Chapter 13 Biodiversity Through Earth History

Chapter 13

Biodiversity Through Earth History

The Fossil Record of Biodiversity

Underlying assumption is that the process of evolution is occurring

• evolution: creation of new species

• random mutation: genetic changes

• natural selection: environmental pressures favor certain characteristics

• adaptation: favored characteristics prevail

• extinction: death of all members of a species


The text starts off with two ideas about how the number of species changes over time that do not seem to pan out

One view is that, perhaps, diversity is a dynamic process:

logistic growth – “S-shaped” growth observed for populations, common model used in population dynamics

rate of change in number = origination - extinction of species on earth rate rate

This does not seem to be the case.

These observations do not appear to be logistic, but do they indicate exponential growth in # of species?

Problem with this: # of species may simply be a function of the amount of sediment available.

Does not appear to be either logistic or exponential



The taxonomic tree for homo sapiens

Data on the number of new appearances of marine invertebrates of a given age (namely, as they appeared) by phyla-classes-orders taxonomic levels. Compare to previous data - no exponential growth is observed when looking at the evidence this way


Marine invertebrate fossils indicating diversity through geological time

At least five major extinctions over the past 500 m. y.

Largest mass extinction at end of Permian, 251 mya. Best researched extinction (dinosaurs) is the Cretaceous-Tertiary (K-T), 65 mya.

Fossil record indicates high taxonomic levels are less affected, and that lower taxonomic levels recover rapidly from mass extinctions

Mass extinctions cause great losses but stimulate origination.

440 my360 my

251 my

206 my

65 my


The Cretaceous-Tertiary (K-T) Mass Extinction(see chapter 1 also)

• Extinction of dinosaurs, many other marine and terrestrial species (both animal and plant)

• Iridium: normally deposited at a slow, known, rate from extraterrestrial sources. Iridium concentrations in the layer at the K-T boundary are so high that it could not have been deposited by normal deposition, must have been a massive, sudden extraterrestrial source.

• Most iridium on earth is in core, concentration in asteroids are much higher than on earth’s surface

The Cretaceous-Tertiary (K-T) Mass Extinction(see chapter 1 also)

• Estimated size of meteorite, from amount of iridium, = 10km diameter

• Example of a “shock” to the earth system. The climate, biosphere, recovered, but at different rates. Biosphere “recovery” was very different from prior biosphere.

The Cretaceous-Tertiary (K-T) Mass Extinction

What caused it, and how did the earth system respond?

This has been the most studied, and perhaps most controversial, extinction event. Big debate in the scientific literature in the 1980s – 90s

Realistic possibilities:

• Sudden sea level or climate change

• Volcanic eruptions

• Meteorite (comet or asteroid, extraterrestrial) impact


Sudden sea level or climate change

Not likely, because some of the times of greatest climate and/or sea level changes have not coincided with mass extinctions

Volcanic eruptions or extraterrestrial impacts

the idea is that the heat and explosion from the impact would kill everything regionally, and that the subsequent cooling from dust / soot would kill many things globally. Volcanic eruptions could result in global cooling from sulfur aerosols in the stratosphere.

Impact would also send hot particles across the globe starting fires; destroy the ozone layer for several years; caused tsunamis; food would be scarce for survivors; CO2 would have been released increasing temperatures for thousands of years; toxics across the globe; slow recovery time of biosphere.


Meteorite impact theory

Luis Alvarez’s 13 predictions which indicate that the geological evidence is consistent with the meteorite impact theory (only some listed here)

• Iridium layer should be at the K-T boundary worldwide

• Frequency of large meteorites is sufficient to explain the record

frequency of collisions of large objects with Earth.



Luis Alvarez’s 13 predictions that show that the geological evidence is consistent with the meteorite impact theory

•Both plants and animalsshould have been affected

•clays in the iridium layershould be identical worldwide, be differentthan the layers aboveand below, and have an extraterrestrialsignature

Evidence for mass extinction of plants



Luis Alvarez’s 13 predictions that show that the geological evidence is consistent with the meteorite impact theory

•There should be evidenceof high temperature and high pressure at impact.Sphericals (from cooling of molten droplets in air)and shocked quartz (only forms when quarts is underhigh pressure) are there.

•Evidence of wildfires

•Layer should be between a change in fossils




Luis Alvarez says that the geological evidence is inconsistent with the volcano theory

• Volcanoes would notdistribute the spherules globally

• Shocked quartz has neverbeen found in volcanicdeposits, but is often foundin meteorite impact craters

• Volcanic deposits tend to havelow iridium concentrations




Additional evidence:

• Helium isotopes found in the layer are extraterrestrial

• The Chicxulub Crater: - associated with cenotes

- contain spherules and shocked quartz

- enriched in iridium

- in the right age range

Location of the Chicxulub impact crater

Size of impact crater


Did the biological pump shut down for hundreds of thousands of years?

Currently, and prior to K-T, there is adifference in C13/C12 ~2o/oo

Difference is caused by the biologicalpump, algae preferentially take up C12, then sink to bottom and decompose

After K-T, much smaller difference.

Extraterrestrial influences and extinctions

Asteroids: rocky material composed of minerals and metals from dust to 100km in size found mostly in the asteroid belt (between Mars and Jupiter)

Comets: materials that are volatile on earth but solid in space, plus some minerals and metals

found mostly in the Oort Cloud


Both asteroids and comets can be deflected, by collision, into different orbits that might cause them to collide with a planet, including earth.

Jupiter acts as a major gravitational attraction to deflected asteroids, affecting their paths and possibly sending them towards other planets


Is there a 26 million year periodicity in the extinction record?

4 possible explanations:

1. Galactic plane hypothesis

2. Companion star hypothesis

3. Planet X hypothesis

4. Artifact of the way the data are collected

Vertical lines drawn every 26 million years

Evidence supporting meteorite impact and its effect on plants at the K-T boundary

Support for meteorite impact as a cause for the K-T mass extinction: frequency of collisions of large objects with Earth.

Location of the Chicxulub impact crater

Size of impact crater




1. Galactic plane hypothesis: our solar system moves up and down in a 26 million year cycle through the plane of the galaxy. When we pass through the center, we encounter more gas/dust, higher chance of disturbing the orbit of an asteroid or comet




1.

2. Companion star hypothesis: our sun has a companion star (small brown dwarf, <.08 size of sun) in an orbit that passes through the Oort cloud every 26 million years




1.

2.

3. Planet X hypothesis: there isa planet that we have never seen,located in an orbit beyond Pluto,that disturbs the Kuiper Belt




1.

2.

3.

4. Artifact of the way the data are collected:data are collected in stages, perhaps this results in some funnycyclicity in the data

Vertical lines drawn every 26 million years

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Chapter 13 Biodiversity Through Earth History