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Missing Link, The

Viewing Ideas

This whale, horse, snake, and bird—though different in structure and function—are all considered tetrapods. The animals that no longer exhibit four legs are classed as tetrapods because their ancestors once had four legs.

Before Watching

1. Tetrapods are vertebrate animals with four leg-like appendages. Ask students to define tetrapod and list examples of tetrapods that live on land, in water, and in the air. While students are likely to think of mammals or reptiles with four legs, point out that the tetrapod group also includes modern-day birds, snakes, and other animals with fewer or no appendages.

2. As students watch, ask them to note the dates, evidence, and inference related to each find featured in the program.

After Watching

1. Review students' notes about the research finds made from the 1930s to the 1990s. How did the thinking change as to how the first land animals formed limbs? What forces might have led to the evolution of tetrapods?

2. Discuss with students the key dates and finds from the program that they noted. How and why did the thinking change over the years? What do students think about Erik Jarvik's decision to withhold his fossil from study by others for so long? What impact may that decision have had on the research on transitional forms?