the Triassic theropods.

Dr Olsen and his colleagues suggest that the explanation for this rapid increase in size may be a phenomenon called ecological release. This is seen today when reptiles (which, in modern times, tend to be small creatures) reach islands where they face no competitors. 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.

That poses a puzzle of where the impact happened. No large hole in the Earth’s crust seems to be 202 million years 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. Although the continental crust is more or less permanent, the ocean floor is constantly recycled by the tectonic processes that cause continental drift. There is no ocean floor that is more than 200 million years old, so a crater that formed in the ocean would have been swallowed up by now.

However, the third intriguing possibility is that the crater is known, but may have been misdated. The 90 km diameter, late Triassic Manicouagan impact structure of Québec, Canada, is a well-preserved, undeformed complex crater and is thought to be 214 million years old. It is 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. Such an impact would certainly have had a noticeable impact on the world, but there were no records of the rocks from 214 million years ago. It is possible, therefore, that the date of Manicouagan is wrong. That’s the next thing to check.

Questions 1 – 6

Do the following statements agree with the information given in Reading Passage 1?

In boxes 1–6 on your answer sheet, write

TRUE if the statement agrees with the information

FALSE if the statement contradicts the information

NOT GIVEN if the information is not given in the passage

• 1.Paul Olsen and his colleagues believe that asteroid strikes may also lead to a dinosaur species boom.

TRUEFALSENOT GIVEN

• 2.The books and movies like Jurassic Park often exaggerate the size of the dinosaurs.
• 3.Dinosaur footprints are more abundant than dinosaur skeletons.
• 4.Footprints were chosen by Dr Olsen to study because they are more detect-able than the Earth’s magnetic field to track the date with precision over thousands of years.
• 5.The Ichnotaxa indicated that dinosaur footprints provide accurate information about the tracks left by a particular species.
• 6.We can find more iridium on the Earth’s surface than in meteorites.

Questions 7 – 13

Complete the summary below.

Choose NO MORE THAN TWO WORDS from the passage for each answer.

Write your answers in boxes 7–13 on your answer sheet.

答案及解析请付费后查阅：