E. Earth and Space Systems · Rocks, Minerals, and Geological Processes

Fossils: Clues to Earth's Past

E2.5 A1.1 A3.1About 45 minutes

Learning goals

I can explain how fossils form, usually in sedimentary rock.
I can describe different kinds of fossils, like body fossils, trace fossils, moulds and casts.
I can explain what fossils tell us about living things and places long ago.
I can explain why fossils are found in sedimentary rock but rarely in igneous or metamorphic rock.

Words to know

fossil
The remains or traces of a living thing from long ago, kept (preserved) in rock or another material.

preserved
Kept safe from rotting or breaking down for a long time.

body fossil
A fossil of part of a living thing's body, like a shell, bone, tooth or leaf.

trace fossil
A fossil that shows what a living thing did, like a footprint, a burrow or a trail.

mould and cast
A mould is a hollow print left in rock. A cast forms when the mould fills up with minerals or sediment, making a copy of the shape.

palaeontologist
A scientist who studies fossils to learn about life long ago.

Ontario's ancient sea

Imagine walking along a rocky creek in Toronto or Ottawa and finding a rock with a shell pressed into it. A sea shell, hundreds of kilometres from any ocean! How did it get there?

That shell is a fossil, and it is a clue. About 450 million years ago, a warm, shallow sea covered much of what is now southern Ontario. Sea animals lived, died and were buried there. Their remains turned into fossils, and the sea floor mud became rock.

A grey limestone slab covered with fossil sea shells with fan-shaped ribs and short, segmented stem pieces, beside a creek. — These are fossils of brachiopods (ribbed shells) and pieces of crinoid stems, sea animals that lived in the warm sea that once covered southern Ontario. AI-generated image

What is a fossil?

A fossil is the remains or traces of a living thing from long ago, preserved in rock or another material. Most fossils are thousands to millions of years old.

Most living things never become fossils. Usually, dead plants and animals are eaten or rot away. A fossil forms only when things go just right, usually when the living thing is buried quickly in sediment.

How a shell becomes a fossil. 1: A sea animal dies and sinks to the bottom. 2: Mud and sand quickly bury it. 3: More layers pile up; the sediment hardens into rock, and minerals fill or replace the shell. 4: Millions of years later, uplift and erosion wear away the rock above, and the fossil is exposed.

How a fossil forms

A living thing dies, often in or near water.
It is buried quickly by mud or sand. This keeps it away from animals that would eat it and slows down rotting. The soft parts usually rot away; hard parts like shells, bones and teeth last longer.
More layers pile on top. The sediment becomes sedimentary rock (compaction and cementation). Water carrying minerals soaks through. The minerals can fill in the tiny spaces or slowly replace the shell or bone, turning it to stone.
Millions of years later, uplift and erosion wear away the rock above, and the fossil appears at the surface, where someone might find it.

Kind of fossil	What it is	Example
body fossil	part of the body itself, often turned to stone	shells, bones, teeth, petrified wood
mould	a hollow print where a shell or bone used to be	a shell-shaped hollow in limestone
cast	a mould that filled up with minerals or sediment, making a copy	a stone copy of a shell
trace fossil	signs of what an animal did	footprints, burrows, trails, nests
imprint	a thin print of a flat thing	a leaf print in shale
preserved remains	the real body kept in ice, amber or tar	an insect in amber; ice age mammoth remains frozen in the ground in Yukon

Kinds of fossils

A shell pressed into mud leaves a hollow mould. If minerals or sediment later fill the hollow, they make a cast, a stone copy of the shell.

Why sedimentary rock?

Fossils are found almost only in sedimentary rock.

Sedimentary rock forms from sediment that can gently bury a living thing.
Igneous rock forms from melted rock. Anything living would be destroyed by the heat.
Metamorphic rock has been heated and squeezed. Most fossils get squashed or erased.

What fossils tell us

Fossils are like pages from Earth's history book. Palaeontologists (scientists who study fossils) use them to learn:

What lived long ago, including animals and plants that no longer exist, like dinosaurs and trilobites.
How living things have changed over a very long time.
What a place was like. Coral and shell fossils mean the place was once under the sea. Fossil palm leaves in a cold place mean it was once warm.
How old the rock layers are. Lower layers are usually older, so fossils in lower layers usually lived earlier.

In southern Ontario's limestone and shale, people find fossils of sea animals like trilobites, brachiopods (shellfish with two shells), crinoids ("sea lilies", animals related to starfish), and corals. Canada also has amazing dinosaur fossils, especially in Alberta, where Dinosaur Provincial Park is a UNESCO World Heritage Site.

Just for fun

Funny but true: trilobites lived in Earth's seas for more than 250 million years. That's longer than the dinosaurs were around! Not bad for a bug-like sea creature.

Worked example

Let's read the fossil clues

In a cliff, the bottom layer is limestone with coral and shell fossils. The top layer is sandstone with fern leaf prints and footprints of a land animal. What story do these layers tell?

Bottom layers are older. So first, this place had coral and shells: it was under a sea.
Later (higher layer), it had ferns and land animal footprints: the place had become land, maybe a swampy shore.
Footprints are trace fossils; the coral and shells are body fossils; the leaf prints are imprints.

Long ago the place was under the sea. Later, the sea went away and it became land with ferns and animals walking by.

Fossil hunting rules

Ask permission before looking on private land. In provincial and national parks and conservation areas, fossils must be left where they are. Take a photo instead!
Stay away from cliff bottoms and edges: rocks can fall.
Some special fossils are protected by law. If you find something unusual, take a photo, note the place, and tell a museum.
In the activity, plaster gets warm as it sets and must never go down the drain. A grown-up mixes it. Don't touch it while it is wet, and never put your hand in it to make a print.

Hands-on activity

Make a mould and a cast

You need: modelling clay, play dough or salt dough (1 cup flour, ½ cup salt, about ½ cup water), a small object with a clear shape (a seashell, a leaf, a plastic toy dinosaur), a little cooking oil, and either more dough or plaster of Paris (with a grown-up), a paper cup.

Flatten a ball of dough into a thick pancake.
Rub a tiny bit of oil on the object so it won't stick. Press it firmly into the dough, then lift it out carefully. The hollow print is a mould.
Cast option A (easy): let the mould dry for a day. Press a fresh ball of dough (or clay) into it, then peel it out. The bump that copies the object's shape is a cast.
Cast option B (with a grown-up): the grown-up mixes plaster of Paris in a paper cup and pours it into the mould. Wait until it is completely hard (follow the package; usually about an hour), then peel away the dough.
Make a trace fossil too: walk a toy animal's feet across another dough pancake.
Swap with a family member: can they guess what made each "fossil"? Is it a body fossil (mould or cast) or a trace fossil?

Research question: With a grown-up, look up a museum in Canada with fossils (for example, the Royal Ontario Museum in Toronto, the Canadian Museum of Nature in Ottawa, or the Royal Tyrrell Museum in Alberta). Find one fossil from their collection and write 3 facts about it.

Your turn

What is a fossil?

Why must a living thing usually be buried quickly to become a fossil?

Which parts of an animal usually become fossils? Why?

What is the difference between a mould and a cast?

Is a dinosaur footprint a body fossil or a trace fossil?

In which type of rock are fossils usually found?

Coral fossils are found in rocks near Ottawa. What does that tell you?

More practice: warm-up

What do we call a scientist who studies fossils?

Name one kind of trace fossil.

True or false: Most living things become fossils.

Which Canadian province is famous for dinosaur fossils?

What covered much of southern Ontario about 450 million years ago?

More practice: on your own

Why are there almost no fossils in igneous rock?

A fossil is found in a layer below another fossil. Which animal probably lived first? Why?

Give two things fossils can tell us about the past.

Your play dough "fossil" is a hollow shell shape. Then you press clay into it and get a shell-shaped bump. Name each one.

Why should you leave fossils in a provincial park and take a photo instead?

Challenge

Explain, step by step, how a trilobite that died 450 million years ago could end up in a rock in your hand today.

A palaeontologist finds fossils of fish in a layer, then fossils of land plants in the layer above. What changed in that place?

Why might a palaeontologist want to know exactly where a fossil was found, even which layer?

Check your understanding

Quiz

A fossil is...

Fossils are found mostly in...

A footprint preserved in rock is a...

A hollow, shell-shaped print in rock is a...

Shell and coral fossils in Ontario rocks tell us that...

Why do hard parts become fossils more often than soft parts?

For parents and teachers

Big idea (E2.5): Fossils are preserved remains or traces of past life, formed mostly in sedimentary rock when organisms are buried rapidly and mineralized. They reveal past life forms, changes over time, past environments and the relative ages of rock layers. Southern Ontario's Ordovician limestones and shales (about 450 million years old) are rich in marine fossils such as trilobites, brachiopods, crinoids and corals.

How to help: Visit a museum (the ROM, the Canadian Museum of Nature) or look closely at limestone building stone and stone walls, which often show shell fragments. The research question supports A1.1. Talk about the job of a palaeontologist, and the many other people who work with fossils (museum preparators, educators) (A3.1).

Common difficulties:

Thinking all fossils are dinosaurs. Most are shells and small sea animals.
Mixing up mould and cast. Mould = hollow; cast = filled copy.
Thinking fossils are "just bones". In most body fossils, minerals have filled or replaced the original material.

Safety: Plaster of Paris heats up while setting and should be mixed by an adult, kept off skin while wet, and never poured down drains. Fossil collecting is not allowed in provincial parks and conservation areas; on private land, ask permission. Stay back from cliff faces.