Fossils—the remains of creatures that lived long ago—offer clues to the distant past. In this lesson, you will find out what scientists have learned about the connection between fish and land animals thanks to one fossil named Tiktaalik (tick-TAH-lick). Like a fish, Tiktaalik could swim and breathe under water. But like a land animal, it also may have been able to walk on land and breathe air.

Goals
Here are the big ideas you will learn about in this lesson:

• Transitional fossils help scientists establish how living things are related
• Physical features and behaviors may change over time to help living things survive where they live

Strategies
You will be using many reading and writing strategies to complete this lesson. The key strategies that you will focus on are:

• Categorizing basic facts and ideas
• Making inferences
• Identifying and using text features
• Determining important information
• Sequencing events

• species
• tetrapod
• transition

Animal life began in the sea. It should come as no surprise then that primitive, or ancient, fish had characteristics perfect for living in water. But over time, some primitive fish that lived in shallow waters shuffled their way onto land. Land provided new sources of food and safety from other animals.

Scientists use the word "transition" to explain the physical and behavioral changes that occur when one life form evolves, or develops, into a different life form. Legs for moving around and lungs for breathing air do not evolve overnight. In fact, it took tens of millions of years for certain kinds of primitive fish to transition into tetrapods—animals with four limbs and a backbone that live on land.

Name one living thing that lives on land, and another that lives in water. For each, write down a characteristic that helps it live in its environment.

Paleontologists (pay-lee-un-TAH-lo-jists) are scientists who discover and study fossils. (Paleo- means ancient or old, and -ologist means a person who studies a subject or field.) Paleontologists look at bone structure and skin covering and explore questions such as:

• How did different animals move around?
• Why did some animals have feathers or fur, while others had scales or smooth skin?
• How did certain body parts in one species become modified, or changed, in a new species?

To explain evolution and how different forms of life are related, paleontologists often use a tree as a model. Each branch shows where a different life form evolved from the earlier life form. While paleontologists would like to place every species that ever lived in this model, the "tree of life" contains gaps.

A gap can only be filled in when an important new transitional fossil is found—a fossil that has characteristics of an ancestor (an earlier species) as well as those of a descendant (a species that come after it).

A recent fossil discovery helped paleontologists fill in one of the gaps in the tree of life. Tiktaalik, the fossilized creature, represents just one step in the transition of life from water to land, but a very important step.

Click "View" on the left to watch the video.

Then show what you've learned by answering these three questions. Be sure to save all your answers before you go to the next screen.

You will now check your understanding of the vocabulary words for this lesson by placing them in sentences. Click "View" on the left to begin the activity.

Before moving to the next screen, choose two words from the vocabulary list and write a new sentence for each word.

Vocabulary words: fossil, characteristic, transition, tetrapod, species, amphibian, and evolution

The transitional fossil Tiktaalik represents a species that was partway between fish with fins and amphibians. Read more about the species' gradual transition from water to land and why Tiktaalik was such an important discovery.

Click "View" on the left to read the passage.

When you finish the passage, retype the following life forms in the order in which they appeared in evolutionary history (left to right, left being the earliest): Tiktaalik, amphibians, primitive fish.

As the video and passage explained, transitional fossils like Tiktaalik show how two groups of animals that may seem different from each other can still be related. Tiktaalik shares characteristics of the group that came before it in time and the group that followed.

In this activity, you will sort information on a concept map according to which of the three life forms (primitive fish, amphibians, and Tiktaalik) a characteristic best belongs. You may want to watch the video again or review the passage before doing the activity (see below).

Click "View" on the left to begin the activity. Your teacher will review the answers with you.

Transitional Fossil

Video: 3m 15s

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The "Fishapod" Tiktaalik

Document

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In this lesson, you have learned how a recently discovered fossil named Tiktaalik represents an important step in the transition of animal life from water to land. You will now write an essay that shares your ideas and your learning.

Begin by reviewing your notes. Click "review my work" above and view or print out the screen that pops up.

Final Assignment Instructions

• Click Tiktaalik: A Fish Out of Water—Final Assignment (PDF).
• Choose one of the topics in the PDF and write an essay.
• Organize your response so that it includes at least three supporting details that you have learned from the reading passage, video, and other materials in this lesson.
• Use at least three vocabulary words (or form of the words) as part of your supporting details.