Transcript The Fossil Record and Geological Time

CHAPTER 25
PHYLOGENY AND SYSTEMATICS
Section A2: The Fossil Record and Geological Time
(continued)
3. The fossil record is a substantial, but incomplete, chronicle of
evolutionary history
4. Phylogeny has a biogeographical basis in continental drift
5. The history of life is punctuated by mass extinctions
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3. The fossil record is a substantial, but
incomplete, chronicle of evolutionary history
• The discovery of a fossil depends on a sequence of
improbable events.
• First, the organism must die at the right place and time
to be buried in sediments favoring fossilization.
• The rock layer with the fossil must escape processes that
destroy or distort rock (e.g., heat, erosion).
• The fossil then has only a slight chance that it will be
exposed by erosion of overlying rock.
• Finally, there is only a slim chance that someone will
find the fossil on or near the surface before it is
destroyed by erosion too.
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• A substantial fraction of species that have lived
probably left no fossils, most fossils that formed
have been destroyed, and only a fraction of
existing fossils have been discovered.
• The fossil record is slanted toward species that existed
for a long time, were abundant and widespread, and
had hard shells or skeletons.
• Still, the study of fossil strata does record the sequence
of biological and environmental changes.
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4. Phylogeny has a biogeographical
basis in continental drift
• The history of Earth helps explain the current
geographical distribution of species.
• For example, the emergence of volcanic islands such as
the Galapagos, opens new environments for founders
that reach the outposts, and adaptive radiation fills many
of the available niches with new species.
• In a global scale, continental drift is the major
geographical factor correlated with the spatial
distribution of life and evolutionary episodes as mass
extinctions and adaptive radiations.
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• The continents drift about Earth’s surface on
plates of crust floating on the hot mantle.
Fig. 25.3a
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• About 250 million years ago, all the land masses
were joined into one supercontinent, Pangaea, with
dramatic impacts on life on land and the sea.
• Species that had evolved in isolation now competed.
• The total amount of shoreline was reduced and shallow
seas were drained.
• Interior of the continent was drier and the weather more
severe.
• The formation of Pangaea surely had tremendous
environmental impacts that reshaped biological diversity
by causing extinctions and providing new opportunities
for taxonomic groups that survived the crisis.
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• A second major
shock to life
on Earth was
initiated about
180 million years
ago, as Pangaea
began to break
up into separate
continents.
Fig. 25.4
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• Each became a separate evolutionary arena and
organisms in different biogeographic realms
diverged.
• Example: paleontologists have discovered matching
fossils of Triassic reptiles in West Africa and Brazil,
which were continguous during the Mesozoic era.
• The great diversity of marsupial mammals in Australia
that fill so many ecological roles that eutherian
(placental) mammals do on other continents is a
product of 50 million years of isolation of Australia
from other continents.
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5. The history of life is punctuated by mass
extinction
• The fossil record reveals long quiescent periods
punctuated by brief intervals when the turnover of
species was much more extensive.
• These brief periods of mass extinction were
followed by extensive diversification of some of
the groups that escaped extinction.
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• A species may become extinct because:
• its habitat has been destroyed,
• its environment has changed in an unfavorable direction
• evolutionary changes by some other species in its
community may impact our target species for the worse.
• As an example, the evolution by some Cambrian
animals of hard body parts, such as jaws and shells, may
have made some organisms lacking hard parts more
vulnerable to predation and thereby more prone to
extinction.
• Extinction is inevitable in a changing world.
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• During crises in the history of life, global
conditions have changed so rapidly and
disruptively that a majority of species have been
swept away.
• The fossil record
records five to
seven severe
mass extinctions.
Fig. 25.5
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• The Permian mass extinction (250 million years
ago) claimed about 90% of all marine species.
• This event defines the boundary between the Paleozoic
and Mesozoic eras.
• Impacting land organisms as well, 8 out of 27
orders of Permian insects did not survive into the
next geological period.
• This mass extinction occurred in less than five
million years, an instant in geological time.
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• Factors that may have caused the Permian mass
extinction include:
• disturbance to marine and terrestrial habitats due to the
formation of Pangaea,
• massive volcanic eruptions in Siberia that may have
released enough carbon dioxide to warm the global
climate
• changes in ocean circulation that reduced the amount
of oxygen available to marine organisms.
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• The Cretaceous mass extinction (65 million years
ago) doomed half of the marine species and many
families of terrestrial plants and animals,
including nearly all the dinosaur lineages.
• This event defines the boundary between the Mesozoic
and Cenozoic eras.
• Hypotheses for the mechanism for this event
include:
• The climate became cooler, and shallow seas receded
from continental lowlands.
• Large volcanic eruptions in India may have
contributed to global cooling by releasing material into
the atmosphere.
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• Walter and Luis Alvarez proposed that the impact
of an asteroid would produce a great cloud that
would have blocked sunlight and severely
disturbed the climate for several months.
• Part of the evidence for the collision is the widespread
presence of a thin layer of clay enriched with iridium,
an element rare on Earth but common in meteorites
and other extraterrestrial debris.
• Recent research has focused on the Chicxulub crater, a
65-million-year-old scar located beneath sediments on
the Yucatan coast of Mexico.
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Fig. 25.6
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• Critical evaluation of the impact hypothesis as the
cause of the Cretaceous extinctions is ongoing.
• For example, advocates of this hypothesis have argued
that the impact was large enough to darken the Earth
for years, reducing photosynthesis long enough for
food chains to collapse.
• The shape of the impact crater implies that debris
initially inundated North America, consistent with
more severe and temporally compacted extinctions in
North America.
• Less severe global effect would have developed more
slowly after the initial catastrophe, consistent with
variable rates of extinction around the globe.
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• Although the debate over the impact hypothesis
has muted somewhat, researchers maintain a
healthy skepticism about the link between the
Chicxulub impact event and the Cretaceous
extinctions.
• Opponents of the impact hypothesis argue that changes
in climate due to continental drift, increased
volcanism, and other processes could have caused
mass extinctions 65 million years ago.
• It is possible that an asteroid impact was the sudden
final blow in an environmental assault on late
Cretaceous life that included more gradual processes.
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• While the emphasis of mass extinctions is on the
loss of species, there are tremendous opportunities
for those that survive.
• Survival may be due to adaptive qualities or sheer
luck.
• After a mass extinction, the survivors become the
stock for new radiations to fill the many
biological roles vacated or created by the
extinctions.
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