How are fossils preserved, and what makes a fossil useful for dating rocks?
Fossils: the conditions that favour preservation (rapid burial, anoxia, hard parts, fine sediment); the modes of preservation (moulds and casts, permineralisation, carbonisation, and preservation in amber or ice); the properties of a good index (zone) fossil (abundant, widespread, easily recognised, short stratigraphic range); the distinction between body and trace fossils.
A focused answer to the OCR H414 dot point on fossils. Covers the conditions favouring preservation, the modes of preservation (moulds and casts, permineralisation, carbonisation, amber and ice), the properties of a good index or zone fossil, and the distinction between body and trace fossils.
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What this dot point is asking
OCR wants you to state the conditions that favour fossil preservation, describe the modes of preservation (moulds and casts, permineralisation, carbonisation, amber and ice), give the properties of a good index (zone) fossil, and distinguish body fossils from trace fossils.
The answer
Conditions that favour preservation
Fossilisation is rare because most remains are destroyed first. Preservation is favoured by:
- Rapid burial, which protects remains from scavengers and erosion.
- Anoxia (low oxygen), which slows decay by bacteria.
- Hard parts (shells, bones, wood), which resist decay better than soft tissue.
- Fine sediment, which buries the organism gently and completely.
Because this combination is uncommon, only a tiny fraction of organisms ever fossilise, which makes the fossil record incomplete.
Modes of preservation
- Mould and cast. The buried shell dissolves, leaving a hollow mould of its shape; if that cavity is later filled, the infill is a cast.
- Permineralisation (petrification). Minerals from groundwater fill the pore spaces of a hard part (for example wood or bone), turning it to stone while preserving fine detail (for example petrified wood).
- Carbonisation. Volatile elements are driven off under pressure, leaving a thin carbon film that preserves the outline (common for leaves and graptolites).
- Preservation in amber or ice. Whole organisms (for example insects in amber, mammoths in permafrost) can be preserved almost intact, including soft parts.
Index (zone) fossils
An index (zone) fossil is used to date and correlate rocks. A good one has four properties:
The short range is the most important property, because the shorter the time a species existed, the more precisely its presence pins down the age. Examples used in the course include graptolites, ammonites and trilobites.
Body versus trace fossils
- Body fossils are the preserved remains of the organism itself (shells, bones, teeth, leaves).
- Trace fossils record the activity of an organism without its body (footprints, burrows, borings, coprolites). Trace fossils are valuable because they show behaviour and the environment, and unlike body fossils they are rarely transported far from where the organism lived.
Examples in context
Example 1. Ammonites as zone fossils. Ammonites evolved rapidly, were abundant and widespread in Mesozoic seas, and are easily recognised, so their short-ranging species make them excellent zone fossils for correlating Jurassic and Cretaceous strata.
Example 2. Petrified wood. Groundwater rich in silica permineralised buried logs, replacing the organic structure with stone while preserving cell-level detail, a classic example of permineralisation.
Try this
Q1. State two conditions that favour the preservation of a fossil. [2 marks]
- Cue. Any two of: rapid burial; low oxygen (anoxia); the presence of hard parts; fine sediment.
Q2. Explain why a short stratigraphic range makes a fossil a good index fossil. [2 marks]
- Cue. A short range means the species existed for only a short time, so its presence pins the age of the rock to a thin slice of time, giving a precise age.
Q3. Distinguish a body fossil from a trace fossil. [2 marks]
- Cue. A body fossil is the preserved remains of the organism (shell, bone); a trace fossil records its activity (footprint, burrow) without the body.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H414/01 20194 marksState the four properties of a good index (zone) fossil, and explain why a short stratigraphic range is the most important for dating rocks precisely.Show worked answer →
List the four properties, then justify the range.
The four properties. A good index fossil is (1) abundant (so it is likely to be found), (2) geographically widespread (so it allows correlation between distant areas), (3) easily recognisable (so it is not misidentified), and (4) has a short stratigraphic (geological) range (so it existed for only a short time).
Why a short range matters most. The shorter the time a species existed, the more precisely its presence pins down the age of the rock. A fossil with a long range tells you only a broad interval, whereas a short-ranging fossil narrows the age to a thin slice of time, giving a precise zone (biozone). This is why a short range is the key property for accurate dating.
Markers reward all four properties and the explanation that a short range gives a precise age.
OCR H414/01 20184 marksDescribe how a mould and a cast of a shell form, and explain why preservation as a fossil is rare.Show worked answer →
Describe the two-stage process, then the reasons for rarity.
- Mould
- The shell is buried in sediment, which hardens around it. The shell then dissolves away, leaving a hollow cavity that preserves the shape of the shell: a mould (an external mould records the outside, an internal mould the inside).
- Cast
- If that cavity is later filled by minerals or fresh sediment, the infill takes the shape of the original shell: a cast.
- Why preservation is rare
- Most organisms are destroyed before they can be buried: soft parts decay, scavengers consume remains, and erosion or chemical processes destroy hard parts. Preservation requires rapid burial in fine sediment, low oxygen (to slow decay) and the presence of hard parts, a combination that is uncommon, so only a tiny fraction of organisms become fossils.
Markers reward the mould-then-cast sequence and at least two reasons preservation is rare (decay, scavenging, lack of rapid burial).
Related dot points
- Relative dating: the principles used to order geological events (superposition, original horizontality, cross-cutting relationships, included fragments and faunal succession); the recognition of way-up evidence; the application of these principles to construct the geological history of a cross-section, including faults, intrusions and unconformities.
A focused answer to the OCR H414 dot point on relative dating. Covers superposition, original horizontality, cross-cutting relationships, included fragments and faunal succession, way-up evidence, and how to apply these principles to reconstruct the geological history of a cross-section with faults, intrusions and unconformities.
- The geological record: the hierarchy of the geological time scale (eon, era, period, epoch) and the major divisions (Precambrian and the Phanerozoic eras); correlation of strata by lithostratigraphy (matching rock units) and biostratigraphy (matching fossils and biozones); the use of marker horizons such as volcanic ash bands; the distinction between rock units (systems) and time units (periods).
A focused answer to the OCR H414 dot point on the geological time scale and correlation. Covers the eon, era, period and epoch hierarchy and the major divisions, correlation by lithostratigraphy and biostratigraphy, the use of marker horizons such as ash bands, and the distinction between rock units and time units.
- Evolution and the fossil record: evidence for evolution from the fossil record (morphological change through time, transitional forms, adaptive radiation); the models of evolutionary change (gradualism versus punctuated equilibrium); mass extinctions and their causes and effects (for example the end-Permian and end-Cretaceous events); the incompleteness and biases of the fossil record.
A focused answer to the OCR H414 dot point on evolution. Covers the fossil evidence for evolution (morphological change, transitional forms, adaptive radiation), the gradualism and punctuated equilibrium models, mass extinctions (the end-Permian and end-Cretaceous events) and their causes and effects, and the incompleteness and biases of the fossil record.
- Radiometric dating: radioactive decay of unstable parent isotopes to stable daughter isotopes; the concept of half-life as a constant; the use of parent-to-daughter ratios to calculate absolute ages; the main isotopic systems (uranium-lead, potassium-argon and carbon-14) and their suitable age ranges; the assumptions and limitations of radiometric dating; the combination of absolute and relative dating.
A focused answer to the OCR H414 dot point on radiometric dating. Covers radioactive decay of parent to daughter isotopes, half-life as a constant, calculating absolute ages from parent-to-daughter ratios, the uranium-lead, potassium-argon and carbon-14 systems and their ranges, the assumptions and limitations, and combining absolute with relative dating.
- Sedimentary environments: the concept of facies as a body of rock reflecting a particular depositional environment; sedimentary structures (bedding, cross-bedding, graded bedding, ripple marks and desiccation cracks) and their interpretation; the characteristics of the main environments (fluvial, deltaic, shallow marine, deep marine and desert); the construction and interpretation of sedimentary logs to reconstruct environmental change.
A focused answer to the OCR H414 dot point on sedimentary environments. Covers facies, sedimentary structures (bedding, cross-bedding, graded bedding, ripple marks, desiccation cracks) and their interpretation, the fluvial, deltaic, shallow-marine, deep-marine and desert environments, and how sedimentary logs reconstruct environmental change.
Sources & how we know this
- OCR A Level Geology (H414) Specification — OCR (2017)