In 1982 David Raup and John Sepkoski, American paleontologists, plotted the number of extinctions per million years for families of invertebrate and vertebrate marine organisms during the last 560 million years. Their graph reveals very high levels of extinction for five geologic periods. The occurrence of mass extinction is well established.

In 1984 Raup and Sepkoski published a paper suggesting that mass extinctions occur at regular intervals spaced 26 million years apart. Many theories attempting to explain this periodicity have been proposed. The idea that mass extinctions occur at these regular intervals is still heavily debated.

First, we will look at proposed causes for mass extinction. Second, we will look at various data as they relate to the geologic time scale. Finally, you will use the data to develop your own theory of mass extinction.

Glaciation

Glaciation occurs as a result of global cooling. Much of the Earth’s water can be locked up in ice sheets that expand over oceans and land. Evidence for glaciations comes from deposits containing glacial sediments and the disappearance of warmer climate species from the fossil record. The global cooling and the drop in sea levels obviously disrupt many ecological niches. Ocean salinity and oxygen content may also change during periods of glaciations. It is clear that glaciation is associated with climate change.

Asteroids

In 1980 Luis Alvarez hypothesized that a large extra terrestrial impact had caused the great Cretaceous extinction. A large asteroid could trigger global fires, earthquakes, tidal waves, atmospheric dust, acid rain, and global warming. Atmospheric dust could cause a nuclear winter in which the Sun’s light is blocked out to such an extent that plants have problems photosynthesizing. Evidence for this impact comes from the presence of a rare element called Iridium found in a layer at the K-T boundary (Cretaceous-Tertiary boundary). Iridium is rare in Earth’s crust, but can be common in asteroids and volcanoes fed by the Earth’s mantle. Shock quartz or quartz grains that are formed from high pressures are also found in these layers as well as a form of carbon formed under intense heat and pressure. Finally, in 1981 a large crater 65 million years old and of the correct size to fit Alvarez’s theory was found in the Yucatan Peninsula of Mexico. The name of this crater is Chicxulub. Some have suggested that asteroids could cause a large distribution of the element nickel, which can prevent plants from photosynthesizing.

Sea Level Changes

Marine regressions or sea level changes can cause major disruptions in ecological niches. The movement of the Earth’s crustal plates can cause sea level changes. As two plates come together seaways can slowly drain away. Sea levels drop and rise as glacial periods come and go. Sea level changes can also affect the salinity and gas content of the water.

Volcanic Activity

Volcanic activity can fill the air with large volumes of dust and gases causing nuclear winter and climate changes. Volcanic activity like asteroids can produce iridium. In the geologic past there have been large basalt floods called basalt traps that form during massive volcanic activity on land.

Other Theories

Cosmic radiation could increase to dangerous levels from a nearby supernova. This cosmic radiation could cause mutation and increased cancer rates among organisms.

Many theories are specific to the demise of the dinosaurs. One theory holds that mammals could have eaten the dinosaurs’ eggs. However, this theory has difficulty explaining why mammals had coexisted with dinosaurs for millions of years. Another theory is that flowering plants were evolving when dinosaurs started dying out and may have been poisonous to them. But this would not explain why many other organisms both terrestrial and aquatic became extinct.

Theories to Explain Periodicity

The idea that mass extinction occurs at regular intervals is heavily debated. There have been many attempts to explain the proposed periodicity of mass extinctions. these include: comet showers, the existence of a planet X, the existence of a companion star to our Sun called Nemesis, sudden overturns of the Earth’s mantle, Earth’s oscillation through the Milky Way galactic plane, meteor impacts, basalt floods, climatic cooling, marine regressions and species-species interactions (Interactions between species may occasionally lead to an instability that cascades through an ecosystem). Some believe that these periods between extinctions just represent the time it takes for extinction sensitive species to evolve.

Steven Stanley in his book Extinction identifies five important extinction processes to keep in mind:

• Extinction occurs on land and sea.
• On the land, animals suffer extinctions repeatedly while plants seem to be
more extinction resistant.
• There is preferential disappearance of tropical life forms in mass extinction.
• Some groups experience extinction repeatedly (trilobites & ammonoids).
• There might be a periodicity to mass extinction.

You have two assignments:

1. Using the data provided look for patterns to create your own theory to explain the cause or causes of the “Big Five”. You will not be making any hypotheses as to the proposed periodicity of extinction.

2. Research and summarize what type of organisms were effected by these mass extinctions.

When you have completed your assignment check your hypotheses with those of other scientists by going to the following web site:

Extinctions: Cycles of Life and Death Through Time

The Five Worst Extinctions in Earth's History

Fossil Types

Bibliography for Echoes of Life Through Time