Perhaps one of the mostdramatic and importantchangesthattookplace in the
Mesozoic era occurredlate in that era, among the smallorganismsthatpopulate the
uppermost, sunlitportion of the oceans--the plankton. The term "plankton" is a broadLine one, designating all of the smallplants and animalsthatfloatabout or weaklypropel
(5) themselvesthrough the sea. In the latestages of the Mesozoic era. during the Cretaceousperiod, there was a greatexpansion of planktonthatprecipitatedskeletons or shellscomposed of two types of mineral: silica and calciumcarbonate. Thisdevelopmentradicallychanged the types of sedimentsthataccumulated on the seafloor, because,
while the organicparts of the planktondecayedafter the organismsdied, theirmineralized
(10) skeletonsoftensurvived and sank to the bottom. For the firsttime in the Earth's longhistory, verylargequantities of silicaskeletons, whichwouldeventuallyhardenintorock,
began to pile up in parts of the deep sea. Thickdeposits of calcareousoozemade up of
the tinyremains of the calciumcarbonate-secretingplanktonalsoaccumulated as neverbefore. The famouswhitechalkcliffs of Dover, in the southeast of England, are just one
(15) example of the hugequantities of suchmaterialthatamassedduring the Cretaceousperiod; there are manymore. Just why the calcareousplanktonwere so prolificduring
the latterpart of the Cretaceousperiod is not fullyunderstood. Suchmassiveamounts
of chalkysedimentshaveneversincebeendepositedover a comparableperiod of time.
The highbiologicalproductivity of the Cretaceousoceansalso led to idealconditions
(20) for oil accumulation. Oil is formedwhenorganicmaterialtrapped in sediments is slowlyburied and subjected to increasedtemperatures and pressures, transforming it intopetroleum. Sedimentsrich in organicmaterialaccumulatedalong the margins of the
TethysSeaway, the tropicaleast-westoceanthatformedwhenEarth's singlelandmass
(known as Pangaea) splitapartduring the Mesozoic era. Many of today's important oil
(25) fields are found in thosesediments--in Russia, the MiddleEast, the Gulf of Mexico, and
in the states of Texas and Louisiana in the UnitedStates.
【題組】31. Whatdoes the passagemainlydiscuss?
(A) How sedimentswerebuilt up in oceansduring the Cretaceousperiod (B) How petroleum was formed in the Mesozoic era
(C) The impact of changes in oceanicanimal and plantlife in the Mesozoic era
(D) The differencesbetweenplanktonfound in the present era and Cretaceousplankton
【題組】35. According to the passage, the mostdramaticchange to the oceanscaused by planktonduring
the Cretaceousperiodconcerned (A) the depth of the water (B) the makeup of the sediment on the oceanfloor (C) the decrease in petroleum-producingsediment (D) a decline in the quantity of calcareousooze on the seafloor
【題組】36. The "whitechalkcliffs of Dover" are mentioned in line 14 of the passage to
(A) showwhere the planktonsedimentfirstbegan to build up
(B) provide an example of a planktonbuildupthatscientistscannotexplain (C) provide an example of the buildup of planktonsediment (D) indicate the largestsingleplanktonbuildup on Earth
It is estimatedthatover 99 percent of all speciesthateverexistedhavebecomeextinct. Whatcausesextinction? When a species is no longeradapted to a changedenvironment, it may perish. The exactcauses of a species’ deathvaryfromsituationLine to situation. Rapidecologicalchange may render an environmenthostile to a species.
For example, temperatures may change and a species may not be able to adapt. FoodResources may be affected by environmentalchanges, whichwillthencauseproblems
For a speciesrequiringtheseresources. Otherspecies may becomebetteradapted to an
Environment, resulting in competition and, ultimately, in the death of a species.
The fossilrecordrevealsthatextinction has occurredthroughout the history of Earth.
(10) Recentanalyseshavealsorevealedthat on someoccasionsmanyspeciesbecameextinct
at the sametime—a massextinction. One of the best-knownexamples of massextinctionoccurred 65 millionyears ago with the demise of dinosaurs and manyotherforms of life.
Perhaps the largestmassextinction was the one thatoccurred 225 millionyears ago,
Whenapproximately 95 percent of all speciesdied, Massextinctions can be caused by
(15) a relativelyrapidchange in the environment and can be worsened by the closeinterrelationship of manyspecies. If, for example, somethingwere to happen to destroymuch of the plankton in the oceans, then the oxygencontent of Earthwoulddrop,
affectionevenorganisms not living in the oceans. Such a changewouldprobablylead to a massextinction.
One interesting, and controversial, finding is thatextinctionsduring the past 250
Millionyearshavetended to be moreintenseevery 26 millionyears. Thisperiodicextinctionmight be due to intersection of the Earth’s orbitwith a cloud of comets, but
thistheory is purelyspeculative. Someresearchershavealsospeculated tat extinction
may often be random. That is, certainspecies may be eliminated and others may survive
(25) for no particularreason. A species’ survival may havenothing to do with its ability or
inability to adapt. If so, some of evolutionaryhistory may reflect a sequence of essentiallyrandomevents.
【題組】25. Why is “ plankton” mentioned in line 17?
(A) To demonstrate the interdependence of differentspecies (B) To emphasize the importance of foodresources in preventingmassextinction.
(C) To illustrate a comparisonbetweenorganismsthatlive on the land and thosethatlive in the ocean (D) To point out thatcertainspeciescouldneverbecomeextinct.