The Major Mass Extinctions Of The Phanerozoic
Image from http://www.carleton.ca/~tpatters/teaching/intro/extinction/extinction1.html.
ACTIVITY 1: Read the Fact Sheets and answer the following:
- List 4 theories that try to account for the extinction of
the dinosaurs in the Cretaceous crisis.
- When was this in years before
present (YBP)? What percentage of life became extinct?
- Explain why one theory is favoured.
- Prepare a chart of all the crises or extinctions, in order
- from most recent to earliest - by name, years before present,
and percentage loss.
- Compare this chart with the graph of ice ages in Section
4.2. Check the time line of both graphs carefully. Is there
a relationship between these graphs? State your findings and
be prepared to defend them.
 The Major Mass Extinctions Of The Phanerozoic: Fact Sheets
MEZOZOIC ERA
The Great Cretaceous Mass Extinction
The sudden disappearance of the dinosaurs has long been known
to paleontologists. Numerous attempts have been made to explain
this, with two major theories supporeted the greatest collection of
evidence. Sixty-five million years ago nearly 75-80% of all species
of life on Earth were wiped out in an incredibly short period
of time. In addition to dinosaurs, pterosaurs, belemnoids, many
species of plants (such as the plant that produced Aquilapollenites type
pollen), ammonites, marine reptiles and bivalves also disappeared.
Among the various hypotheses put forward were
- allergies by dinosaurs to
pollen produced by flowering plants
- increased radiation levels
that led to male dinosaur sterility
- diseases brought by migrating
animals
- a gradual climate deterioration until the dinosaurs
were unable to exist.
The first hypothesis ignores the fact that flowering plants
and dinosaurs had coexisted for over 70 million years before the mass extinction.
Increased radiation levels would
not cause the extinction of many of the plant species, nor do
we know if male dinosaurs carried their gonads outside their
bodies.
The migration hypothesis does have some merit. Introduction
of a new animal group to an area will cause ecosystems to change
as well as possibly bring disease. Sheep in Iceland, for example,
were severely affected by a virus brought with new sheep
from Europe. The Icelandic sheep had no resistance to this virus,
although the European newcomers did.
The suggestion that dinosaurs became extinct because they were
unable to cope with a cooling had merit if all dinosaurs were
little more than large ectothermic lizards. The modern view of
dinosaurs suggests they were quite well adapted to their environments.
Although the diversity of dinosaurs shows a drop approaching
the end of the Cretaceous, dinosaurs that did exist, such as Tyrannosaurus
rex and Triceratops, were well suited to their environments.
The Jurassic Extinctions
The Jurassic saw two minor mass extinctions. The first
occurred about one-third of the way through the period, during
the early Jurassic. This first Jurassic extinction affected approximately
80% of the bivalves as well as other shallow water organisms.
At the close of the Jurassic, about 140 million years ago, a
second minor mass extinction happened. This affected mostly the
ammonoids, marine reptiles, bivalves, and certain dinosaur groups
such as the stegosaurs and the giant sauropods. Only one stegosaur
escaped this extinction, Dravidosaurus, a small stegosaurid
restricted to the Cretaceous of India, which was an island at
that time. The causes of both of these extinctions are not clear.
The Triassic Extinction
At the end of the Triassic a minor mass extinction occurred.
The cause of this is still in dispute, although the consensus
opinion favors global cooling associated with an asteroid impact
or comet showers. The victims of this event included the labyrinthodont
amphibians, conodonts, and all marine reptiles except ichthyosaurs.T
hecodonts, brachiopods, gastropods, and molluscs also suffered
a loss of diversity as did the synapsids. As with larger mass
extinctions, the Triassic die off opened ecological spaces into
which the dinosaurs and other creatures moved during the Jurassic.
PALEOZOIC ERA
The Great Permian Extinction
The Permian is geologically marked by the final assembly of
Pangaea, glaciation in the souther extreme of Gondwana, and the
greatest mass extinction in Earth history that occurred at the
close of the Permian. The Permian world was marked by a nearly
pole-to-pole supercontinent, Pangaea, that was surrounded by
Panthalassa, the world sea. A small ocean, Tethys (today akin to the Mediterranean Sea) was also present.
The end of the Permian, also the end of the Paleozoic era, was
marked by the greatest extinction of the Phanerozoic eon. Despite
its magnitude, the terminal Permian extinction has not received
the amount of publicity or research that the more famous, but
lesser, terminal Cretaceous extinction has. During the Permian
extinction event, whose causes remain controversial, over 95%
of marine species went extinct, while 70% of terresdtrial taxonomic
familes suffered the same fate. The fusulinid foraminiferans
went completely extinct, as did the trilobites. Brachiopod genera
declined from 60 to 10. The majority of extinctions seem to have
occurred at low paleolatitudes, possibly suggesting some event
involving the ocean.
Plants seem to have missed the great extinction. What floral
changes occurred during the Permian occurred earlier in the period,
when the gradual drying out of the continent led to the evolution
and spread of better adapted dry forms such as gymnosperms
and seed ferns to replace the swamp trees of the Carboniferous
such as treelike lycopods and sphenopsids. One major plant group
disappeared, the swamp gymnosperm Cordaites.
Victims of the Permian extinction:
- fusulinid foraminifera trilobites rugose and tabulate
corals
- blastoids acanthodians placoderms
- pelycosaurs
The cause (or causes) of the Permian extinction remain in dispute.
- The Siberian Traps are a massive lava flow in eastern Asia
that may have contributed to (or even caused) the Permian event
by triggering a massive, sudden glaciation as well as other
environmental consequences of volcanic eruptions. The period
of greatest eruption in the traps coincides with the mass extinction.
Ages of the lava flows suggest the traps formed over a one
million year interval. The traps are near the city of Tura
and also occur in Yakutsk, Noril'sk and Irkutsk, covering an
area of slightly less than 2 million square kilometers (an
area larger than Europe). The original volume of the traps
is estimated at between 1 million and 4 million cubic km.
- The Cretaceous-Tertiary extinction that marks the close of
the Mesozoic era has been hypothesized as a result of a large
meteor or comet strike. Evidence of a similar (and larger)
impact at the close of the Permian is not strongly supported,
although some indirect evidence suggests an impact did occur
during the Permian, although possibly not at the time of the
extinction crisis.
- Climate change, possibly caused by glaciation and/or volcanic
activity, has been associated with many mass extinctions. It
seems likely that climate change is a consequence of the cause
of extinction rather than the root cause itself.
- Formation of Pangaea has been invoked as a cause for the
extinction. However, the formation of Pangaea had occurred
well before the mass extinction. The mountain building and
associated tectonism would also have been extremely long term
in duration, possibly 10 million years according to some studies
of the snowball garnets from Vermont. Pangaea's presence may
have led to extreme environments with hotter interior areas
of the continent and colder polar areas, possibly producing
glaciation.
- There is evidence of a sudden drop in sea level at the end
of the Permian. This could be attributed to the sudden cooling
of the climate that produced glaciers at the southern polar
areas in Gondwana. Glacial tillites and other geological evidence
of late Permian age occur in Australia, Siberia and in the
North Sea are interpreted as proof of the existence of large
continental glaciers at the close of the Permian.
- Poisoning of the ocean has been suggested due to an apparent
drop in carbon isotope data obtained from marine sediments
formed at the time of the extinction. The cause of this apparent
dropoff in the photosynthetic rate in the seas has not yet
been determined.
Whatever cause (or combination of causes), the terminal Permain
extinction was a massive and severe crisis for life. Many groups
of organisms went extinct at that time. Surviving groups diversified
during the Triassic period and gradually a more modern world
developed.
The Carboniferous Extinction
Glaciation in Gondwana during the late Carboniferous time contributed
to decline in marine environments and fluctuating sealevels that
alternately formed and destroyed coastal coal swamp environments.
There appears to have been no mass extinction associated with
this glaciation, unlike the Devonian mass extinction.
The Devonian Extinction
Near the end of the Devonian another mass extinction occurred.
This one was more severe on marine creatures than on the newly
established terrestrial forms. The corals were quite seriously
decimated, and the return of extensive reef building did not
happen until the Triassic with the evolution of a new group of
reef-building corals, the scleractinians. Brachiopods, trilobites
and primitive fish groups either were diminished or completely
snuffed out by this extinction event.
Global cooling tied to Gondwanan glaciation has been proposed
as the cause of the Devonian extinction, as it was also suspected
of causing the terminal Ordovician extinction. Rocks in parts
of Gondwana suggest a glacial event. The forms of marine life
most affected by the extinction were the warm water to tropical
ones. Another hypothesis for this extinction is that an asteroid
impact caused a global cooling. There are several impact sites
known to be of the right potential age to have been involved
in this extinction. Neither hypothesis, glaciation or impact,
is unequivocally supported by the available data.
Global cooling tied to Gondwana glaciation has been proposed
as the cause of the Devonian extinction, as it was also suspected
of causing the terminal Ordovician extinction. Rocks in parts
of Gondwana suggest a glacial event. The forms of marine life
most affected by the extinction were the warm water to tropical
ones. Another hypothesis for this extinction is that an asteroid
impact caused a global cooling. There are several impact sites
known to be of the right potential age to have been involved
in this extinction. Neither hypothesis, glaciation or impact,
is unequivocally supported by the available data.
The Paleozoic Era literally translates as the "time of ancient
life" and spans the time period between 544 and 245 million years
ago. The Paleozoic is the first of three eras within the Phanerozoic
Eon (the time of visible life). While life originated during
the Archean and increased in complexity during the earlier Proterozoic,
the Paleozoic Era is marked by the spread of animals with hard
preservable parts such as shells and exoskeletons. This
has led to what is popularly known as the Cambrian Explosion,
the sudden appearance of a stupendous array of animal life, much
of which is not closely related to modern forms. Despite extinctions
at various times, the Paleozoic is notable for the increasing
modernization of life. By the end of the Paleozoic, almost all
major groups of life had developed. The Paleozoic ended in the
greatest mass extinction event in world history. During this
massive die-off nearly 96% of all marine species went extinct.
During the Paleozoic we see several major advances in life.
The aformentioned Cambrian Explosion is the first. The evolution
of plants from some group of green alge during the Ordovician
is another, since these plants moved from water onto land, paving
the way for vertebrate animals to follow. The first vertebrates, amphibians,
were little more than legged fish, although their remote descendants
would come to rule the land as reptiles, the first truly
terrestrial vertebrates.
The Silurian World
Silurian climates were characterized by a warming and return
to moderate. Silurian is not marked by a major mass extinction
event.
The Ordovician Mass Extinction
The end of the Ordovicianperiod saw another mass extinction,
accounting for the second most severe loss of marine species
during the Phanerozoic. This mass extinction saw the loss of
one third of all brachiopod and bryozoans, and the loss of groups
of conodonts, trilobites, and graptolites. The falling sea-levels
caused by the growth of the continental glaciers caused much
of the reef-building fauna to become either locally or globally
extinct. Over one hundred families of marine invertebrates disappeared.
However, this loss paved the way for an adaptive radiation during
the next period, the Silurian.
The Cambrian Extinctions
The Cambrian period is marked by as many as four mass extinctions.
The first of these mass extinctions resulted in the disappearance
of the archaeocyathids and a major group of trilobites. The later
extinctions limited the diversity of conodonts, brachiopods,
and other triolobite groups. The Cambrian is bracketed by large
scale glaciation at the close of the Proterozoic and by a similar
occurrence early in the Ordovician. Development of continental
glaciers would have several consequences: cooling of the planet
and a drop in sea level. Both of these might have happened quickly
enough to result in a mass extinction.
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