Introduction:
Transitional Fossils

Fossils are the preserved remains or traces of ancient life forms. Although very few organisms end up as fossils, every once in a while, scientists discover a new kind of fossil. So what can scientists actually learn from such fossils?

The purpose of this activity is to help you understand how scientists can analyze the similarities and differences between fossils to better understand the evolution of life on Earth.

Next: A Slow but Steady Transition to Land

Animal life began in the sea, so it's no surprise that very ancient animals were well suited to living in water. To survive on land, an animal would need different characteristics, suited to the conditions there. For example, it would need limbs strong enough to lift itself off the ground and to move around. It would also need a way to breathe air, rather than take in water through its gills to get oxygen, like fish.

Physical characteristics like legs and air-breathing lungs do not evolve overnight. In fact, scientists have found that the transition of animals from water to land took place over tens of millions of years.

In the box below, list three examples of animals that live on land and three examples of animals that live in the water. Next, list a characteristic that makes each animal well suited to life on land or water.

Click "save notes" before moving on to the next page.

Next: Transitional Fossil: Watch This!

Paleontologists (pay-lee-un-TAH-lo-jists) are scientists who discover and analyze fossils. By comparing physical features of fossilized organisms, such as their bone structure and skin covering, paleontologists can determine whether different species may be related to one another.

Now watch a video about a recent fossil discovery that supports the idea that the transition of life from sea to land happened gradually. This fossilized animal, Tiktaalik, has features shared by fish, which spend their entire lives in water, and amphibians, which spend part of their lives in water and part on land. Tiktaalik represents just one stage of the transition, but it's a very important one.

Note: You will hear the following words in the video segment. Click on each word to see its definition.

• evolutionary theory
• Inuit people
• transitional fossil
• tree of life

After watching the video, describe which characteristics suggest that Tiktaalik is related to fish, and which suggest that it is related to amphibians. Click "save notes" before moving on to the next page.

Next: Try This!

As you saw in the video, transitional fossils like Tiktaalik help show how animals can evolve from one distinct group of species to another. These fossils display characteristics of those that came before and those that came later. In this activity, you will categorize information about the physical characteristics of three life forms: fish, amphibians, and Tiktaalik. You may assign a characteristic to more than one category (in fact, you'll have to).

Then use the "Connect" tool to draw some connections between the related characteristics of Tiktaalik, fish, and/or amphibians. Use the Comments space to indicate how these characteristics might provide evidence of a transition from one form to another. (Complete directions for how to do this appear in the Flash Interactive activity.)

Click "View" on the left to begin the activity. Be sure to print your notes before you exit the activity. Your notes will not be saved.

If you want, you can watch the video again.

Transitional Fossil

QuickTime Video
Length: 3m 15s

Next: Other Interesting Discoveries

You've just learned how Tiktaalik provides clues about a key transition in the history of life: from fish to amphibians. Here are some other interesting discoveries found from studying transitional fossils.

• Primitive bird fossils show that feathers did not evolve for flight. By evaluating their structure and function, scientists believe that feathers actually evolved on reptiles for warmth. This of course suggests that birds evolved from reptiles.
• You can hear yourself chewing because parts of the human ear structure evolved from reptilian jawbones. That's right, this means that humans also descended from reptiles!
• Soon after its discovery, a fossilized bird called Archaeopteryx was proclaimed the "missing link" between reptiles and birds. This fossil exhibited common bird characteristics, including feathers and wings. It also had reptilian characteristics, including a long bony tail, fingers with claws on the front edge of the wing, and teeth in its jaws.
• The transitional fossil Ambulocetus (the "walking whale") links wolves to whales in evolution. This connection suggests that whales and other marine mammals actually descended from land animals. (From sea to land and back again!)

Next: Read and Write About It!

In the video, you learned about the discovery of the fossilized skeleton of an intermediate form between fish with fins and land animals with four limbs. Because Tiktaalik is part-fish and part-tetrapod (four-legged animal), it's been called a "fishapod." In the following reading, learn more about the transition from water to land, a gradual process that began in creatures still living in the water. Then, you will have a chance to share your ideas with others.

• Print out your notes. (After you print, your notes will be erased. Make sure you are done before printing.)
  
  
• Print out and read The "Fishapod" Tiktaalik (Version A) (PDF) or The "Fishapod" Tiktaalik (Version B) (PDF). Check with your teacher if you are not sure which version to use.
• After reading, take a look at your chart and make any changes or add new information.
• Then, select and complete a writing assignment on page 2 of the printout.

Note: The following words will appear in the readings. Click on each word to see its definition.

• ancestors
• aquatic
• descendants
• precursors