Fossils and Earth’s Past

A sea shell on top of a mountain? A footprint in a rock? Fossils are clues left by living things from long ago. Let’s find out how they form, what kinds there are, and what they tell us about Earth’s past.

This lesson takes about 10 minutes. You can take a break after each part.

Before you start

Fossils and Earth’s Past

Learning objectives

  • I can explain how a fossil forms in layers of rock, and that deeper layers are usually older.
  • I can tell body fossils, trace fossils, molds and casts apart.
  • I can use fossils to describe living things and places from long ago.

How a fossil forms

Buried in layers

A fossil is what is left of a living thing from long ago, kept in rock.

When an animal dies, mud and sand can cover it. Layer after layer, over a very long time, they turn into rock. The deepest layers are usually the oldest.

Four numbered pictures joined by arrows. 1: a dead fish sinks to the bottom of the water. 2: layers of mud and sand cover its skeleton. 3: deep under the layers, water seeps through the bones. 4: much later, the rock is on dry land and a young explorer finds the fish skeleton in the stone.
A dead fish is buried under mud and sand. Over a very long time, the layers turn to rock and the skeleton becomes a fossil.

Loading the questions…

Fossils of all kinds

Bodies, traces, molds and casts

Some fossils are parts of a body: bones, teeth, shells. Others are traces: footprints or tracks that show what an animal did.

A shell can leave a hollow print in mud: a mold. If the mold fills up and hardens, it makes a cast.

Nine fossils on stands, in three rows. Row 1: two large teeth, a spiral shell, a three-toed footprint pressed in stone. Row 2: wavy tracks left by a worm, a trilobite mold next to its cast, the print of a fern leaf. Row 3: an insect trapped in golden amber, a slice of wood turned to stone, a baby mammoth frozen in ice.
Some fossils are parts of a living thing, like teeth or shells. Others are traces, like footprints. Some are molds and casts.
Three numbered pictures of children at a table. 1: a boy presses a shell into soft clay. 2: a girl lifts the shell; its print stays hollow in the clay. 3: a boy pours white plaster into the print. Below each step, a circle zooms in: shell in the clay, hollow print, white plaster shell.
The hollow print in the clay is a mold. The plaster that fills it makes a cast, with the shape of the shell.

Loading the questions…

What fossils tell us

Clues about living things and places

Fossils show living things from long ago. Some, like the mammoth, no longer live on Earth.

They also show how places have changed: sea shells on a mountain mean the land was once under the sea.

A young hiker high on a rocky mountain holds a spiral shell fossil. More shell fossils stick out of the rocks around him. In a thought bubble, the same shells and fish live under the sea, long ago.
Sea shells found on a mountain show that this land was once under the sea.
Three pairs side by side. A mammoth skeleton next to an elephant, a fern leaf print in stone next to a green fern, a horseshoe crab fossil next to a living horseshoe crab. Numbered circles zoom in on the same parts of each pair: head, body, legs or tail.
A fossil can be compared with a living thing of today. The mammoth looks like an elephant; the fern and the horseshoe crab have hardly changed.

Loading the questions…

Review game

Final assessment

To master the lesson, you need at least 60% correct answers.

Loading the assessment…

Summary

  • A fossil is what is left of a living thing from long ago, kept in rock.
  • Fossils form when living things are buried in layers that turn into rock; deeper layers are usually older.
  • Fossils can be body parts, traces, molds or casts.
  • Fossils tell us about living things and places of long ago: sea shells on a mountain mean the land was once under the sea.

A few related activities

Fossils and Earth’s Past — Review gameGameFossils and Earth’s Past — Review gameReview game for the lesson “Fossils and Earth’s Past”: 8 questions drawn at random every round. Free science…See the activityInherited Traits and the EnvironmentLessonInherited Traits and the EnvironmentWhy does a puppy look like its parents, but not exactly? Why is a flamingo pink? Some traits…See the activityWeather and ClimateLessonWeather and ClimateSunny, rainy, windy… How do scientists measure the weather? And how do they know what weather to expect…See the activity