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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.
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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…
Cambrian explosion- the most remarkable burst of evolution. During this time, all phyla as well as many classes and orders evolve. Higher taxa have greater resiliency 15% After a 100 million years, diversity levels off. Mass extinction- when 25% or more of extant families disappear. These occur at terminal Ord.-440Ma, Dev.-360Ma, Perm.-251Ma, Triassic-206Ma, and Cret.-65Ma. 50% 85%
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 10km-wide 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.
