Iridium: A member of the platinum group
of metallic elements. There
is very little iridium found in the Earth's crust but larger
amounts
found in meteors and asteriods. However, there is an unusually
large
amount of iridium found in rocks deposited at the
Cretaceous-Tertiary boundary. This increase in iridium could have
resulted from collision with an asteriod or could have been brought
up from deeper layers in the Earth by volcanic activity.
Extinction: The total disappearance of a
species.
Local extinction: The disappearance of a
species from part of its
geographic range.
Mass extinction: The simultaneous
elimination of large numbers of
species on a global geographic scale. Such mass extinctions have
occurred from 5-12 times in the history of the Earth. The rate at
which species have disappeared during mass extinctions greatly
exceeds the rate of background extinction.
Background extinction: The rate of
extinction of species over a
long
period of time. Species are outcompeted and replaced by better
adapted species on a continuous basis. This relatively slow process
of species replacement results in the background extinction rate.
Taxon cycle: A cycle of immigration, adaptation, and extinction of species on islands. The cycle has several stages.
Hypotheses of background extinction:
Human-mediated extinction: Extinction of
species associated with
human immigration and colonizations. Humans have directly
exterminated vulnerable species. Humans have also indirectly caused
extinction of species by introducing competitively strong pests or
by the elimination of habitat.
Epicontinental seaway: A large inland sea
that covered the central
part of North America, separating the western part of the continent
from the eastern part. This seaway regressed during the early part
of the Tertiary Epoch.
Correlation: The cooccurrence of two
events. The coincidence of the
two events does not prove a cause and effect relationship between
them. For instance, the fact that most car accidents occur close to
home does not mean that people are particularly careless near home
or that long trips are safe. The coincidence occurs because most
car
trips begin at home and are relatively short.
Familial extinction: The number of
families of organisms going
extinct in a million years.
Tethys Sea: The tropical sea that covered
the area now occupied by
the Mediterranean Sea.
Catastrophe: A single event that set in
motion a chain of other
events, therby causing major biological changes and extinctions
within at most a few thousand years.
Asteroid impact theory: An hypothesis
suggesting that the mass
extinction which occurred at the Cretaceous-Tertiary boundary
resulted from the impact of an extraterrestrial body such as an
asteriod. The impact would have left a large amount of iridium in
rocks from the boundary. The impact would also have cast up into
the
atomsphere a large amount of dust, suppressing photosynthesis and
causing a temporary collapse in the food chain of many species.
This
lack of food would have led to a mass extinction of species.
Marine paleotemperature curve: A record
of the temperature in the
oceans in ancient times. There is evidence to indicate that there
was a cooling trend across the Cretaceous-Tertiary boundary. There
is some conflicting evidence to the contrary that indicates that
the
trend was a warming one.
Noncatastrophe theories: Extinction could have resulted from any one of several terrestrial events, including
Catastrophe theories: Extinction could have been initiated by severe short term disruptions, including
Continental drift: Movement of continents
from one place to
another,
caused by convection currents of molten material under the Earth's
crust. There are 4 bodies of evidence that show that the continents
have been oriented in different places in the past.
Climatic cooling: A gradual decrease in
the mean temperature of the
Earth. Cooling temperatures, and sometimes major glaciation events,
are associated with most of the episodes of mass extinction.
Ice Age: Periods of glaciation during the
Pleistocene Epoch. As a
result of the cooling, much sea water was locked up in the ice caps
of the Earth, leading to a drop in the level of the ocean. Also,
the
latitude at which organisms could survive moved toward the equator.
Cooling hypothesis: The species that went
extinct were those
adapted
to warm water or to a narrow temperature range, and they could not
escape the cooling pulse associated with glacial expansion. Pacific
species migrated as water temperature changed, and so could find a
habitat for survival. In the Atlantic, Mediterranean and Caribbean
areas acted as basins from which marine species could not escape
the
changing temperatures.
Foraminifera: Ameba-like protozoans with
a protective shell,
usually
of calcium carbonate but sometimes of silica. Most of them dwell on
the bottom of the sea.
Globigerines: A group of planktonic
foraminiferans, common in cool
water. They became dominant during the Oligocene (about 30 million
years ago), lending some support to the cooling hypothesis.
Psychrosphere: A layer of cold water in
the deep sea, thought to
have formed during the late Eocene (about 40 million years ago) by
the flow of frigid ocean water from the poles toward the equator.
Trilobites: A group of extinct arthropods
first known from the
Precambrian. The body was divided by 2 longitudinal furrows into 3
lobes. The trilobites were dominant marine animals during the
Cambrian, but suffered several extinction events, culminating at
the
end of the Devonian Period.
Ammonoids: Swimming cephalopod molluscs
(like squid with shells;
a
modern type is the nautilus) carrying a chambered spiral shell.
They
arose during the Devonian Period and went extinct with the
dinosaurs. They were probably swimming predators.
Acritarchs: Single-celled phytoplankton,
apparently the first
organisms to evolve cells with nuclei. They were abundant in the
Precambrian, during which they suffered the first known mass
extinction at a time of extensive glaciation 650 million years ago.
They later recovered to become the largest group of fossil
phytoplankton preserved in Paleozoic rocks.
Last updated on January 30, 2004
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