Perhaps one of the most dramatic and important changes that took place in the
Mesozoic era occurred late in that era, among the small organisms that populate the
uppermost, sunlit portion of the oceans--the plankton. The term "plankton" is a broad
Line one, designating all of the small plants and animals that float about or weakly propel
(5) themselves through the sea. In the late stages of the Mesozoic era. during the Cretaceous
period, there was a great expansion of plankton that precipitated skeletons or shells
composed of two types of mineral: silica and calcium carbonate. This development
radically changed the types of sediments that accumulated on the seafloor, because,
while the organic parts of the plankton decayed after the organisms died, their mineralized
(10) skeletons often survived and sank to the bottom. For the first time in the Earth's long
history, very large quantities of silica skeletons, which would eventually harden into rock,
began to pile up in parts of the deep sea. Thick deposits of calcareous ooze made up of
the tiny remains of the calcium carbonate-secreting plankton also accumulated as never
before. The famous white chalk cliffs of Dover, in the southeast of England, are just one
(15) example of the huge quantities of such material that amassed during the Cretaceous
period; there are many more. Just why the calcareous plankton were so prolific during
the latter part of the Cretaceous period is not fully understood. Such massive amounts
of chalky sediments have never since been deposited over a comparable period of time.
The high biological productivity of the Cretaceous oceans also led to ideal conditions
(20) for oil accumulation. Oil is formed when organic material trapped in sediments is slowly
buried and subjected to increased temperatures and pressures, transforming it into
petroleum. Sediments rich in organic material accumulated along the margins of the
Tethys Seaway, the tropical east-west ocean that formed when Earth's single landmass
(known as Pangaea) split apart during the Mesozoic era. Many of today's important oil
(25) fields are found in those sediments--in Russia, the Middle East, the Gulf of Mexico, and
in the states of Texas and Louisiana in the United States.
【Group】31. What does the passage mainly discuss?
(A) How sediments were built up in oceans during the Cretaceous period
(B) How petroleum was formed in the Mesozoic era
(C) The impact of changes in oceanic animal and plant life in the Mesozoic era
(D) The differences between plankton found in the present era and Cretaceous plankton
【Group】35. According to the passage, the most dramatic change to the oceans caused by plankton during
the Cretaceous period concerned
(A) the depth of the water
(B) the makeup of the sediment on the ocean floor
(C) the decrease in petroleum-producing sediment
(D) a decline in the quantity of calcareous ooze on the seafloor
【Group】36. The "white chalk cliffs of Dover" are mentioned in line 14 of the passage to
(A) show where the plankton sediment first began to build up
(B) provide an example of a plankton buildup that scientists cannot explain
(C) provide an example of the buildup of plankton sediment
(D) indicate the largest single plankton buildup on Earth
It is estimated that over 99 percent of all species that ever existed have become
extinct. What causes extinction? When a species is no longer adapted to a changed
environment, it may perish. The exact causes of a species’ death vary from situation
Line to situation. Rapid ecological change may render an environment hostile to a species.
For example, temperatures may change and a species may not be able to adapt. Food
Resources may be affected by environmental changes, which will then cause problems
For a species requiring these resources. Other species may become better adapted to an
Environment, resulting in competition and, ultimately, in the death of a species.
The fossil record reveals that extinction has occurred throughout the history of Earth.
(10) Recent analyses have also revealed that on some occasions many species became extinct
at the same time—a mass extinction. One of the best-known examples of mass extinction
occurred 65 million years ago with the demise of dinosaurs and many other forms of life.
Perhaps the largest mass extinction was the one that occurred 225 million years ago,
When approximately 95 percent of all species died, Mass extinctions can be caused by
(15) a relatively rapid change in the environment and can be worsened by the close
interrelationship of many species. If, for example, something were to happen to destroy
much of the plankton in the oceans, then the oxygen content of Earth would drop,
affection even organisms not living in the oceans. Such a change would probably lead to a mass extinction.
One interesting, and controversial, finding is that extinctions during the past 250
Million years have tended to be more intense every 26 million years. This periodic
extinction might be due to intersection of the Earth’s orbit with a cloud of comets, but
this theory is purely speculative. Some researchers have also speculated tat extinction
may often be random. That is, certain species may be eliminated and others may survive
(25) for no particular reason. A species’ survival may have nothing to do with its ability or
inability to adapt. If so, some of evolutionary history may reflect a sequence of essentially
【Group】25. Why is “ plankton” mentioned in line 17?
(A) To demonstrate the interdependence of different species
(B) To emphasize the importance of food resources in preventing mass extinction.
(C) To illustrate a comparison between organisms that live on the land and those that live in the ocean
(D) To point out that certain species could never become extinct.