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Sari on Science: Explain the formation of fossils with this fun, at-home activity

Photo courtesy of Sari Custer.

Who doesn’t love dinosaurs? My daughter and I both love these prehistoric creatures, so imagine her surprise when she asked me if she could meet a Tyrannosaurus rex, and I got to say, ‘“Yes!” In preparing for Arizona Science Center’s exhibition, “Victoria the T. rex,” which opened Nov. 17, I used this activity to help explain to Carson how fossils are formed, and how we can still see the remains of dinosaurs and prehistoric plants, even though they are millions of years old.

Did you know? While the Tyrannosaurus rex lived more than 65 million years ago, the oldest known fossils are actually of cyanobacteria in layered formations called stromatolites. They date back 3.5 billion years!

  • Crafting clay that will harden (playdough works, too)
  • Plastic dinosaurs or other toys with feet
  • Leaves (real leaves work best)
  • Shells (available at your local craft store if you don’t have beach souvenirs)
  • Optional: A drinking glass that’s flat on the bottom
  1. Flatten out four balls of clay into small disc shapes (about 3 inches across). Make them a little thick.
  2. Choose a leaf. Use the cup to press the leaf into one of the clay discs, then carefully remove the leaf.
  3. Choose a dinosaur toy. Have the toy “walk” across the second clay disc and leave tracks.
  4. Choose a shell. Press the shell into the third clay disc and cover with the last disc. Gently press down.
  5. Make a notch in the same place on both discs so you can line them up again later. Gently pull the top disc off, then remove the shell.
  6. Leave your clay discs to dry, then compare them.
  7. Use your two hardened “shell” discs and line up the notches. Place a soft ball of clay between the imprints and press down.
  8. Gently pull apart and observe the results.
What’s happening?

Fossils are actually pretty rare. Most plants, animals, and other organisms completely decay after they die. Conditions have to be just right for a fossil to form, and the remains must be quickly covered by sediment (sand, mud, tar, etc.). We are more likely to see the hard parts: bones, teeth, shells, and sometimes feathers, after minerals seep into them and they harden with the rock layers.

In this activity, we are making different types of fossils: tracks, imprints, and with the shell, a mold and cast. Animal tracks, leaf imprints, and burrows are what we call trace fossils, because they are just “traces” of the organism they came from, not the actual plant or animal. If an organism completely dissolves in sedimentary rock, it can leave an impression in the rock, called a mold. If that mold gets filled with other minerals, it becomes a cast and looks like the original organism, even though it is not.

How did your shell cast turn out? Which of these fossils looks more like the original plant or animal? Do you know of any other types of fossils? Where else do we use/see molds and casts?

Give fossil-making a try, then visit Victoria: the T. rex, the largest Tyrannosaurus rex to tour the world and the second most complete T. rex ever discovered. This immersive, interactive exhibition promises to transport you to the time of the dinosaurs!

RELATED: More “Sari on Science” projects

Read this month’s Raising Arizona Kids Magazine!


Sari Custer
Sari Custerhttp://azscience.org
Sari Custer is a lifelong science junkie, Chief Science and Curiosity Officer at Arizona Science Center, and mom to daughter Carson (6). Follow her on Twitter and Instagram @SariOnScience and find Arizona Science Center at azscience.org


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