How are fossils preserved and used to correlate and date rocks?
The conditions and modes of fossil preservation, the principle of faunal succession, and the use of zone (index) fossils to correlate and relatively date strata.
A focused answer to WJEC and Eduqas A-Level Geology F3 on fossils, covering the conditions and modes of fossil preservation, the principle of faunal succession, and how zone (index) fossils are used to correlate strata between areas and to relatively date rocks.
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What this dot point is asking
WJEC wants you to know how fossils form (the conditions and modes of preservation), to state the principle of faunal succession, and to use zone (index) fossils to correlate and relatively date strata. Fossils are central to Component 2: they let geologists match rocks of the same age in different places and build the relative timescale.
The answer
How fossils form
A fossil is the remains or trace of a once-living organism preserved in the rock record. Preservation is the exception, not the rule, because most organisms decay or are destroyed. Good preservation requires the right conditions: possession of hard parts (shells, bones, teeth, wood), rapid burial (to protect from scavengers, currents and weathering), and low-oxygen (anoxic) conditions that slow bacterial decay. Soft tissues are only preserved exceptionally, in settings such as anoxic muds, amber or permafrost.
Modes of preservation
The common modes are: permineralisation (mineral-rich groundwater fills the pore spaces of a hard part with silica, calcite or pyrite, hardening it); mould and cast (the original hard part dissolves to leave a cavity, a mould, which may be filled to make a cast); replacement (the original material is dissolved and replaced, molecule for molecule, by another mineral); carbonisation (volatile elements are driven off, leaving a thin carbon film, typical of leaves and graptolites); and trace fossils (preserved evidence of activity, such as burrows, tracks and coprolites, rather than the body itself).
Zone fossils and correlation
A zone (index) fossil is used to date and correlate rocks. The best zone fossils have a short vertical (stratigraphic) range (they lived for only a short time, so they date a narrow interval precisely), a wide horizontal (geographical) range (found over large areas, so they correlate distant rocks), and are abundant, easily preserved and distinctive. Correlation is the matching of rocks of the same age in different places using their fossils (or other markers), even where the rock types differ.
Examples in context
Graptolites in Welsh slates. Carbonised graptolites are the classic zone fossils of the Lower Palaeozoic rocks of Wales, their rapid evolution and wide spread allowing fine subdivision of otherwise featureless slates. Ammonites in the Jurassic. Fast-evolving, widespread ammonites zone the Jurassic in great detail and correlate marine rocks across Europe. Trace fossils of environment. Burrow types reveal the energy and oxygenation of an ancient sea floor, adding palaeoenvironmental information that body fossils alone may not give.
Try this
Q1. State three conditions that favour the preservation of a fossil. [3 marks]
- Cue. Possession of hard parts, rapid burial, and low-oxygen (anoxic) conditions.
Q2. Why must a good zone fossil have a short vertical range? [2 marks]
- Cue. A short vertical (stratigraphic) range means the organism existed for only a brief span of time, so its presence pins the rock to a narrow, precise time interval.
Q3. Explain what correlation means in stratigraphy. [2 marks]
- Cue. Matching rocks of the same age in different places, typically using their fossils, even where the rock types differ.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC Eduqas 20184 marksState the features that make a fossil a good zone (index) fossil and explain why each is important.Show worked answer →
List the features and justify each, because the marks reward the reasoning, not just the list.
A good zone fossil has a short stratigraphic (vertical) range, so it lived for only a short span of geological time and therefore pins a narrow time interval.
It has a wide geographical (horizontal) distribution, so it is found over a large area and can correlate rocks between distant places.
It is abundant and easily preserved, so it is commonly found, and it has distinctive, recognisable features, so it cannot be confused with other species.
Markers reward short vertical range (for precise dating), wide horizontal range (for correlation), abundance, easy preservation and distinctiveness, each linked to its purpose.
WJEC Eduqas 20224 marksExplain how the soft tissue of an organism is rarely preserved and describe two modes by which hard parts are commonly fossilised.Show worked answer →
Soft tissue decays rapidly after death through scavenging and bacterial action, so it is only preserved exceptionally, for example by very rapid burial in oxygen-poor (anoxic) conditions that exclude scavengers and slow decay.
Hard parts such as shells and bones survive far more often, and two common modes are permineralisation and the formation of moulds and casts.
In permineralisation, mineral-rich groundwater fills the pore spaces of the hard part with minerals such as silica or calcite, hardening and preserving it. In mould-and-cast preservation, the original hard part dissolves to leave a cavity (a mould), which may later be filled by sediment or minerals to form a cast.
Markers reward the rapid decay of soft tissue and the exceptional conditions needed, plus a correct description of two preservation modes of hard parts.
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Sources & how we know this
- WJEC Eduqas A-level Geology specification — WJEC Eduqas (2017)