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The Era of Fishes: The Devonian
How did life make the leap from water to land?

A dramatic shift happened during the Devonian Period: life began to leave the oceans and colonize the land. Plants grew tall, creating the first forests. Insects and spiders scurried through new ecosystems. And some fish (special fish with sturdy fins) began to crawl out of the water and explore the muddy shores. In this lesson, your learner will discover how a fish became the ancestor of every four-legged animal that ever walked on land, including humans. It's a story of adaptation, exploration, and transformation that changed life on Earth forever.

S2S_Lesson16_THE_DEVONIAN_PERIOD by Selene

Key Ideas

  • By the end of this time, the first four-legged animals walked on land: the ancestors of all land animals today, including us.
  • Plants began to grow tall and turned the land green with the first forests. The "Green Period": Seedless plants evolved into the first large trees, creating complex forest ecosystems.
  • The "Age of Fishes": Fish diversified into many forms, including sharks and lobe-finned fish.
  • Some fish began to develop sturdy fins that worked like legs to crawl in shallow water. The Tetrapod Transition: Lobe-finned fish evolved limbs with digits (fingers/toes), leading to the first amphibians.
  • Atmospheric Change: The growth of massive forests began to change Earth's atmosphere by increasing oxygen levels.

Spines
  • DK's Science as You've Never Seen it Before:
✏️  Notebooking Activity
Label three features of Tiktaalik that make it a unique transitional species.

Cosmic TimelineWhere we are: December 20
The Devonian Period falls on December 20th. Read the script below before the lesson.

Read aloud: December 20th on our Cosmic Calendar. The Devonian Period, sometimes called the Age of Fishes, falls around 419 to 359 million years ago in real time. Fish with jaws are spreading through the seas. And at the edges of those seas, something remarkable is beginning. Some fish are developing fins strong enough to push themselves onto land. We are just eleven days from the end of the calendar, and the first true land vertebrates haven’t arrived yet. That’s how late in the story that step happens. December 20th. Jawed fish rule the water, and the land is about to get its first permanent residents.

Timeline EntriesLabel the next page in your timeline “The Devonian Period: 419 Million Years Ago”. The workbook prompt asks learners to draw Tiktaalik hauling itself out of a Devonian swamp on its stumpy fins, with the first fern forests rising behind it on the shore.
Discussion Questions
  1. What changed on the land during this time? 
    Sample answer: It went from being rocky and bare to being covered in green plants and the first big trees.
  2. What did some fish start to do with their fins? 
    Sample answer: They used them like little legs to crawl through shallow water and mud.
  3. What kind of animals were the first to live in the new forests? 
    Sample answer: Small crawlers like spiders and early insects.
  4. Why is the Devonian often called the "Age of Fishes"? 
    Sample answer: Because fish went through a huge explosion of diversity, including the rise of sharks and the lobe-finned fish that led to land animals.
Digging Deeper
  • How did the evolution of roots and tall trees affect the Earth's surface? 
    Sample answer: Roots helped break up rocks into soil and prevented erosion, while trees changed the climate by breathing in carbon dioxide.
  • What anatomical features define the transition from fish to tetrapod? 
    Sample answer: The development of a neck, stronger ribcages to support weight, and fins evolving into limbs with wrists and digits.

Vocabulary
  • Devonian — A period of Earth's history (about 419-359 million years ago) known as the 'Age of Fishes' and marked by the first forests.
  • Tetrapod — A four-limbed vertebrate; the first tetrapods evolved from lobe-finned fish and were ancestors of all land vertebrates.
  • Lobe-Finned Fish — Fish with strong, fleshy fins supported by bones similar to limb bones; ancestors of the first land animals.
  • Tiktaalik — A fossil 'fishapod' — a transitional species between fish and four-limbed land animals — about 375 million years ago.
  • Amphibian — A vertebrate that can live both in water and on land; the first amphibians evolved from lobe-finned fish.
  • Vascular Plant — A plant with tubes that transport water and nutrients; their evolution allowed tall trees and the first forests.
  • Seed — A structure containing a plant embryo and stored food; seeds can survive dry conditions, unlike spores.

Species to ResearchThe Devonian is called the Age of Fish and the beginning of life on land. Here are some species to research:
  • Tiktaalik— The famous “missing link” fish with limb-like fins; a key transitional fossil between fish and four-limbed vertebrates.
  • Dunkleosteus— A massive armored fish with bone-crushing jaws; one of the top predators of the Devonian seas.
  • Acanthostega— An early tetrapod with eight fingers; lived mostly in water but had true limbs, not fins.
  • Ichthyostega— One of the first true land vertebrates; had both fish and tetrapod features.
  • Rhynie chert organisms— Beautifully preserved early land plants and arthropods from a Scottish Devonian hot spring deposit.
  • Eusthenopteron— A lobe-finned fish whose fins had bone structures very similar to vertebrate limbs.

SCIENTIST SPOTLIGHT: Jenny ClackJenny Clack was a British paleontologist who transformed our understanding of one of the most important transitions in the history of life: how vertebrates moved from water to land. She discovered and studied Acanthostega, a remarkable Devonian animal that had four limbs with digits but clearly still lived primarily in water. This showed that legs evolved before animals left the water, not as an adaptation for walking on land. Her work overturned decades of assumptions about how and why the transition from sea to land happened. She worked in a field dominated by men, faced significant barriers throughout her career, and continued producing landmark research until her death in 2020. She is considered one of the most important vertebrate paleontologists of the twentieth century.

Videos:
Digging Deeper Activity:
Look up images of Acanthostega and compare its limbs to those of a fish and a modern salamander. What features are fish-like? What features are tetrapod-like? Why does it matter for our understanding of evolution that legs came before the move to land?

Sources and More Information:

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