Download The history of life is punctuated by mass extinction

PHYLOGENY AND SYSTEMATICS
Part 2
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
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 and recognize the fossil on or near the surface
before it is destroyed by erosion too.
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.
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 such as
mass extinctions and adaptive radiations.
• The continents drift about Earth’s surface on
plates of crust floating on the hot mantle.
Plate Boundaries
Plate Boundaries
The San Andreas Fault
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.
• A second major
shock to life
on Earth was
initiated about
180 million years
ago, as Pangaea
began to break
up into separate
continents.
Each continent 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,
These land masses were connected during the
Mesozoic era when these reptiles thrived.
• The great diversity of marsupial mammals in Australia
that fill so many ecological roles that placental
mammals do on other continents is a product of 50
million years of isolation of Australia from other
continents.
The history of life is punctuated by mass
extinction
The fossil record reveals long quiet periods
punctuated by brief intervals when the turnover of
species was much more extensive.
• These brief periods of mass extinction were
followed by adaptive radiation and extensive
diversification of some of the groups that escaped
extinction.
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.
• 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.
THE PERMIAN EXTINCTIONS
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.
What Caused the permian extinctions ?
Factors that may have caused the Permian mass
extinction include:
• disturbance to marine and terrestrial habitats due to the
formation of Pangaea,
• Global warming - 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.
The cretaceous extinctions
goodbye to the dinosaurs - hello mammals
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:
• Global Cooling - 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 and
blocked sunlight needed by plants and other photosynthetic
species
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.
• Regardless of the cause, global cooling contributed to
the large scale extinction of gymnosperm plants, which
were the main source of food for many dinosaur
species.
The shape of the impact crater indicates a fireball and spread of
debris in a geographic pattern that matches the rate of extinction
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.
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 which 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.
Adaptive radiation of surviving species
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.
• Example: after the extinction of the dinosaurs, there was a
large adaptive radiation of mammals and the subsequent
diversification into many new species.