-
Extinction of the Dinosaurs
Paleozoic Era 334 to 248 million years
ago
Mesozoic Era 245 to 65 million
years ago
—
Triassic Period
—
Jurassic Period
—
Cretaceous Period
Cenozoic Era 65 million years ago to
the present
Paleontologists
have argued for a long time that the demise of the
dinosaurs was caused by climatic
alterations
associated
with
slow
changes
in
the
positions
of
continents
and
seas
resulting
from
plate
tectonics. Off and on throughout the
Cretaceous (the last period of the Mesozoic era,
during which dinosaurs
flourished),
large
shallow
seas
covered
extensive
areas
of
the
continents.
Data
from
diverse
sources,
including
geochemical evidence preserved in seafloor
sediments, indicate that the Late Cretaceous
climate
was
milder
than
today’s.
The
days
were
not
too
hot,
nor
the
nights
too
cold.
The
summers
were
not
too
warm, nor the winters too frigid. The
shallow seas on the continents probably buffered
the temperature of
the nearby air,
keeping it relatively constant.
At
the
end
of
the
Cretaceous,
the
geological
record
shows
that
these
seaways
retreated
from
the
continents
back
into
the
major
ocean
basins.
No
one
knows
why.
Over
a
period
of
about
100,000
years,
while
the
seas
pulled
back,
climates
around
the
world
became
dramatically
more
extreme:
warmer
days,
cooler
nights;
hotter
summers,
colder
winters.
Perhaps
dinosaurs
could
not
tolerate
these
extreme
temperature changes and became extinct.
If true, though, why did
cold-blooded animals such as snakes, lizards,
turtles, and crocodiles survive the
freezing winters and torrid summers?
These animals are at the mercy of the climate to
maintain a livable
body temperature.
It’s hard to understand why they would not be
affected, whereas din
osaurs were left
too
crippled to cope, especially if, as
some scientists believe, dinosaurs were warm-
blooded. Critics also point
out that
the shallow seaways had retreated from and
advanced on the continents numerous times during
the
Mesozoic, so why did the dinosaurs
survive the climatic changes associated with the
earlier fluctuations but
not
with
this
one?
Although
initially
appealing,
the
hypothesis
of
a
simple
climatic
change
related
to
sea
levels is insufficient
to explain all the data.
Dissatisfaction
with
conventional
explanations
for
dinosaur
extinctions
led
to
a
surprising
observation
that, in turn,
has suggested a new hypothesis. Many plants and
animals disappear abruptly from the fossil
record as one moves from layers of rock
documenting the end of the Cretaceous up into
rocks representing
the beginning of the
Cenozoic (the era after the Mesozoic). Between the
last layer of Cretaceous rock and
the
first layer of Cenozoic rock, there is often a
thin layer of clay. Scientists felt that they
could get an idea of
how long the
extinctions took by determining how long it took
to deposit this one centimeter of clay and they
thought they
could determine the time it took to deposit the
clay by determining the amount of the element
iridium (Ir) it contained.
Ir has not been common at Earth’s since
the very beginning of the planet’s history.
Because it usually
exists
in
a
metallic
state,
it
was
preferentially
incorporated
in
Earth’s
core
as
the
planet
cooled
and
consolidated.
Ir
is
found
in
high
concentrati
ons
in
some
meteorites,
in
which
the
solar
system’s
original
chemical
composition is preserved.
Even
today,
microscopic
meteorites continually
bombard
Earth,
falling
on both land and
sea. By measuring how many of these meteorites
fall to Earth over a given period of time,
scientists can estimate how long it
might have taken to deposit the observed amount of
Ir in the boundary
clay.
These
calculations
suggest
that
a
period
of
about
one
million
years
would
have
been
required.
However,
other
reliable
evidence
suggests
that
the
deposition
of
the
boundary
clay
could
not
have
taken
one million years. So
the unusually high concentration of Ir seems to
require a special explanation.
In
view
of
these
facts,
scientists
hypothesized
that
a
single
large
asteroid,
about
10
to
15
kilometers
across, collided with Earth, and the
resulting fallout created the boundary clay. Their
calculations show that
the impact
kicked up a dust cloud that cut off sunlight for
several months, inhibiting photosynthesis in
plants;
decreased surface temperatures
on continents to below freezing; caused extreme
episodes of acid rain; and
significantly
raised
long-term
global
temperatures
through
the
greenhouse
effect.
This
disruption
of
food
chain and climate would
have eradicated the dinosaurs and other organisms
in less than fifty years.
Paragraph 1: Paleontologists have
argued for a long time that the demise of the
dinosaurs was caused
by climatic
alterations associated with slow changes in the
positions of continents and seas resulting from
plate
tectonics.
Off
and
on
throughout
the
Cretaceous
(the
last
period
of
the
Mesozoic
era,
during
which
dinosaurs
flourished),
large
shallow
seas
covered
extensive
areas
of
the
continents.
Data
from
diverse
sources, including geochemical evidence
preserved in seafloor sediments, indicate that the
Late Cretaceous
climate was milder than
today’s. The days were not too hot, nor the nights
too cold. The summers were not
too
warm, nor the winters too frigid. The shallow seas
on the continents probably buffered the
temperature
of the nearby air, keeping
it relatively constant.
1.
According to paragraph 1, which of the following
is true of the Late Cretaceous climate?
○Summers were very warm and winters
were very cold.
○Shallow
seas on the continents caused
frequent
temperature changes.
○The climate was
very similar to today’s climate.
○The climate did not change
dramatically from season to season.
Paragraph 2: At the end of
the Cretaceous, the geological record shows that
these seaways retreated
from the
continents back into the major ocean basins. No
one knows why. Over a period of about 100,000
years,
while
the
seas
pulled back, climates
around
the
world
became
dramatically
more
extreme:
warmer
days,
cooler
nights;
hotter
summers,
colder
winters.
Perhaps
dinosaurs
could
not
tolerate
these
extreme
temperature changes and became extinct.
2.
Which of the following reasons is suggested in
paragraph 2 for the extinction of the dinosaurs?
○Changes in the lengths of the days and
nights during the late Cretaceo
us
period
○Droughts caused by the movement
of seaways back into the oceans
○The change from mild to severe
climates during the Late Cretaceous
period
○An extreme decrease
in the average yearly temperature over 10,ooo
years
Paragraph
3:
If
true,
though,
why
did
cold-blooded
animals
such
as
snakes,
lizards,
turtles,
and
crocodiles survive the
freezing winters and torrid summers? These animals
are at the mercy of the climate
to
maintain
a
livable
body
temperature.
It’s
hard
to
understand
why
they
would
not
b
e
affected,
whereas
dinosaurs
were
left
too
crippled
to
cope,
especially
if,
as
some
scientists
believe,
dinosaurs
were
warm-
blooded. Critics also
point out that the shallow seaways had retreated
from and advanced on the continents
numerous times during the Mesozoic, so
why did the dinosaurs survive the climatic changes
associated with
the earlier
fluctuations but not with this one? Although
initially appealing, the hypothesis of a simple
climatic
change related to sea levels
is insufficient to explain all the data.
3. Why does the author
mention the survival of “
snakes,
lizards, turtles, and crocodiles
” in
paragraph 3?
○To argue that
dinosaurs may have become extinct because they
were not cold
-blooded animals
○To question the adequacy of the
hypothesis
that climatic change related
to sea levels caused the
extinction of
the dinosaurs
○To present examples of
animals that could maintain a livable body
temperature more easily than
dinosaurs
○To
support
a
hypothesis
that
these
animals
were
not
as
sensitive
to
climate
changes
in
the
Cretaceous
period as they are today
4.
The word “
cope
” in the
passage is closest in meaning to
○
adapt
○
move
○
continue
○
compete
5. According to paragraph 3, which of
the following is true of changes in climate before
the Cretaceous
period and the effect of
these changes on dinosaurs?
○Climate
changes associated with the movement of seaways
before the Cretaceous period did not
cause dinosaurs to become extinct.
○Changes in climate before the
Cretaceous period caused seve
re
fluctuations in sea level, resulting
in
the extinction of the dinosaurs.
○Frequent changes in climate before the
Cretaceous period made dinosaurs better able to
maintain
a livable body temperature.
○Before the Cretaceous period there
were few changes in
climate, and
dinosaurs flourished.