Lesson 14

14

The Cambrian Explosion

Why did so many different kinds of animals appear on Earth in such a short period of time?

The Cambrian Period saw a rapid increase in the variety of animals preserved in the fossil record. Shells, eyes, limbs, and different body plans show how life in the oceans changed as animals developed new ways to feed and survive.

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Key Ideas

• The Cambrian Explosion was a time when an enormous variety of complex animal life suddenly appeared in the oceans.
• Animals evolved eyes, shells, claws, and armor for the first time.
• Increased oxygen levels and competition for food drove rapid evolution.
• The Burgess Shale is a famous fossil site that preserved Cambrian creatures in stunning detail.
• Most of today's major animal groups (phyla) trace back to the Cambrian.

Spines

• DK's Science as You've Never Seen it Before:
• Mammoth Science: Fossils pg. 140, The Eye pg. 64
• Visual Timelines: Life on Earth pg. 20-25
• Alternative: Oak National Academy: Endoskeletons
• Alternative: Kiddle: Exoskeletons
• Alternative: Britannica Student: Fossils
• Alternative: Kiddle: Cambrian Explosion

Vocabulary

• Cambrian Explosion: A period ~541 million years ago when enormous variety of complex animal life suddenly appeared in the fossil record.
• Trilobite: An ancient armored sea creature; one of the first animals with eyes, and one of the most successful for 270 million years.
• Exoskeleton: A hard outer skeleton on the outside of an animal's body that provides support and protection.
• Predator: An animal that hunts and eats other animals; the first true predators appeared during the Cambrian.
• Biodiversity: The variety of life in a given area or on Earth as a whole.
• Natural Selection: The process by which individuals with traits better suited to their environment survive and reproduce more.

Discussion Questions

1. Why was the Cambrian Explosion such an important moment?
   Sample Answer: Almost every major animal body plan that exists today first appeared during the Cambrian Explosion. In just a few million years, life went from simple soft-bodied organisms to an enormous variety of complex animals with eyes, shells, legs, and specialized body parts.
2. How were Cambrian animals different from earlier life?
   Sample Answer: Earlier life was mostly soft-bodied and simple. Cambrian animals had hard shells, compound eyes, claws, jointed legs, and much more complex body structures suited for hunting and defense.
3. Why do you think hard shells and armor became so important?
   Sample Answer: Hard shells provided protection from predators that also evolved during the Cambrian. As predators became more effective hunters, animals with protective coverings survived better and passed those traits to their offspring.
4. What does the Burgess Shale tell us about ancient life?
   Sample Answer: It preserved Cambrian creatures in stunning detail, so it shows us what many of these animals looked like, including Anomalocaris.

Digging Deeper

1. What does "explosion" mean here? It wasn't actually an explosion, so what was it?
   Sample Answer: It was not a blast. It describes how quickly, in geologic time, a huge variety of complex animals appeared in the fossil record.