What are fossils and how are they formed? Revision Notes | GCSE OCR Foundation Biology

What this lesson covers

Students learn what fossils are and explore the three main ways fossils can form: from parts that haven't decayed, through mineral replacement, and as p...

• Explain what fossils are and how they provide evidence of past life
• Describe the three main ways fossils can be formed
• Identify the conditions needed for fossil formation
• Evaluate why fossils are rare and incomplete records of past life

Key ideas to keep in view

Use this lesson to stay inside B5.2 Natural selection and evolution while connecting the detail back to the wider B5: Genes, inheritance and selection topic.

• B5: Genes, inheritance and selection
• B5.2 Natural selection and evolution
• GCSE OCR Foundation Biology

Lesson notes preview

Read the core explanations from What are fossils and how are they formed? before testing yourself from memory.

• Imagine finding a perfect dinosaur skeleton buried in rock, or discovering ancient footprints preserved in stone. These are fossils - windows into life from millions of years ago.
• The preserved remains or traces of organisms that lived millions of years ago, found in rocks.
• Fossils are incredibly rare. Think about it - when you see a dead bird or mouse, what normally happens? Bacteria break it down completely within weeks. For a fossil to form, this normal decay process must be stopped or slowed down dramatically.
• Trap: Many students think fossils are just old bones. Actually, fossils can be shells, leaves, footprints, or even preserved poo (called coprolites). They're any evidence of past life preserved in rock.
• There are three main ways organisms can become fossils. Each requires special conditions that prevent normal decay from destroying the remains completely.
• Method 1: No Decay ConditionsSometimes organisms are preserved because one or more conditions needed for decay are missing. Bacteria need oxygen, warmth, and moisture to break down dead organisms. Remove these, and decay stops.
• Examples include insects trapped in amber (no oxygen), mammoths frozen in ice (too cold), or organisms buried in acidic peat bogs (wrong pH for bacteria).
• Method 2: MineralisationHere's where it gets clever. As the organism slowly decays, minerals from surrounding water seep in and replace the original material bit by bit. The shape stays the same, but it's now made of rock instead of bone or wood.
• This is how we get petrified wood - the wood fibres are replaced by minerals like quartz, creating a perfect stone copy of the original tree.
• Method 3: Trace FossilsSometimes the organism itself doesn't survive, but evidence of its activity does. Footprints in mud, burrows in sand, or root traces in soil can be preserved when sediment fills them and hardens into rock.

Continue through the topic

Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.