Fossils — AQA GCSE Combined Science
Test yourself on Fossils with AQA GCSE practice questions.
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Fossils explained
A fossil is any preserved trace of an organism that lived millions of years ago, and fossils are found within rocks.
Read the full explanation
The word ‘remains’ is in quotation marks because a fossil is not always body material: it can be the shape, impression or chemical trace left behind. Most fossils occur in sedimentary rock, where layers of mud, sand or silt bury dead organisms and harden over long periods. The organism must usually be buried quickly, and soft parts normally decay before preservation, so hard parts such as shells, bones, teeth and wood are most commonly fossilised. For example, a dinosaur skeleton in sandstone or an ammonite in limestone shows how rock preserves evidence of past life. Because the rock layers form in order, fossils give a record of organisms that existed long before humans, supporting ideas about evolution.
Fossils may be formed:
This statement introduces the different ways fossils can form. Fossils form when an organism or its traces are preserved before they decay completely. Common routes include: rapid burial in sediment, so decay is limited and hard parts remain; preservation of an impression or mould when an organism dissolves and leaves a cavity that may later fill with minerals to make a cast; replacement of hard parts by minerals, turning bone or shell into rock-like material; and preservation in amber, tar or ice, where decay is prevented. Fossils can also be traces such as footprints, burrows or droppings. The method depends on conditions: hard parts, quick burial, low oxygen and stable temperatures all help. For example, a shell buried in mud may dissolve and leave a mould, while an insect trapped in amber may survive almost unchanged.
from parts of organisms that have not decayed because one or more of the conditions needed for decay are absent
Decay is carried out by microorganisms, which need warmth, moisture and oxygen to break down dead material. If any one of these conditions is missing, decay slows or stops, so parts of an organism can survive for a very long time and become fossils. This explains exceptional preservation: an insect trapped in amber is sealed from oxygen and microorganisms; a mammoth frozen in permafrost is too cold for decomposers; a body in a bog may be preserved because acidic, waterlogged conditions prevent microbial activity. The fossil record is therefore biased towards organisms that were buried rapidly in conditions where decay could not occur.
when parts of the organism are replaced by minerals as they decay
Some fossils form when hard parts such as bone, shell or wood are slowly replaced by minerals. As the original material decays or dissolves, mineral-rich water seeps into the remains and minerals such as calcium carbonate or silica are deposited in the spaces. Over long periods the minerals harden and preserve the shape and structure of the original part, even though its chemical composition has changed. This produces the familiar stony fossils found in sedimentary rock. The process requires burial, water carrying dissolved minerals, and long periods of time, and it usually preserves hard parts rather than soft tissue.
as preserved traces of organisms, such as footprints, burrows and rootlet traces.
Not every fossil is a body fossil. Sometimes the organism itself is absent and what survives is a trace of its activity, preserved in rock. A dinosaur footprint forms when a foot presses into soft mud; if that mud later hardens and is buried, the impression can survive as sedimentary rock. A burrow shows where an animal tunnelled through sediment, and a rootlet trace records where a plant root once grew, leaving a cast or impression after the root decayed. These are called trace fossils. They form only when the sediment sets before the trace is destroyed, and they are later exposed by erosion or quarrying. Trace fossils reveal behaviour and movement, which body fossils often cannot, so they add evidence about how organisms lived.
Many early forms of life were soft-bodied, which means that they have left few traces behind. What traces there were have been mainly destroyed by geological activity. This is why scientists cannot be certain about how life began on Earth.
The fossil record is incomplete, especially for the earliest life. Early organisms were largely soft-bodied, so they had no hard shells, bones or teeth to resist decay and fossilise. Their remains were usually broken down before they could be buried, so few traces formed. Even those rare traces have mostly been destroyed by geological activity: heat and pressure from burial, melting of rock, and plate movements can erase fossils over billions of years. As a result, the evidence available for the origin of life is fragmentary. Scientists can propose hypotheses, such as life forming in deep-sea vents or shallow pools, but they cannot be certain, because the direct evidence has largely been lost.
We can learn from fossils how much or how little different organisms have changed as life developed on Earth.
Fossils are preserved remains or traces of organisms from the past, and comparing them with living species reveals the extent of change over geological time. Some lineages show little change: horsehoe crabs in rocks hundreds of millions of years old closely resemble modern forms, so they are often called living fossils. Others show major change: comparing ancient whale fossils with modern whales reveals the gradual loss of hind limbs and the shift to flippers. Fossils also document extinction, showing that many organisms no longer exist. By arranging fossils by rock age, we can see whether a body plan stayed stable or was modified, and how life on Earth developed.
Students should be able to extract and interpret information from charts, graphs and tables such as evolutionary trees.
Evolutionary trees are branching diagrams showing how species or groups are related through common ancestors. The branching point, or node, represents a common ancestor; the tips represent species alive today or at a chosen time. Reading a tree involves tracing lines from a species back to a node, then following the other branch to find its closest relative. The number of branching points between two species indicates how recently they shared an ancestor, not how similar they look. Charts and tables may also show fossil ages or numbers of species, and these data can be read and compared to support conclusions about relationships and change over time.
Your focus
- Define a fossil as preserved evidence of an organism from millions of years ago.
- Describe how fossils are found in rocks, particularly sedimentary rock.
- Give an example of a fossil and explain why its remains were preserved.
Show all 24 objectives
- Describe at least two ways in which fossils may be formed.
- Explain how conditions such as rapid burial or low temperature prevent decay.
- Distinguish between body fossils, moulds, casts and trace fossils.
- State the conditions microorganisms need for decay.
- Explain how the absence of one or more of these conditions can preserve parts of organisms.
- Apply this explanation to named examples such as amber, ice or bog preservation.
- Describe how minerals replace parts of an organism during fossil formation.
- Explain why hard parts are more likely than soft parts to be preserved in this way.
- Relate mineral replacement to the formation of fossils in sedimentary rock.
- State that trace fossils are preserved traces of organism activity, giving footprints, burrows and rootlet traces as examples.
- Describe how a footprint, burrow or rootlet trace can be preserved in sedimentary rock.
- Explain what trace fossils can reveal about an organism that a body fossil might not.
- Explain why soft-bodied early organisms left few fossil traces.
- Describe how geological activity destroys fossils and creates gaps in the fossil record.
- Explain why the origin of life on Earth cannot be stated with certainty from the available evidence.
- Compare a named fossil with its living relative and state whether the organism has changed much or little.
- Use fossil evidence to describe how a lineage changed as life developed on Earth.
- Explain how fossils provide evidence that some organisms have become extinct.
- Identify common ancestors and closest relatives from an evolutionary tree.
- Read and quote values accurately from a chart, graph or table about fossils or species.
- Use data from a chart, graph or table to justify a conclusion about evolutionary relationships.
Fossils exam tips
Marking Points
- States that a fossil is evidence of an organism that lived millions of years ago.
- Explains that fossils are found in rocks, most commonly sedimentary rock formed from layers of sediment.
- Recognises that ‘remains’ can include body parts, impressions, moulds, casts or chemical traces rather than only whole organisms.
- Links preservation to rapid burial and to the decay or loss of soft parts, leaving hard parts more likely to fossilise.
- Uses a named example, such as an ammonite in limestone or a dinosaur bone in sandstone, to illustrate the idea.
- Identifies rapid burial in sediment as a route to fossil formation, limiting decay and scavenging.
- Describes preservation of hard parts such as bones, shells, teeth or wood, which resist decay better than soft tissue.
- Explains moulds, casts or impressions, where an organism leaves a cavity or shape that may later be filled by minerals.
- Recognises preservation in amber, tar or ice, where decay is prevented and even soft parts may survive.
- Includes trace fossils such as footprints, burrows or droppings as evidence of organism activity.
- Decay is caused by microorganisms such as bacteria and fungi, not by the organism simply 'drying out' on its own.
- Microorganisms need warmth, moisture and oxygen; removing any one of these conditions prevents or greatly slows decay.
- Absence of oxygen, very low temperature, or very dry conditions can each preserve parts of an organism.
- Examples include insects in amber, mammoths in ice, and soft tissue preserved in acidic bogs.
- Preservation is more likely when the organism is buried quickly, which limits exposure to decomposers.
- Fossils formed this way are rare and give an incomplete record of past life.
- Hard parts such as bones, shells and teeth are the most likely to be mineralised.
- Mineral-rich water must reach the buried remains for replacement to occur.
- As the original material decays or dissolves, minerals are deposited in its place.
- The minerals harden over a long period, preserving the shape and structure of the original part.
- The resulting fossil is made of rock minerals, not the original biological material.
- This process is common in sedimentary rock and helps build the fossil record.
- Trace fossils are preserved evidence of an organism's activity rather than its body parts.
- A footprint forms when a foot deforms soft sediment that later lithifies, preserving the impression.
- A burrow is a tunnel or hole made in sediment by an animal and later filled or cast by mineral material.
- Rootlet traces record the position and branching pattern of plant roots after the root tissue has decayed.
- Trace fossils can indicate behaviour such as walking, burrowing or anchoring, which body fossils may not show.
- Preservation requires the sediment to harden before the trace is eroded, disturbed or destroyed.
- Soft-bodied organisms decay quickly and lack hard parts, so they fossilise far less often than organisms with shells or skeletons.
- Fewer fossils form when there are no hard parts to resist decay and scavenging before burial.
- Geological activity such as heat, pressure, melting and plate movement destroys many fossils over long periods.
- The earliest fossils are therefore rare and fragmentary, leaving gaps in the fossil record.
- Because the evidence is incomplete, scientists cannot be certain exactly how or where life began on Earth.
- Scientific explanations of the origin of life remain hypotheses supported by indirect evidence rather than direct fossil proof.
- Fossils are remains or traces of organisms preserved in rocks, ice or amber, and their age can be estimated from the rock layer in which they are found.
- Comparing the structure of a fossil with a living organism shows whether the body plan has changed a lot or only a little over time.
- Organisms such as horseshoe crabs have changed very little over hundreds of millions of years, whereas whale ancestors show major changes in limb structure.
- Fossils provide evidence that some species became extinct, because their fossils appear in older rocks but no living examples exist.
- Sequencing fossils from oldest to youngest allows trends in body structure to be identified as life developed on Earth.
- Identify the branching points on an evolutionary tree and state that each represents a common ancestor of the lineages above it.
- Trace lines from two species back to their shared node to decide which species are most closely related.
- Interpret a table or graph of fossil ages or species numbers by reading values accurately and describing the trend shown.
- Use evidence from the chart, graph or table to support a conclusion about relationships or change, quoting specific values or labels.
- Recognise that a species at the tip of a branch is not necessarily more advanced than one on another branch; branching shows relatedness, not progress.
Examiner Tips
- 💡Define the term fossil in one sentence, then add where fossils are found and why that rock type preserves them.
- 💡Use the phrase ‘millions of years ago’ when giving the timescale, rather than vague words such as ‘a long time ago’.
- 💡Include one named example of a fossil and the rock it is found in to make the answer concrete.
- 💡Use the command word ‘may be formed’ to justify giving more than one method, each with a brief explanation.
- 💡Name the material or condition involved, such as sediment, minerals, amber, tar or ice, to show precise knowledge.
- 💡Link each method to why decay is prevented or limited, for example rapid burial, low oxygen or low temperature.
- 💡Name the specific missing condition in the example you are given, such as 'no oxygen' for an insect in amber.
- 💡Link each example back to the microorganisms that would normally cause decay.
- 💡Use comparative language such as 'slows' or 'prevents' rather than saying decay 'never happens' in all conditions.
- 💡Use the phrase 'mineral-rich water' to show you know what carries the minerals into the remains.
- 💡Describe the sequence: burial, decay or dissolving, mineral deposition, hardening.
- 💡Link the process to sedimentary rock, where most mineral-replaced fossils are found.
- 💡Name the type of trace and the organism group that made it, for example an animal footprint or a plant rootlet trace.
- 💡Describe the sequence: soft sediment is disturbed, then hardens and is buried, then is exposed by erosion.
- 💡Contrast trace fossils with body fossils in one sentence to show you understand the difference.
- 💡Link each reason for uncertainty to a mechanism: soft bodies decay, and geological activity destroys rock and fossils.
- 💡Use the phrase fossil record is incomplete when explaining why scientists cannot be certain.
- 💡Distinguish evidence from explanation: state what is missing, then say why that limits certainty.
- 💡When describing change, name the specific feature compared, such as limb bones or shell shape, rather than saying the organism 'changed'.
- 💡Use the command word: 'describe' needs features and trends, while 'explain' needs a reason linking the fossil evidence to change over time.
- 💡Quote data from any chart or table, such as 'the fossil is 300 million years old', to support your comparison.
- 💡Name the node and the two species when explaining a relationship, for example 'species A and B share node X, so they are closely related'.
- 💡When a graph or table is given, quote a value with its unit and use it in your answer rather than describing the shape vaguely.
- 💡Check the axis labels and keys before reading values, because evolutionary trees and data tables often use symbols or abbreviations.
Common Mistakes
- Thinking every fossil is the actual body of the organism: correct this by explaining that many fossils are impressions, moulds, casts or mineral replacements.
- Believing fossils are found in any rock type: correct this by linking most fossils to sedimentary rock, where layers bury and preserve remains.
- Assuming all organisms fossilise: correct this by noting that decay, scavengers and geological processes destroy most remains, so fossils are rare.
- Listing only one method when the question asks how fossils may be formed: correct this by giving at least two distinct routes, such as rapid burial and preservation in amber.
- Confusing a mould with a cast: correct this by stating that a mould is a cavity left by a dissolved organism, while a cast forms when that cavity fills with minerals.
- Claiming soft parts always decay before fossilisation: correct this by noting that amber, tar and ice can preserve soft parts because decay is prevented.
- Thinking decay stops only when an organism is completely frozen; correction: any missing condition, such as lack of oxygen or moisture, can prevent decay.
- Confusing 'not decayed' with 'never alive'; correction: the preserved parts are still the remains of once-living organisms.
- Assuming all fossils are bones or shells; correction: amber, ice and bog deposits can preserve soft parts such as skin, hair or whole insects.
- Saying the organism turns into rock instantly; correction: mineral replacement happens gradually over a very long time.
- Thinking soft parts are usually preserved this way; correction: hard parts such as bone and shell are much more likely to be replaced by minerals.
- Believing the original material remains unchanged; correction: the original material decays or dissolves and is replaced by minerals.
- Thinking a footprint is part of the animal's body: correct this by stressing that the body has gone and only the impression or cast remains.
- Assuming any mark in a rock is a fossil: correct this by requiring evidence that a once-living organism produced the trace.
- Confusing a burrow with a rootlet trace: correct this by linking burrows to animal activity and rootlet traces to plant roots.
- Saying no early life fossils exist at all: correct this by saying few traces survive, not none.
- Claiming soft-bodied organisms cannot fossilise: correct this by explaining that fossilisation is much less likely, not impossible.
- Treating a hypothesis about the origin of life as proven fact: correct this by linking uncertainty to missing and destroyed evidence.
- Thinking every fossil is a complete skeleton: most fossils are fragments, impressions or traces, so conclusions are based on the features preserved.
- Assuming all organisms change at the same rate: some lineages remain similar for long periods while others change substantially.
- Believing fossils show the exact age of an organism without evidence: age is inferred from the rock layer or dating methods, not from the fossil alone.
- Reading an evolutionary tree from left to right as a timeline of progress: the branching pattern shows common ancestry, and branch tips can represent the same time period.
- Assuming species at the top of a tree are more evolved: position on the page does not indicate superiority or greater complexity.
- Confusing similarity of appearance with closeness of relationship: analogous features can look alike without indicating a recent common ancestor.