8. Dinosaur Extinction Theories

Summary

Cartoon by Chris Madden

Ark cartoon - Chris MaddenWhat caused the extinction of the dinosaurs? Was it a massive meteor? Was it the result of tremendous volcanic activity that covered the globe in volcanic ash? Was it the effects of gradual climate change? Was it the result of plate tectonics? Students are given a set of evidence cards that state scientific discoveries about rocks from 65 million years ago and prediction cards that discuss the predicted effects of different events such as a meteor impact, a rise in carbon dioxide levels, and plate tectonics. Teams of students are charged with assembling this information into an overarching theory that best explains the evidence at hand. At the end of the period or the following period, teams present their theories to the whole group and enter into a debate about the cause or causes of the great dinosaur extinction.
 

Objectives
Can sort through evidence and come up with a scientific theory that best fits the data.
Can recognize whether evidence is consistent with a scientific theory.
Can use geologic evidence to propose theories about past life on earth.

 

Vocabulary
Theory
Extinction
Mass extinction
K-T boundary
Iridium

Attachment Size
8extinction_theories.doc 63 KB
extinction_handout.doc 33.5 KB
evidence_cards_v2.doc 47.5 KB

8. Extinction Theories - Logistics

Time
5 minute introduction
40-50 minutes to come up with theories
45-55 minutes to present theories and discuss conflicting theories

Grouping
Groups of 3 students

Materials

  • 1 set of evidence cards per team
  • 1 envelope per team
  • 1 “Dinosaur Extinction” worksheet per team

Setting
classroom

8. Extinction Theories - Background

Teacher Background
There have been many mass extinctions throughout the history of the Earth. A mass extinction may be defined as an episode in geologic history where over half of the species in existence become extinct in a relatively short amount of time (just a few million years). The worst mass extinction came at the end of the Paleozoic Era 245 million years ago when nearly 95% of plant and animal life in the seas disappeared. Another mass extinction may be happening today. Evidence from the fossil record shows that, on average, only 10-100 species become extinct per year. Some estimates show that current rates of extinction are as high as 27,000 species per year.

Probably the most famous mass extinction happened 65 million years ago when the dinosaurs disappeared. This is generally called the K-T extinction since it occurred at the boundary between the Cretaceous (K) and Tertiary (T) periods. Whatever triggered the extinction of the dinosaurs also caused the death of nearly 60-70% of all the other species on Earth. Interestingly, not all groups of organisms were affected equally. Ocean species were hit harder than land-based species, with 90% of them becoming extinct. Birds were the only survivors of the dinosaur lineage. Interestingly, mammals, lizards, snakes, and other smaller terrestrial creatures were hardly affected. For some reason, ferns actually expanded and thrived during this time.

So what caused the dinosaur extinction? The clues can be found in the rocks that date from 65 million years ago. Some pieces of evidence are agreed upon by nearly all scientists:

  1. Around the time of the K-T extinctions, there was global climate change. What was once a warm, mild climate changed to one that was more varied (sometimes very hot, sometimes very cold).
  2. In India, there was extensive volcanic activity, known as the Deccan Traps. Rocks that date from 65-68 million years ago in southern India are almost entirely igneous (volcanic). Almost 200,000 square miles was covered in lava over a period of 3 million years (an area equivalent to the entire mid-western United States). In some places, the lava beds are a mile deep.
  3. A scientist called Alan Hildebrand found evidence of what is most likely an enormous asteroid impact site in the Yucatan region of Mexico. He called the 110 mile wide crater Chicxulub. The crater has been dated as 65.3 million years old (300,000 years before the mass extinctions). The size of the crater is comparable to that which would have been caused by an asteroid 6 miles wide.
  4. Rocks that date from 65 million years ago contain high amounts of soot
  5. Rocks that date from 65 million years ago contain unusual amounts of the rare element iridium. The levels of iridium contained in the rocks from this time period are roughly 30 times the normal levels. The most likely sources of high levels of iridium are:
    • from outer space in the form of cosmic dust from a nearby exploding supernova
    • from outer space carried to Earth by an asteroid or meteor
    • from eruptions of massive volcanoes.
  6. Major changes in the organization of the continents were occurring around 65 million years ago due to plate tectonics. There is evidence that the oceans were receding. For example, a shallow sea once covered what is now the mid-western United States. This sea drained away over several million years around the time of the dinosaur extinctions as the Colorado Plateau rose.

Yet, even with an abundance of evidence, there is no consensus among scientists as to what happened at this time. Generally speaking, scientists are divided between two camps:

Gradualists – These scientists believe that the fossil record indicates a gradual decline over 5-10 million years. This time frame is more consistent with long term events such as plate tectonic forces and massive volcanic activity. These scientists believe that plate tectonic forces caused extensive volcanic activity in India and perhaps elsewhere that resulted in dense clouds of soot being released into the air. The soot darkened the skies resulting in global climate change and the reduced survival of plants, algae and plankton. In addition, the volcanoes likely released large quantities of carbon dioxide, further aggravating the climate through global warming and acid rain.

Catastrophists – These scientists believe that the fossil record indicates a sudden decline that is more consistent with a catastrophic event such as a massive asteroid impact. This theory was first proposed in 1980 by Walter Alvarez and is often referred to as the Alvarez Theory. He based his claims on the high iridium levels in rocks of that time period – suggesting that the isotopic profile of the iridium is more consistent with an extraterrestrial origin (a meteor or comet impact) than a volcanic origin. This is consistent with the presence of shocked quartz (metamorphically transformed quartz resulting from intense shock waves) in the rocks of that age. The resulting blast would have destroyed everything within 250-300 miles, including the object. Trillions of tons of debris (like dust, smoke, and steam) would have been thrown into the atmosphere when the object vaporized, darkening the sky around the globe in just a few weeks. Earthquakes, tsunamis, and wildfires would almost certainly have been triggered. The darkness may have only persisted for a few years but the effects on plant life would have been devastating.

The biggest problem with the catastrophic theory is that no conclusive crater has been identified. The most promising is evidence of an ancient crater called Chicxulub that was discovered in the Yucatan peninsula of Mexico. It is widely believed that Chicxulub is indeed the result of a massive asteroid nearly 6 miles across. Unfortunately, the crater itself is dated to 300,000 years before the K-T extinctions themselves. Could there have been another meteor impact or even a series of impacts? An even larger crater, the Shiva crater, was reported by Sankar Chatterjee under the Arabian Sea, off the coast of India. It is called the Shiva cater and dates from 65.0 million years ago. The Shiva crater is about 370 miles across and 7.5 miles deep. However, what created the crater is unknown. If it were made by an asteroid or meteoroid, the object must have been at least 25 miles wide. Other geologists claim the Shiva crater is the result of a sinkhole in the Earth’s surface, not an asteroid.

This ongoing debate offers an exciting opportunity for students to sort through the clues and propose a theory to explain the extinction of the dinosaurs. The key to this activity is for students to begin by organizing the evidence into sets of related information and then use the evidence to support a logical theory. Since there is no right answer students have an opportunity to engage in a true scientific debate over the same set of data that paleontologists, geologists, and astronomers argue over. Furthermore, there are endless directions in which the debate may travel, opening endless opportunities for further exploration.

Student Prerequisites
Students should have an understanding of how fossils form and have experience with the geologic time scale. It is helpful to have a good foundation in the rock cycle and stratigraphy so as to better understand how the evidence provided may have been gathered.

8. Extinction Theories - Getting Ready

Getting Ready

  1. Make a copy of the “Dinosaur Extinction” handout for each team of students.
  2. Copy and cut out a set of extinction cards for each team of students. Putting the cards on cardstock paper allows you to use the same cards year after year.
  3. Put a set of extinction cards into each envelope.

8. Extinction Theories - Lesson Plan

Lesson Plan

  1. Rather than start class with any warm-up or initial discussion, begin by dividing the class into teams and giving each team a copy of the handout. It is important to keep kids from thinking about what they might have heard previously about the dinosaur extinction 65 million years ago. Many have heard about the catastrophic meteor/asteroid/comet theory and are likely to use evidence to justify their previous beliefs than to use evidence to arrive at a theory.
  2. Read through the first page of the handout together, stopping to answer any questions and define any vocabulary the students may be unfamiliar with (mass extinction, theory, etc.).
  3. Emphasize that a theory must be supported by evidence. Encourage students to throw out everything they think they know about the dinosaur extinction and instead seriously consider ANY possible explanation that is suggested by the evidence. The evidence is not equal in importance, so it’s not necessarily the number of evidence cards that matters but the conclusiveness of the evidence that matters. Thus, discourage students from using the strategy of the theory with the most number of evidence cards must be true.
  4. Students may need an example of what a theory looks like, particularly, the way a theory is composed of a series of logically linked statements. For instance, a theory might say “The dinosaurs, and the majority of other organisms that existed at the time, became extinct because space aliens landed on Earth. An alien virus was released, wiping out all the green life forms including all the dinosaurs and plants. This resulted in the collapse of the food web (since all the producers were wiped out) and an increase in the amount of carbon dioxide in the atmosphere (since plants no longer were using carbon dioxide for photosynthesis).”
  5. Once students understand the overall goal of this exercise, give them their cards and allow them to begin. Circulate around the room and help groups that get stuck or cannot agree. Make sure that students do not let their previous assumptions interfere with the process and keep students focused on the evidence cards.
  6. If teams finish early, force them to make their theory as clear as possible and challenge them by pointing out flaws in their logic or evidence that does not fit their ideas. For instance, point out that the Chicxulub crater was formed 300,000 years before the mass extinctions. Force students to think their ideas through completely and explain why they favor some evidence more than others.
  7. The following class period, or when all groups have finished, give each group a chance to present their theory, and their supporting evidence, to the rest of the class. Allow a question and answer period following each group’s presentation. Make sure that questions from other students are also grounded in the evidence and not personal attacks.
  8. When all groups have finished, allow an open debate about the causes of the dinosaur extinction. If necessary, clarify the various theories by writing them up on the board. Use this opportunity to discuss what pieces of evidence are stronger than others and why. If it appears that all teams favor one explanation (as happened in one of my classes), play the devil’s advocate and challenge students with evidence supporting an opposing viewpoint. Always come back to the evidence cards and keep the discussion focused on looking for a theory that best explains the evidence at hand.
  9. 10 minutes before the end of class, close the debate and describe the current state of affairs is in the scientific community – that scientists are very divided into the gradualist and catastrophic camps. Allow students to suggest areas of research that could help settle the debate: more conclusive information about how quickly or slowly the extinctions occurred, more clear evidence of asteroid or meteor impacts, more detailed radioactive dating information, etc.

8. Extinction Theories - Going Further

Going Further

  1. Have students research other mass extinctions and compare them to the rate of extinction today. Have students write a position paper arguing whether we are currently in a period of mass extinction or not.
  2. Have students investigate the conflicting theories explaining the Permian extinction, the largest mass extinction known. As they did in this activity, have then summarize one theory about the causes of the Permian extinction and the evidence that supports that idea.

8. Extinction Theories - Sources and Standards

Sources
There are many excellent websites that discuss the K-T extinction:

  • The UC Museum of Paleontology has a superb discussion of the ongoing scientific debate.
  • Wikipedia offers a great deal of information about both of the major theories and the evidence in favor of each.
  • PBS also provides a balanced discussion of the major theories.
  • Finally, the BBC has a good article about the dating of the Chicxulub crater.

Standards
Grade 7
Earth and Life History (Earth Sciences)
4. Evidence from rocks allows us to understand the evolution of life on Earth. As a basis for understanding this concept:
a. Students know Earth processes today are similar to those that occurred in the past and slow geologic processes have large cumulative effects over long periods of time.
b. Students know the history of life on Earth has been disrupted by major catastrophic events, such as major volcanic eruptions or the impacts of asteroids.
e. Students know fossils provide evidence of how life and environmental conditions have changed.
g. Students know how to explain significant developments and extinctions of plant and animal life on the geologic time scale.