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15
The Ordovician and Silurian Seas
What was life like in the ancient oceans before anything lived on land?

After the Cambrian Explosion, life in the oceans continued to diversify and specialize. Corals began building massive reefs that became underwater cities full of life. Simple fish appeared and evolved jaws and fins. In this lesson, your learner will explore two fascinating periods when the oceans were the center of all life on Earth, filled with creatures that competed, hunted, and built elaborate homes together. This is the story of how coral reefs became some of the most productive ecosystems on the planet.

S2S_Lesson15_LIFE_IN_THE_ORDOVICIAN_SILURIAN_SEAS by Selene

Key Ideas

  • The Ordovician and Silurian seas were rich in marine biodiversity, including corals, trilobites, brachiopods, and the first jawed fish.
  • Coral reefs are homes and cities for lots of sea creatures, providing structure and shelter.
  • Early fish showed major evolutionary advances like jaws and paired fins, allowing them to become more effective hunters.

Spines
  • DK's Science as You've Never Seen it Before: A Place to Live pg. 182-183
✏️  Notebooking Activity
Look at the two-column chart in the workbook. Fill in examples of jawless fish and jawed fish from this period. For each type, add a note explaining why their body structure was an advantage.

Cosmic CalendarWhere we are: December 17 to 19
We move through the third week of December. Read the script below before the lesson.

Read aloud: We’re moving through December on our Cosmic Calendar, and notice how quickly the dates are changing now. We went from December 14th to December 17th through 19th in just one lesson. The Ordovician and Silurian periods cover roughly 530 to 420 million years ago in real time. The ocean is full of life now, fish are beginning to develop, and the first brave plants are testing the land. The Ordovician ended with a major extinction event, and yet life bounced back and kept pushing forward. That resilience is a theme you’re going to see again and again on this calendar. We’re in the third week of December. The story is moving fast now.

Timeline EntriesLabel the next page in your timeline “Life in the Ordovician & Silurian Seas: 485 Million Years Ago”. The workbook prompt asks learners to draw an Ordovician coral reef packed with brachiopods, crinoids, and jawless fish like Arandaspis swimming above the reef in the shallow sea.
Discussion Questions
  1. What were the first homes in the ocean like?
    Sample answer: They were coral reefs, which acted like big underwater cities for many animals.
  1. What kinds of animals lived in the old oceans? 
    Sample answer: There were corals, shellfish, and eventually the first fish.
  2. Why were fish important to the sea? 
    Sample answer: They were new animals that could swim around instead of just staying in one spot.
Digging Deeper
  • What do you think "marine biodiversity" means in the context of the Paleozoic era
    Sample answer: It refers to the wide variety of life forms, such as trilobites, brachiopods, and early fish, that filled the seas.)
  • How did coral reefs function as complex ecosystems? 
    Sample answer: Reefs provided structural complexity, offering protection and diverse habitats that supported many different species at once.
  • What was the evolutionary significance of the first jawed fish? 
    Sample answer: Jaws allowed fish to become predators and eat a wider variety of food, which changed the food chain.

Species to ResearchThe Ordovician and Silurian seas were teeming with life. Here are some species learners can research:
  • Arandaspis— One of the earliest known jawless fish; an armored swimmer from the Ordovician seas of what is now Australia.
  • Orthoceras— A straight-shelled cephalopod; a relative of modern nautiluses and a top Ordovician predator.
  • Crinoids (Sea Lilies)— Filter-feeding echinoderms that formed dense Ordovician reefs; still alive today in deep oceans.
  • Eurypterids (Sea Scorpions)— Large, scorpion-like arthropods that could reach 2 meters; among the largest invertebrate predators ever.
  • Brachiopods— Two-shelled filter feeders that dominated Paleozoic seafloors; often mistaken for clams but unrelated.
  • Cooksonia— One of the earliest land plants; a tiny, leafless, branching stem that appeared in the Silurian.


Vocabulary
  • Ordovician — A period of Earth's history (about 485-444 million years ago) marked by rich marine life, including the first coral reefs and early fish.
  • Silurian — The period following the Ordovician (about 444-419 million years ago) when jawed fish appeared and life began colonizing land.
  • Coral Reef — A complex underwater structure built by corals; provides habitat for enormous numbers of sea creatures.
  • Marine — Related to or living in the ocean; during the Ordovician and Silurian, nearly all life was marine.
  • Vertebrate — An animal with a backbone; the first vertebrates were jawless fish in the Ordovician seas.
  • Invertebrate — An animal without a backbone; trilobites, brachiopods, and sea scorpions dominated Ordovician oceans.
  • Ecosystem — A community of living things interacting with each other and their physical environment.

Sources

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