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[Other sources] - The Dinosaurs Footprints and Extinction
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The Dinosaurs Footprints and Extinction
[Other sources] - The Dinosaurs Footprints and Extinction
Xem tóm tắt bài đọc
The Dinosaurs Footprints and Extinction
Everybody knows that the dinosaurs were killed by an asteroid. 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 Science.
Dinosaurs first appear in the fossil record 230 million years ago, during the Triassic period. But they were mostly small, and they shared the earth with lots of
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
The researchers looked at 18 so-called ichnotaxa. These are recognizable types of footprint that cannot be matched precisely with the species of animal that left them. But
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
The surprise is how rapidly the new ichnotaxa appeared. Dr Olsen and his colleagues suggest that the explanation for this rapid increase in size may be a phenomenon
That leaves the question of where the impact happened. No large crater in the earth’s crust seems to be 202 million years old. It may, of course, have
Bài đọc giải thích cực kỳ chi tiết với YouPass
other
sorts
of
reptile.
It was in the subsequent Jurassic, which began 202 million years ago, that they overran the planet and turned into the monsters depicted in the book and movie "Jurassic Park".
(Actually, though, the dinosaurs that appeared on screen were from the still more recent Cretaceous period.)
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.
as
are
the
skeletons
themselves.
Dr Olsen and his colleagues therefore concentrated on prints, not bones.
The prints in question were made in eastern North America, a part of the world full of rift valleys similar to those in East Africa today.
Like the modern African rift valleys, the American ones contained lakes, and these lakes grew and shrank at regular intervals because of climatic changes caused by periodic shifts in the earth’s orbit.
That regularity, combined with reversals in the earth’s magnetic field, means rocks from this place and period can be dated to within a few thousand years.
As a bonus, squishy lake-edge sediments are ideal for recording the tracks of passing animals.
Dividing the labour between themselves, the ten authors of the paper were able to study such tracks at 80 sites.
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.
Five of the ichnotaxa the researchers looked at were made by dinosaurs.
Four of these 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.
of
the
Cretaceous,
when
the
beasts
disappeared
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 the impact theory, they started looking for other relevant 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".
That matched the theory nicely.
Many modern ferns are opportunists.
They cannot compete against plants with leaves, but if a piece of land is cleared by, say, a volcanic eruption, they are often the first things to set up shop there.
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 dramatic has happened.
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.
called
ecological
release.
This is seen today when reptiles 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.
been
overlooked.
Old craters are eroded and buried, and not always easy to find.
Alternatively, it may have vanished.
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 200 million years old, so a crater that formed in the ocean would have been swallowed up by now.
There is a third possibility, however.
This is that the crater is known, but has been misdated.
The Manicouagan crater in Quebec is thought to be 214 million 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.
Dr Paul Olsen and his colleagues believe that asteroid knock may also lead to dinosaurs' boom.
2 Books and movie like Jurassic Park often exaggerate the size of the dinosaurs.
3 Dinosaur footprints are more adequate than dinosaur skeletons.
4 The prints were chosen by Dr Olsen to study because they are more detectable than earth magnetic field to track a date of geological precise within thousands years.
5 Ichnotaxa showed that footprints of dinosaurs offer exact information of the trace left by an individual species.
6 We can find more iridium in 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.
