Lesson 23

23

The End of the Dinosaurs

What could wipe out the most successful group of animals that ever lived?

An asteroid impact about 66 million years ago disrupted the climate and food webs, contributing to a mass extinction. The lesson covers the evidence for the impact, the loss of non-avian dinosaurs, and the traits that helped some organisms survive.

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Key Ideas

• The K-Pg extinction 66 million years ago was caused by an asteroid roughly 10 km across striking the Yucatan Peninsula, triggering firestorms, nuclear winter, and global food chain collapse.
• About 75% of all species went extinct, including every non-avian dinosaur. Evidence includes a worldwide iridium layer, shocked quartz, tektites, and the Chicxulub Crater.
• Survivors shared key traits: small size, flexible diets, ability to burrow or live in aquatic environments, and low energy needs. Small mammals, birds, and crocodilians made it through.
• Birds are avian dinosaurs, the only dinosaur lineage to survive the K-Pg. Dinosaurs are not extinct. They are at your bird feeder.
• With the giants gone, the small survivors (especially mammals) inherited an empty world. The Cenozoic Era, the Age of Mammals, was about to begin.

Spines

• DK's Science as You've Never Seen it Before: N/A
• Visual Timelines: Life on Earth pg. 86–87
• Alternative: Kiddle: Major Extinction Event
• Dinosaur Extinction

Vocabulary

• K-Pg Extinction: Cretaceous-Paleogene mass extinction (~66 mya) wiping out non-avian dinosaurs and ~75% of species.
• Asteroid: A rocky body orbiting the Sun; the Chicxulub asteroid triggered the K-Pg extinction.
• Impact Crater: A depression formed when an asteroid strikes a planet; the Chicxulub crater is 150+ km across.
• Iridium: A metal rare at Earth's surface but common in asteroids; found worldwide in the 66-mya rock layer.
• Nuclear Winter: Months or years of darkness and cold caused by debris blocking sunlight after a massive impact.
• Non-Avian Dinosaur: All dinosaurs except the lineage that evolved into birds; went extinct at the K-Pg boundary.
• Avian: Relating to birds; birds are avian dinosaurs, the only surviving dinosaur lineage.

Discussion Questions

1. What traits helped some animals survive the K-Pg extinction while non-avian dinosaurs did not?
   Sample Answer: Small size, a flexible diet, a burrowing or aquatic lifestyle, and low metabolism all helped. Large size, a specialized plant-based diet, and high daily food needs all hurt. The asteroid did not target large animals, but those traits made them far less able to survive months of food scarcity.
2. How do scientists know an asteroid caused the K-Pg extinction, not a volcano or disease?
   Sample Answer: Multiple independent lines of evidence point to the same event: iridium found worldwide, shocked quartz that needs impact pressures, tektites (impact glass), and the crater itself. No single piece is conclusive, but all of them together are overwhelming.
3. Non-avian dinosaurs went extinct. But avian dinosaurs (birds) survived. Does that mean dinosaurs are really extinct?
   Sample Answer: Technically no. Birds are avian dinosaurs, and the line from a Jurassic theropod to a modern bird is direct and unbroken. What went extinct were the non-avian dinosaurs.
4. Catastrophes reshape ecosystems and open the door for new life. Can you think of any other examples, in history or nature, where a disaster created opportunity?
   Sample Answer: Answers will vary. Examples include forest fires creating clearings where new plants grow, the Great Dying opening the world for dinosaurs, and the K-Pg opening it for mammals.

Digging Deeper

1. Impact Crater Simulation: Fill a large shallow tray with flour or fine sand. Drop rocks of different sizes from different heights into the tray. Observe how the crater size changes with rock size and drop height. Notice how material ejects outward. Compare your craters to satellite images of the Chicxulub crater's shape. How does crater size relate to the energy of the impact?
   Sample Answer: Answers will vary. Bigger rocks and higher drops make bigger craters because they carry more energy.
2. Survival Traits: Who Would Make It? Create a two-column chart: 'Survived K-Pg' and 'Did NOT Survive K-Pg.' Add animals to each column. For each, write one or two sentences explaining which traits helped or hurt them. Then look for patterns: what do the survivors have in common? What do the extinctions have in common?
   Sample Answer: Answers will vary. Survivors tend to be small, flexible eaters that can burrow or live in water. Extinctions tend to be large animals with high food needs.
3. Research: What Early Mesozoic Mammals Looked Like. Research what early Mesozoic mammals (like Morganucodon or Hadrocodium) looked like and how they lived. What do fossils tell us about their behavior, diet, and senses? Why were small size and nocturnal habits advantages for mammals living in the dinosaur world? How did those traits help them survive the K-Pg?
   Sample Answer: Answers will vary.