GCSE · Biology · AQA · Spec 8461

Fossils

A footprint, a beetle in amber and a dinosaur bone with no bone left in it are all fossils. How can all three count?

Biology · Fossils

How did this one get here?

Pick a find, then pick the way you think it formed.

Still to sort

Didn't decay (0)

The organism's own parts are still there, because something stopped them decaying.

Where the line is: The test is the material: if it is still the organism's own tissue, decay was stopped. If it only has the shape and is now made of minerals, it belongs in the next column.

Replaced by minerals (0)

As the parts slowly decayed, minerals took their place — the shape stays, the original material goes.

Where the line is: It looks like bone, wood or shell, but it is rock. Nothing of the original organism is left, yet the shape is a copy of a real body part — which is what separates it from a trace.

Preserved trace (0)

No part of the organism at all — just a sign that it was there.

Where the line is: A trace was never part of the body. It is a mark the living organism made — a print, a tunnel, a gap where a root grew.

7 of 7 still to sort.

A fossil is any remains of an organism from millions of years ago, found in rock. Sort each find by how it formed, then read why.

Exam line: When you say how a fossil formed, name the route AND give the evidence for it: 'the original material is gone but the shape is kept, so it was replaced by minerals'.

Watch out: A fossil does not have to contain any of the organism. A footprint is still a fossil — it is a trace the animal left behind.

What do you really think?

Is that what a fossil is?

Your friend says: "A fossil is just a really old bone. There are loads of them because everything that dies ends up as a fossil sooner or later."

Which of these is closest to what you think right now?

My friend's right that a dinosaur fossil is basically its real bone, just very old.Give it enough time and anything that dies will turn into a fossil.A fossil has to have part of the organism in it — otherwise it's just a mark in the rock.Most dead things just decay away. A fossil is the exception — real parts, a mineral copy, or just a trace.

How sure are you?

I'm sureI think soI'm guessing

My friend's right that a dinosaur fossil is basically its real bone, just very old.

This is a common misconception

Often it isn't bone at all. In many fossils the bone was replaced by minerals as it decayed, so what you are holding is rock in the shape of a bone. And some fossils — footprints, burrows — never contained any bone in the first place.

Give it enough time and anything that dies will turn into a fossil.

This is a common misconception

Time on its own does the opposite: it gives decay the chance to finish the job. When the conditions for decay are all there, a dead organism rots away and leaves nothing. A fossil only forms when something stops decay, or when minerals replace the parts before they have gone.

A fossil has to have part of the organism in it — otherwise it's just a mark in the rock.

This is a common misconception

A mark in the rock can be a fossil. Footprints, burrows and rootlet traces are preserved traces of organisms: they contain none of the organism, but they are real evidence that it lived there.

Most dead things just decay away. A fossil is the exception — real parts, a mineral copy, or just a trace.

This matches the evidence

Exactly. A fossil is any remains of an organism from millions of years ago found in rock, and it only forms when decay is stopped or overtaken, or when the organism left a trace behind.

Fossil = remains of an organism from millions of years ago, found in rock. Decay normally destroys a dead organism, so a fossil only forms when that decay is stopped or overtaken — or when the organism leaves a trace.

Exam line: Say 'remains of organisms from millions of years ago, found in rocks' — 'remains' covers original parts, mineral copies and traces.

The missing first chapter

Reason it through

Why can't scientists be certain how life on Earth began?

Link 1 of 4

First link · your turn

Start at the very beginning. What were many of the earliest living things like — bones and shells, or something else?

Locked — reveal the link above first

Read the tree

Sketch it as you read — it takes ten seconds and makes the answer jump out.

Picture an evolutionary tree. Time runs upwards, from long ago at the bottom to today at the top. One ancestor at the bottom splits into two lines: one line runs straight up to species W. The other line splits again — one branch runs up to species X, and the other splits one last time, near the top, into species Y and species Z. Which two species are most closely related?

Your turn to write

Write it, then mark it

Describe how fossils provide evidence for evolution, and give one limitation of the fossil record. [4 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Fossils are the remains (or traces) of organisms from millions of years ago, found in rocks (1 mark)
Comparing fossils (with each other or with living organisms) shows how much or how little organisms have changed as life developed (1 mark)
This change over time is evidence for Darwin's theory of evolution (1 mark)
Limitation: the record is incomplete — e.g. many early organisms were soft-bodied and left few traces, and most traces were destroyed by geological activity (1 mark)

Model answer

Fossils are the remains of organisms from millions of years ago, found in rocks. By comparing fossils with each other and with organisms alive today, scientists can see how much or how little different organisms have changed as life developed on Earth. This record of change over time is evidence for Darwin's theory of evolution. One limitation is that the fossil record is incomplete: many early forms of life were soft-bodied and left few traces, and most of the traces there were have been destroyed by geological activity.

Remains → compare → change over time → evidence for evolution. Then one honest limit: the record has gaps, especially at the start.

Exam line: Say what fossils SHOW — how much or how little organisms have changed — not just that they are 'old things in rock'.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Three very different ways a living thing can end up in rock — and what fossils can (and can't) tell us about the history of life.

What you need to know

Fossils are the remains of organisms from millions of years ago, found in rocks.
A fossil can form in three ways: parts that did not decay because a condition for decay was missing; parts replaced by minerals as they decayed; or preserved traces such as footprints, burrows and rootlet traces.
Many early forms of life were soft-bodied and left few traces, and most of those traces have been destroyed by geological activity — so scientists cannot be certain how life began.
Fossils show how much or how little organisms have changed as life developed, which is evidence for Darwin's theory of evolution.
On an evolutionary tree, species that share the most recent common ancestor are the most closely related.

The big picture

Fossils are the remains of organisms from millions of years ago, found in rocks. They form in three ways: parts that did not decay because a condition needed for decay was missing; parts that were replaced by minerals as they decayed; and preserved traces such as footprints, burrows and rootlet traces. Many early forms of life were soft-bodied and left few traces, and most of those traces have been destroyed by geological activity, so scientists cannot be certain how life began. Fossils show how much or how little organisms have changed as life developed, which makes the fossil record evidence for Darwin's theory of evolution.

Key points

1A fossil is any remains of an organism from millions of years ago found in rock — not just bone.

2No decay: decay needs conditions such as oxygen, moisture and warmth; if one is missing (sealed in amber, frozen in ice), the organism's own parts can survive.

3Mineral replacement: as parts slowly decay, minerals take their place, keeping the shape but none of the original material.

4Traces: footprints, burrows and rootlet traces contain no part of the organism, but still count as fossils.

5The fossil record is incomplete, especially for early soft-bodied life, so the origin of life is uncertain.

6Branch points on an evolutionary tree are common ancestors; the most recent shared branch point means the closest relationship.

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Worked example

Problem

A scientist finds a fossil fish skeleton. Every bone is in place and perfectly shaped, but tests show it is made entirely of minerals — none of the original bone remains. How did this fossil form? Explain your answer.

⚠ Watch out

Calling every fossil 'the animal's bone preserved in rock'. Many fossils contain no original material because minerals replaced it, and trace fossils such as footprints never contained any part of the organism.

🧠

Memory hook

STOPPED, SWAPPED or STAMPED: decay was stopped, the parts were swapped for minerals, or the organism left its stamp behind — a footprint, a burrow, a root trace.

✓

Check yourself

Cover the page. Name the three ways a fossil can form and give one example of each. Then explain why a burrow counts as a fossil even though nothing of the animal is inside it.

Flashcards

(13)

What is a fossil?

The remains of an organism from millions of years ago, found in rock.

What are the three ways a fossil can form?

Parts don't decay (a condition for decay is missing); parts are replaced by minerals as they decay; or a trace of the organism is preserved.

Why can an insect in amber or an animal frozen in ice still have its original body?

One or more conditions needed for decay were missing — no oxygen or moisture in the sealed resin, too cold in the ice — so the parts didn't decay.

What happens to a bone during mineral replacement?

As it slowly decays, minerals take its place. The shape is kept, but none of the original bone is left.

Give three examples of preserved traces.

Footprints, burrows and rootlet traces.

Is a footprint really a fossil if none of the animal is in it?

Yes. A fossil can be a preserved trace of an organism — it doesn't have to contain any part of its body.

Why did many early forms of life leave few fossils?

They were soft-bodied, with no hard parts to last, so they left few traces behind.

What happened to most of the traces early life did leave?

They have mainly been destroyed by geological activity.

Why can't scientists be certain how life on Earth began?

Early life left few traces and most were destroyed, so the evidence from the very start is missing.

What can comparing fossils tell us about organisms?

How much or how little different organisms have changed as life developed on Earth.

How does the fossil record link to Darwin's theory?

It shows organisms changing over time, which is evidence for Darwin's theory of evolution.

On an evolutionary tree, what is a branch point?

A common ancestor — the point where one line splits into two.

How do you tell which two species on an evolutionary tree are most closely related?

Find the pair that share the most recent common ancestor — the branch point nearest the tips.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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How did this one get here?

Is that what a fossil is?

Write it, then mark it

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