Hands-On Activity: Make a Fossil
Big Idea:
The Cambrian Explosion gave us one of the greatest gifts in paleontology: fossils. Before the Cambrian, most life was soft-bodied and left almost no trace. Then, around 541 million years ago, animals began evolving hard parts: shells, exoskeletons, teeth, and spines. Those hard parts could fossilize. Suddenly, for the first time in the history of life, the rock record started talking. This activity lets learners recreate the two most common types of fossil preservation so they can understand why the Cambrian Explosion did not just change life on Earth, it changed what we are able to know about life on Earth.

Materials:

Materials for Option A (Impression Fossil):

• Modeling clay or air-dry clay
• A natural object to press: a shell, leaf, small bone, or textured rock
• Rolling pin or smooth cup (to flatten clay)

Materials for Option B (Mold and Cast Fossil):

• Everything from Option A
• Plaster of Paris
• Water
• A small disposable cup for mixing
• Cooking spray or petroleum jelly (to help release the cast)
• Stir stick or old spoon

Setup Notes: If using air-dry clay for Option A, the impression can be left to dry and kept. For Option B, coat the clay mold lightly with cooking spray before pouring plaster so the cast releases cleanly. Plaster of Paris sets in about 20 to 30 minutes but should be left for at least an hour before removing. Mix plaster to a thick cream consistency, not too watery.

What to Do:

Step 1: Set the Scene
"Before the Cambrian Explosion, almost nothing fossilized. The Ediacaran creatures we looked at in the last lesson were soft and squishy, like living mats and fronds, and soft bodies almost never survive long enough to become fossils. Then the Cambrian Explosion happened, and animals started growing hard parts: the trilobite's tough exoskeleton, Anomalocaris's grasping claws, the shells of early mollusks. Hard parts change everything. They can be buried, mineralized, and preserved for hundreds of millions of years. Today we are going to see exactly how that works."

Step 2: Choose Your Object
Have your learner pick an object to press into the clay. Good suggestions are a shell, a leaf with clear veins, a small bone, a pinecone scale, or a textured rock. Ask them: does this object have hard parts or soft parts? Think about a trilobite with its tough exoskeleton versus a jellyfish with nothing but soft tissue. Which one would leave a mark like this?

Step 3: Prepare the Clay
Roll or press the clay into a flat, even slab about half an inch thick. It should be smooth on top so the impression will be clear. Think of the clay as the soft seafloor sediment of a Cambrian ocean, waiting to capture whatever sinks down into it.

Step 4: Make the Impression
Press the object firmly into the clay, then carefully peel it away. Look at what is left. This shape in the clay is the mold. In real life, this is what forms when a shell or bone presses into soft sediment before it hardens into rock over millions of years. This is how we know the shape of a trilobite's body in such precise detail.

Step 5: Option A, Stop Here
If doing Option A, set the clay aside to dry. The impression is your fossil. Talk about what information it preserves and what it does not. A trilobite impression would show us the shape of its shell but nothing about its soft organs, its color, or how it moved.

Step 6: Option B, Make the Cast
Lightly coat the inside of the clay impression with cooking spray or petroleum jelly. Mix plaster of Paris with water in your disposable cup until it is the consistency of thick cream. Pour it carefully into the impression and let it sit undisturbed for at least 30 to 60 minutes. Once set, gently peel the clay away to reveal the cast. This is what happens when minerals seep into the space left by an organism over millions of years and slowly replace it. Many trilobite fossils we find today are exactly this: the original shell dissolved long ago, and minerals filled the space it left behind.

Step 7: Compare and Discuss

• What details did your impression capture? What did it miss?
• If you had pressed a jellyfish into the clay instead of a hard object, what do you think would happen?
• The trilobite had a hard exoskeleton. Hallucigenia and Opabinia were much softer. What does that tell us about which Cambrian creatures we know the most about and which ones we might be missing entirely?
• Why do you think the Cambrian Explosion, the moment animals started growing hard parts, was such a turning point for paleontology?

What's Really Happening (Caregiver Explanation):
Before the Cambrian Explosion, almost nothing fossilized because almost nothing had hard parts. The Ediacaran creatures that came before were soft-bodied, and soft bodies decay quickly. When Cambrian animals began evolving exoskeletons, shells, teeth, and spines, they accidentally made themselves preservable. A trilobite's tough outer shell could be buried in sediment, slowly replaced by minerals over millions of years, and survive long enough for us to find it today. That is why the Cambrian Explosion is such a turning point not just for the history of life, but for our ability to read that history.

Fossilization is still an incredibly unlikely process even with hard parts. An organism must die in a place with sediment, be buried quickly before scavengers and bacteria can break it down, and remain undisturbed while the sediment slowly turns to rock. Even then, the fossil must eventually be exposed by erosion and discovered before it erodes away. The Burgess Shale in British Columbia is exceptional because unusual conditions, rapid burial in fine-grained, low-oxygen sediment, preserved soft-bodied creatures in extraordinary detail alongside their hard parts. That is why it has taught us so much about what Cambrian life actually looked like beyond just the shells and exoskeletons.

Digging Deeper:
The Cambrian Explosion did not just flood the oceans with new life. It created the fossil record as we know it. Before hard parts evolved, preservation was almost impossible. Research the Burgess Shale in British Columbia and find out what made its preservation so unusual. Why were soft-bodied creatures like Hallucigenia and Opabinia preserved there when they almost never fossilize anywhere else? Look up Charles Doolittle Walcott, who discovered the Burgess Shale in 1909, and find out what he thought he was looking at. Then look up Stephen Jay Gould and his book Wonderful Life. Gould argued that the Cambrian creatures preserved there were evolutionary experiments so strange they were never repeated. Other paleontologists disagreed. What is the current scientific thinking? What would we not know about the Cambrian Explosion if the Burgess Shale had never been found?