Mammal Survival Simulation
Big Idea:
When non-avian dinosaurs vanished 66 million years ago, they left behind dozens of empty ecological niches, including large predator, large grazer, glider, burrower, swimmer, and fruit eater roles. Early mammals, small, generalist, and flexible, filled these niches. Over millions of years, they evolved new body shapes, diets, and ways of moving to dominate roles previously held by dinosaurs for 165 million years. This activity encourages learners to think like evolution, starting from a small ancestral mammal and building a world.

Materials:

• The lab sheet
• Colored pencils or markers
• Access to books or the internet for the older learner research component

What to Do:

Step 1: Set the Scene
"Imagine you are standing on Earth 66 million years ago, about 10,000 years after the asteroid impact. The dust is clearing. The large dinosaurs are gone. The forests are starting to recover. Small mammals are coming out of their burrows and looking around at a world that has completely changed. There are no large predators hunting them. There are no giant herbivores competing for plants. Every ecological role that used to be filled is now empty. What happens next?"

Step 2: Meet the Ancestor
Introduce the starting point: a small, shrew-like early Paleocene mammal roughly the size of a mouse. It is warm-blooded, has a flexible diet (insects, seeds, fruit, small invertebrates), can burrow underground, and can climb. It is not specialized for anything in particular, and that is exactly what makes it so powerful as a starting point. "This is your ancestor. Everything that comes next starts here."

Step 3: The Six Empty Niches
Present the six ecological niches that need to be filled. For each one, learners think about what body changes the ancestral mammal would need to survive and thrive in that role:

• The Burrower: Lives underground, eats roots, worms, and insects found in soil
• The Swimmer: Lives in or near water, eats fish or aquatic plants
• The Glider: Lives in trees, moves between them through the air, eats insects or fruit
• The Large Grazer: Lives on open plains, eats grasses and low plants, needs to cover large distances
• The Insect Eater: Highly specialized for finding and catching insects, possibly nocturnal
• The Fruit Eater: Lives in forest canopy, eats fruit and leaves, needs good color vision and grasping hands

Step 4: Build the Radiation Map
On the lab sheet, learners draw lines out from the ancestor to each of the six niches, writing the body changes needed for each one. For younger learners this is a drawing and labeling task. For older learners this is an analytical writing task with a research component added: find a real Cenozoic mammal that actually filled each niche and note how long after the extinction it appeared.

Step 5: Discuss

• Did any niche require more changes than others? Which one required the most dramatic transformation from the ancestor?
• Some of these niches were filled by mammals that look almost nothing like each other today. What does that tell you about how much evolution can change a body over millions of years?
• If the asteroid had never hit, do you think mammals would have radiated this way? Why or why not?
• Is there a niche on your map that is still filled today by a mammal you recognize? Which one surprised you most?

What's Really Happening (Caregiver Explanation):The rapid diversification of mammals after the K-Pg extinction is one of the clearest examples of adaptive radiation in the fossil record. When a large extinction event removes dominant species, it creates what scientists call ecological opportunity: resources and roles that are suddenly available to whoever can use them. Early Paleocene mammals were generalists, meaning they were not highly specialized for any one food source or environment. That lack of specialization, which might seem like a disadvantage in a stable world, turned out to be a powerful advantage in a destabilized one. Generalists can pivot. Over the next 10 to 15 million years, mammals evolved into an extraordinary range of body plans, from the ancestors of whales (which evolved from land-dwelling hoofed mammals) to the ancestors of bats, elephants, and primates. The Cenozoic Era is sometimes called the Age of Mammals precisely because of how completely and rapidly they filled the world left behind by the dinosaurs.

Digging Deeper:
The story of how mammals filled empty niches after the dinosaurs is one of the best-documented examples of adaptive radiation in the fossil record, but it raises a deeper question: why did it take so long? The K-Pg extinction happened 66 million years ago, but truly large mammals do not appear in the fossil record until roughly 55 to 50 million years ago, about 10 million years after the extinction. Research what scientists think explains that gap. Then look up the term adaptive radiation and find two other examples of it in the history of life, one from the animal kingdom and one from plants or another group. Compare the pace of each radiation: did they all happen at similar speeds, or did some radiate faster than others? What conditions seem to speed up or slow down adaptive radiation? Finally, look up Indricotherium, the largest land mammal ever to have lived. How large was it, how long after the extinction did it appear, and what does its size tell you about how far mammal evolution traveled from that small shrew-like ancestor?