A FOSSIL is the preserved remains or traces of an organism from the geological past.
Fossils form in several ways:
MINERALISATION:
An organism dies and is quickly buried by sediment (mud, sand).
Soft tissue decays, but hard parts (bones, teeth, shells) remain.
Over millions of years, minerals from the surrounding rock slowly replace the original material.
Eventually all original material is replaced by rock → a fossil.
PRESERVATION in extreme conditions:
AMBER — tree resin traps small organisms (insects, spiders) which are perfectly preserved.
ICE — frozen conditions preserve soft tissue (woolly mammoths found almost intact in Siberia).
TAR PITS — organisms become trapped in natural asphalt and are preserved.
PEAT BOGS — acidic, anaerobic conditions preserve soft tissue (bog bodies found in northern Europe).
TRACE FOSSILS:
Footprints, burrows, bite marks or droppings preserved in rock.
Provide evidence of behaviour as well as body structure.
Why the Fossil Record is Incomplete
The fossil record does not contain every organism that ever lived — it is significantly INCOMPLETE.
Reasons:
SOFT TISSUE rarely fossilises — decay is usually faster than mineralisation.
SMALL OR FRAGILE organisms are less likely to leave fossils.
CONDITIONS must be exactly right — rapid burial, appropriate chemistry, absence of scavengers.
FOSSILS may be DESTROYED by geological processes — volcanic activity, erosion, subduction.
FOSSILS are simply not yet found — much of the Earth's rock has not been excavated.
Despite being incomplete, the fossil record is remarkably consistent with evolution — older fossils are simpler, newer fossils are more complex, and transitional forms have been found.
Extinction
EXTINCTION occurs when every individual member of a species dies — the species is gone forever.
Extinction is permanent — once a species is gone, it cannot return.
Causes of extinction:
CATASTROPHIC EVENTS — e.g. asteroid impact (linked to mass extinction of dinosaurs ~66 million years ago), volcanic eruptions producing climate change.
HABITAT DESTRUCTION — deforestation, draining of wetlands, urbanisation → species lose their living space.
NEW COMPETITORS or PREDATORS — introduced species outcompeting or preying on native species.
NEW DISEASES — disease spreading through a population with no natural immunity.
CLIMATE CHANGE — rapid shifts in temperature or rainfall patterns that species cannot adapt to quickly enough.
Scientists estimate we are currently experiencing a mass extinction event caused primarily by human activity — habitat destruction, climate change, pollution and overexploitation.
⚠️ Common Mistake
Extinction is PERMANENT — a species cannot come back once all individuals are dead. The fossil record being incomplete does NOT disprove evolution — it is expected and understandable given how rare fossilisation is. Most organisms that ever lived left no fossil at all.
📌 Key Note
Fossils form by mineralisation, preservation in amber/ice/tar, or trace fossils. Record is incomplete because soft tissue rarely fossilises and conditions must be exact. Extinction = all individuals of species die — permanent. Causes: catastrophe, habitat loss, competition, disease, climate change, human activity.
🎯 Matching Activity — Match the Fossil Type or Extinction Cause
Match each term to its correct description. — drag the symbols on the right to match the component names on the left.
Mineralisation
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Amber preservation
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Trace fossil
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Habitat destruction
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Introduced species
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Forests cleared, wetlands drained — species lose living space — extinction risk
Footprints, burrows or droppings preserved in rock — evidence of behaviour
Hard parts replaced by minerals over millions of years as the organism is buried in sediment
New predator or competitor arrives in an ecosystem — native species may be outcompeted
Small organisms trapped in tree resin — perfectly preserved for millions of years
🎯 Test Yourself
Question 1 of 2
1. Why are soft-bodied organisms rarely found as fossils?
2. The dodo became extinct in the 17th century. What caused its extinction?
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