Make a Fossil Cast: Be a Real Paleontologist for a Day
A long, long time ago, a dinosaur stepped in soft mud. The mud dried in the sun and turned hard as rock. Rain and wind covered it up with more layers of dirt for millions and millions of years. Then, one day, someone dug it up — and the shape of that footprint was still there! Today you get to make your own fossil, just like the real ones scientists called paleontologists dig up.
What You'll Learn
- What a fossil mold and a fossil cast are - How real fossils form in the ground over millions of years - How to make your own fossil cast at home - One real dinosaur footprint scientists have found
What Is a Paleontologist?
A paleontologist is a scientist who studies fossils — the preserved remains or traces of animals and plants that lived long ago. Some fossils are bones. Some are shells. And some are just shapes, like a footprint or a leaf print pressed into rock. These shape-fossils are super special because they show us exactly what an animal's foot, or skin, or a plant looked like, even though the actual animal is long gone.
Mold and Cast: Two Fossil Words
When a dinosaur stepped in mud, it left an empty dent shaped like its foot. Scientists call that empty dent shape a mold. Over a very long time, dirt and minerals filled that dent back in, hardening into a solid shape that fills the mold perfectly. That filled-in shape is called a cast. Today, you are going to be both the dinosaur AND the scientist: you'll press a shape to make a mold, then pour something in to make a cast, just like nature does — except it will only take one day instead of a million years!
In Dinosaur Valley State Park in Glen Rose, Texas, scientists have found real dinosaur footprints pressed into limestone over 100 million years old, left by dinosaurs like Acrocanthosaurus walking along an ancient muddy shoreline.
How Real Fossils Form
Making a real fossil takes a very specific and lucky set of steps: (1) an animal or footprint gets covered by soft material like mud or sand very quickly, (2) more layers pile on top over thousands of years, pressing down and turning the mud to stone, (3) water carrying minerals seeps in and slowly replaces or fills the shape, and (4) millions of years later, wind, rain, or a digging scientist uncovers it. Because so many things have to go exactly right, only a tiny fraction of all the plants and animals that ever lived became fossils.
What is the difference between a fossil mold and a fossil cast?
Why do only some plants and animals from long ago become fossils?
Make Your Own Fossil Mold and Cast
Ask an adult to help. You will need: modeling clay or play dough, a small object with texture (a shell, a leaf, a plastic toy dinosaur foot), plaster of Paris or flour-salt dough, and a small cup of water. Steps: (1) Flatten the clay into a small disk. (2) Press your object firmly into the clay and lift it out carefully — this dent is your mold. (3) Mix your plaster or dough according to package directions and pour it into the mold. (4) Let it dry completely (check your material's instructions for time). (5) Once hard, gently peel away the clay — you now have a cast! Take a photo or draw a picture of your finished cast next to the original object as your deliverable, and label which part is the 'mold' and which is the 'cast.'
Flashcards — click each card to reveal the answer
You don't need a real fossil to practice this idea — pressing your own hand into wet sand at a beach or playground and watching the shape stay behind is a mold, just like a dinosaur footprint!
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