Lesson 15

15

The Ordovician and Silurian Seas

What was life like in the ancient oceans before anything lived on land?

During the Ordovician and Silurian periods, marine life diversified and reef ecosystems expanded. The lesson covers corals, early fishes, changing food webs, a mass extinction, and the beginnings of life on land.

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Key Ideas

  • The Ordovician and Silurian seas were rich with marine life: corals, trilobites, brachiopods, and the first jawed fish.
  • Coral reefs formed, providing homes and shelter for hundreds of species.
  • The first vertebrates with jaws appeared, becoming much more effective hunters.
  • During the Silurian, life began its first tentative steps onto land.
  • The Ordovician ended with a mass extinction caused by a sudden ice age.

Spines

Vocabulary

  • Ordovician: A period (~485–444 million years ago) marked by rich marine life, including the first coral reefs and early fish.
  • Silurian: The period following the Ordovician (~444–419 million years ago) when jawed fish appeared and life began colonizing land.
  • Coral Reef: A complex underwater structure built by corals providing habitat for enormous numbers of sea creatures.
  • Vertebrate: An animal with a backbone; the first vertebrates were jawless fish in the Ordovician seas.
  • Invertebrate: An animal without a backbone; trilobites and sea scorpions dominated Ordovician oceans.
  • Ecosystem: A community of living things interacting with each other and their physical environment.

Discussion Questions

  1. What kinds of animals lived in these ancient oceans?
    Sample Answer: There were corals, trilobites, brachiopods, and eventually the first jawed fish.
  2. Why were coral reefs such important places for sea life?
    Sample Answer: Reefs worked like big underwater cities, providing homes and shelter for hundreds of species.
  3. Why were fish with jaws such a big evolutionary deal?
    Sample Answer: Jaws allowed fish to become much more effective hunters and eat a wider variety of food, which changed the food chain.
  4. What does "marine biodiversity" mean in this context?
    Sample Answer: It refers to the wide variety of life forms, such as trilobites, brachiopods, and early fish, that filled the seas.

Digging Deeper

  1. Research the Great Ordovician Biodiversification Event. What caused life to diversify so explosively, and what does it tell us about how ecosystems respond when opportunity opens up?
    Sample Answer: Answers will vary based on research. The event tripled the number of marine animal families in a geologically short time.