Transcript Document

Biodiversity through Earth History
The fossil record is the only strong evidence of biodiversity
through time.
Evolution- the descent, with modification (through genetic
mutation) of preexisting life forms.
Natural selection- the unequal survival and reproduction of
organisms, owing to environmental pressures that result in the
preservation of favorable characteristics. This is the most
important component of evolution next to mutations themselves.
Adaptations- characteristics that enhance an organism’s survival
and/or reproductive success.
Extinction- the loss of all individuals of a species.
Rate of change in number of species on earth = origination rate extinction rate
It is estimated that 10 to 25 species evolve each year and that the
same number become extinct. However, it is clear from the fossil
record that some periods of geologic time had much greater rates
of evolution and extinction.
Logistic growth- population increases most rapidly when
numbers are low and then the birth rate decreases and/or the
death rate increases due to competition for food and space,
disease etc. until the birth rate equals the death rate at the
carrying capacity.
Species- all closely related organisms that can potentially
interbreed and produce fertile offspring.
Because we can’t observe animals breeding in the past,
paleontologists use similarity in morphology to define species.
Taxonomy- the systematic organization of living or fossil
organisms into a hierarchy.
Embryonic Development
The embryonic
development of all
vertebrates shows
remarkable similarities. The
drawings in the top row are
of the embryonic stage
called the pharyngula. At
this stage ("I") they all
contain a:
Notochord
Dorsal hollow nerve cord
Post-anal tail
Paired branchial grooves.
There are some problems with
using the fossil record as a true
picture of species diversity…lots
of problems…
We generally use the record of
marine invertebrates.
At first glance it looks as though
species numbers have increased
exponentially through geologic
time…not quite true…
Higher taxa have greater
resiliency
15%
Cambrian explosion- the most
remarkable burst of evolution. During
this time, all phyla as well as many
classes and orders evolve.
After a 100 million years, diversity
levels off.
50%
85%
Mass extinction- when 25% or more
of extant families disappear. These
occur at terminal Ord.-440Ma, Dev.360Ma, Perm.-251Ma, Triassic206Ma, and Cret.-65Ma.
The Cretaceous-Tertiary Mass Extinction
65 million years ago about 50-75% of species became extinct.
Four possible causes have survived the 20th century.
1) Sudden sea level change; 2) Sharp temperature changes; 3)
volcanic eruptions; 4) Meteor impacts
Sea-level change and climate change
This was formerly the number one theory for the K-T extinction.
Now, it remains number one only for the post Ordovician
extinction. In fact some of the largest sea-level fluctuations occur
with few species lost.
Meteorite impact and volcanic eruptions
Iridium anomaly- this metal is rare on the Earth’s surface but is
common deep down and in meteorites. When it was found in 1979
in a clay layer in Grubbio Italy, it was labeled the “smoking gun”
that killed the dinosaurs. Other however thought that volcanism
was responsible.
Supporters of the impact theory provided a number of predictions
that would serve as tests for the theory.
1) An iridium anomaly should be found worldwide. The
anomaly has now been found in 75 locations globally. A 10kmwide body is required to generate this signal.
2) This enrichment should always be found within the same
interval of geologic time. All iridium layers are found within
polarity chron C29R, which is dated from 65.6 to 65.9 million
years. These researchers place the K-T boundary at 65.0 Ma.
3) Large meteorites strike Earth sufficiently frequently to
explain the extinction record. Eugene Shoemaker
demonstrated a strong relationship between frequency and size of
impacts. A 10km bolide should strike the Earth once every 100
Ma. In 1908 a small comet probably exploded over Tunguska
Siberia.
4) On shorter time scales, such events should be rare. In
polarity chron C29R, there are no other iridium enrichments. The
34 million year sandwich around the K-T boundary has no other
anomalies. In fact it is by far the biggest anomaly of geologic time.
5) Plants as well as animals should have suffered as a result
of the meteorite impact. There is evidence of a significant
turnover in the types of vegetation inhabiting the land surface.
Much of this work is based on the spore and pollen record.
6) The gross chemical composition of the boundary clays
should be identical worldwide, given that they all originate
from the same excavated material. Geochemically, material
from around the world is very similar.
7) The boundary clays should differ in composition from
more typical clays deposited above and below the boundary
clays at individual sites. This has been confirmed at a number of
sites.
8) Any chemical or isotopic signature in the boundary clay
will have a significant extraterrestrial component. The iridium
anomaly is the best studied of these anomalies, but others could
also be extraterrestrial.
9) The boundary clays should bear some evidence of the high
temperatures generated during impact. Small spherules of
silicate minerals have been found in the layers. These represent
molten drops of material ejected during an impact.
10) The boundary clays should bear some evidence of the
high pressures generated during impact. Small, fractured
grains of shocked quartz (formed at very high pressures) are often
found in the boundary clays.
11) The K-T event should have generated wildfires that might
have left a sedimentary record of charred material. Charcoal is
found in the boundary layer.
12) The iridium-rich layer should be just above the last
dinosaur fossil. Dinosaur remains have been found within 2m of
the boundary. Due to the Signor-Lipps effect this relationship isn’t
all that meaningful. Dinosaurs are rare in general.
13) The pattern of extinction should show no evidence of
preferential survival of species that were well adapted to the
Cretaceous environment. Most mass extinctions are
nonspecific, particularly the K-T extinction. Almost all land
mammals larger than 25kg (55lbs.) became extinct.
The volcanic theory of mass extinction is supported by much of
the same evidence as the extraterrestrial theory.
Walter Alvarez has proposed evidence against the volcanic theory.
1) Sand-sized spherules, even if ejected by volcanoes, would not
reach ballistic orbits (as do impact ejecta) or be distributed
globally, as the K-T spherules are observed to be.
2) Shocked quartz of the sort found at the K-T boundary has never
been found in deposits of volcanic origin but is common in
deposits with known impact craters.
3) Volcanic ejecta tend to have
very low iridium concentrations.
Fullerenes are 60 carbon
buckyballs that contain
predominantly 3He which is a
strong extraterrestrial signal.
The Smoking Gun: The Chicxulub Crater
In the early 1990s
scientists finally found an
impact structure that was
about the right age for a
K-T bolide impact. If this
structure is 200km in
diameter, it is the largest
on Earth and one of the
largest in the Solar
System.
Horizontal gradient map of the
Bouguer gravity anomaly over
the Chicxulub crater. The
coastline is shown as a white
line.
A striking series of concentric
features reveals the location of
the crater. This image was
constructed from gravity
measurements taken by
Petróleos Méxicanos beginning
in 1948 in the course of
petroleum exploration and more
recent work by the Geological
Survey of Canada, Athabasca
University, the Universidad
Nacional Autónoma de México,
and the Universidad Autónoma
de Yucatán.
Three-dimensional Bouguer
gravity anomaly map over
the Chicxulub crater (North
is up.).
Environmental impacts of the K-T Meteorite Impact
Prior to the impact, there was a low equator to pole temperature
gradient with ice free poles. Dinosaurs lived close to the polar
latitudes but were not as common as a few 10s of millions of
years earlier.
A large meteorite passing through the atmosphere would have
converted nitrogen gas to nitric oxide which would destroy the
ozone layer.
Anything living in the Yucatan would have been destroyed
instantly. A 200km crater would have been excavated tossing
enormous amounts of material into the atmosphere that would
quickly encircle the Earth.
As the impact ejecta passed back through the atmosphere, the air
would be heated and fires ignited.
Environmental impacts of the K-T Meteorite Impact
The soot that enters the atmosphere would be accompanied by
sulfuric acid generated by volatilized anhydrite (CaSO4) in the
Yucatan. Both of these would block sunlight.
Huge tsunamis (~150m) would be generated that would wreck
coastlines for 100s to 1000s of km.
The blocking of the sunlight would kill off plants and cool the
climate quickly. Acidic snow would fall and climate would remain
cold for perhaps 6 months or more.
Thousands of gigatons of carbon dioxide is released as limestone
is volatilized. This could have caused temperatures to increase by
10-15°C.
Strangelove Ocean
Usually 13C is enriched in surface
water because 12C is preferred by
organic matter which sinks to the
bottom of the ocean.
Zachos, Arthur, & Dean, Nature 337:61-64, 1989
Extraterrestrial Influences and Extinction
Comets are icy balls of water, ammonia, methane, carbon
dioxide, as well as rocks and metals.
Oort Cloud- a spherical reservoir of comets that extends more
than one light year from the Sun. These may be disturbed by
passing stars that can result in comets coming from any direction.
Asteroids are composed of minerals and metals. They orbit the
Sun in a region known as the Asteroid Belt between Mars and
Jupiter. If the big ones were to hit the Earth, we would all be
finished.
Siderophile- “iron-loving” elements like iridium are concentrated
in the interiors of metallic/rocky planets and asteroids which is
why it is thought that an asteroid caused the K-T Ir anomaly and
extinction.
Periodicity of Impacts and Extinctions
This is probably not likely to be true for a number of statistical
reasons.