Notes · GeologyUK · A-Levels

Past life and past climates

This chapter treats the fossil record as evidence. It covers how fossils form and why the record is incomplete, how fossils document the evolution and extinction of life, the major fossil groups and their use as zone fossils, the mass extinctions that punctuate the record, and how fossils and sediments are used as proxies to reconstruct past climates and environments.

5 sections~17 min reading time3 competenciesLevel Standard 4 · Advanced 1

T·0777 / 13

Exam profile

AO1 · Describe fossilisation, the evidence for evolution and extinction, and climate proxiesAO2 · Apply the criteria for a zone fossil and use fossils and sediments as climate indicatorsAO3 · Interpret and evaluate fossil and sedimentary evidence for evolution, extinction and past climate

Operators:describeexplaininterpretdeduceanalyseevaluate

basic level

AS-Level expects you to describe how fossils form, the major fossil groups, and how fossils and sediments indicate past climates.

higher level

The full A-Level requires you to evaluate the incompleteness of the record, use fossils as evidence of evolution, and reconstruct climates from combined proxy evidence.

Depth

Text

Contents · 5 sections

Past life and past climates

Fossilisation and the fossil record#

StandardWJEC/Eduqas A level Geology (Component 3) — Past life and past climates

The pathway to becoming a fossil

Fig. 1Fossilisation is a rare chain of events: rapid burial in oxygen-poor sediment, then mineralisation and lithification, and much later uplift, erosion and discovery.

A fossil is any evidence of past life preserved in the rocks, and fossilisation is a rare event requiring a special combination of circumstances. The key requirement is rapid burial in the absence of oxygen, which protects the remains from scavengers, decay and destruction; hard parts such as shells, bones and wood, which resist decay, are far more likely to be preserved than soft tissues. Because these conditions are met only in certain settings — chiefly quiet, sediment-rich water such as a sea floor, lake or delta — the fossil record is strongly biased toward hard-shelled marine organisms and against soft-bodied and land-living creatures.

There are several modes of preservation. In original preservation the actual material survives, as in insects in amber or shells little altered; in permineralisation (petrification) mineral-rich groundwater fills the pore spaces of bone or wood with minerals such as silica; in replacement the original material is dissolved and simultaneously replaced by another mineral; in mould and cast preservation the shell dissolves to leave a mould, which may later fill with sediment to form a cast; and in carbonisation the volatile parts are driven off to leave a thin carbon film, as in many plant fossils. Trace fossils, such as footprints, burrows and borings, record the activity rather than the body of an organism.

The fossil record is therefore incomplete and biased in ways that must always be borne in mind. Soft-bodied organisms are almost absent except in rare exceptional deposits; land organisms are under-represented because land is a site of erosion rather than deposition; and whole environments and time intervals may be missing where no sediment accumulated or where later erosion or metamorphism destroyed the fossils. The record we have is a small, non-random sample of past life, so absence of a fossil is not proof of absence of the organism.

Despite these limitations the fossil record is extraordinarily informative, and the biases are themselves useful once understood. The dominance of marine hard parts means that marine invertebrates give the finest-resolution record and the best zone fossils; the rarity of exceptional soft-bodied faunas makes them especially precious windows onto whole ecosystems; and the incompleteness is a spur to careful, critical interpretation rather than a reason to distrust the record. Weighing what the record can and cannot tell us is a recurring AO3 theme in this chapter.

Worked example

Explaining a mode of preservation

A fossil tree trunk is found turned to stone, with its internal woody structure preserved in silica. Name and explain the mode of preservation.

Identify the mode
The original wood has had its pore spaces filled by minerals from groundwater, which is permineralisation (petrification).
Explain the process
After burial, silica-rich groundwater percolated through the porous wood and precipitated silica in the cell spaces, hardening the wood and preserving its fine structure.
Comment
Because the structure is preserved in detail, permineralisation gives some of the most informative plant fossils.

Result: The trunk was preserved by permineralisation: silica from groundwater filled the wood's pore spaces, preserving its structure.

Typical mistakes

Treating absence of a fossil as proof the organism did not exist; the record is an incomplete, biased sample.
Confusing a mould (the impression left by a dissolved shell) with a cast (sediment that later fills the mould).

Active revision

Explain why a soft-bodied worm living on land is far less likely to be fossilised than a marine shellfish, referring to burial, decay and depositional setting.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — fossils (British Geological Survey)

Fossils as evidence of evolution#

StandardWJEC/Eduqas A level Geology (Component 3) — Past life and past climates

The succession of major groups through time

Fig. 2The ordered first appearance of major groups through the Phanerozoic is strong evidence for evolution; a group is never found below its first appearance.

The fossil record is the direct historical evidence for evolution, showing that life has changed profoundly through time. Because younger rocks lie above older ones, a vertical sequence of strata is also a sequence in time, and the fossils within it document a clear progression: the oldest rocks contain only simple, single-celled organisms; then, from the base of the Cambrian, a great diversity of marine invertebrates appears; later come fish, then land plants and amphibians, then reptiles and the dinosaurs, and finally mammals and, very recently, humans. This ordered succession of ever-changing life is exactly what evolution predicts and is one of its strongest lines of evidence.

Within lineages the record can show gradual morphological change from one form to another, and occasionally preserves transitional forms that bridge major groups — organisms combining features of an ancestral and a descendant group. Such fossils, together with vestigial structures and the fixed order of first appearances (a mammal is never found below the first fish), demonstrate descent with modification. The principle of faunal succession, that fossil species follow one another in a definite and irreversible order, is the practical expression of evolution used for dating and correlation.

Evolution in the record is not always smooth. Long periods of little change (stasis) are punctuated by episodes of rapid change, often following mass extinctions, when the survivors diversify to fill emptied ecological roles (adaptive radiation) — the rapid rise of the mammals after the dinosaurs disappeared is the classic example. The apparent gaps and sudden appearances in the record partly reflect its genuine incompleteness and partly this episodic tempo, and distinguishing the two is part of interpreting the record critically.

The evidence must be read with the record's biases in mind. Because preservation favours hard-shelled marine organisms, evolutionary trends are best documented in groups such as ammonites and trilobites, whose rapidly changing shells provide detailed lineages; the evolution of soft-bodied or terrestrial groups is far patchier. Recognising both the power of the fossil record as evidence for evolution and the limitations imposed by its incompleteness is the balanced understanding the specification expects, and it links directly to the zone fossils and mass extinctions that follow.

Worked example

Interpreting a fossil succession

In a thick, undisturbed marine sequence, ammonite fossils change steadily upward in the complexity of their shell sutures. Explain what this shows and why ammonites record it well.

Use superposition
Because the sequence is undisturbed, upward means younger, so the changing sutures record change through time.
Interpret as evolution
The steady change in a heritable feature (suture complexity) up the sequence is evolutionary change within the ammonite lineage.
Explain the good record
Ammonites had hard shells, were abundant and widespread, and evolved rapidly, so their lineages are preserved in fine detail.

Result: The upward change in suture complexity records ammonite evolution through time, well preserved because ammonites were abundant, widespread and rapidly evolving.

Typical mistakes

Reading gaps in a lineage as disproving evolution; gaps largely reflect the incompleteness of the record.
Assuming evolution is always slow and gradual; the record shows both stasis and rapid change, especially after extinctions.

Active revision

Explain how the fact that mammal fossils are never found in rocks older than the first fish supports the theory of evolution.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — fossils and evolution (British Geological Survey)

Major fossil groups and their use#

StandardWJEC/Eduqas A level Geology (Component 3) — Past life and past climates

The major invertebrate fossil groups

Fig. 3The major invertebrate fossil groups with their form, time of importance and use: free-swimming groups make the best zone fossils, sea-floor groups the best environmental indicators.

A small number of invertebrate groups dominate the fossil record and the examination, and each has a characteristic form, habit and time of importance. Trilobites are extinct marine arthropods with a three-lobed, segmented exoskeleton, abundant in the Palaeozoic and important zone fossils of the Cambrian. Graptolites are colonial, mostly floating organisms whose fossils look like fine saw-blades or tuning forks, abundant and rapidly evolving in the Ordovician and Silurian, and so among the best zone fossils of the Lower Palaeozoic.

Ammonites are extinct coiled cephalopods (relatives of the modern nautilus and octopus) with chambered shells and intricate suture lines; they were free-swimming, abundant, widespread and evolved rapidly, making them the premier zone fossils of the Mesozoic. Brachiopods, superficially clam-like but with two unequal valves each symmetrical about the midline, are common sea-floor dwellers throughout the Palaeozoic. Corals build calcareous skeletons in warm, clear, shallow seas and, besides their stratigraphic use, are valuable indicators of past environment and climate.

The value of a fossil group for correlation depends on how well it meets the criteria for a zone fossil — short-lived species, widespread, abundant, easily preserved and distinctive. Free-swimming and floating groups (graptolites, ammonites) meet these criteria best because they spread quickly across whole oceans and evolved fast, giving fine time resolution; sea-floor dwellers restricted to a particular environment (corals, some brachiopods) are better environmental indicators than precise time markers. Matching a group to the task — dating or environmental reconstruction — is a key skill.

The groups also carry evolutionary and ecological information. Evolutionary trends within a group, such as the increasing complexity of ammonite sutures or changes in trilobite eyes and segmentation, provide the fine detail used to define successive zones. The habit of a group constrains the environment: reef corals mean warm, clear, shallow water; graptolites mean open ocean. So a single fossil can, in the right hands, give an age, an environment and a place in the story of evolution, which is why fluency with these groups underpins so much of practical geology.

Worked example

Choosing a fossil for a purpose

A geologist has two tasks: to determine the precise age of a Lower Palaeozoic marine mudstone, and to decide whether an ancient limestone formed in a warm shallow sea. Recommend a fossil group for each and justify the choice.

For precise age
Graptolites are ideal for dating the Lower Palaeozoic mudstone: they were widespread floaters that evolved rapidly, so each species marks a short, recognisable time interval.
For environment
Corals are ideal for the limestone: they live only in warm, clear, shallow seas, so their presence indicates that environment.
State the principle
Free-swimming, fast-evolving groups make the best time markers; environmentally restricted groups make the best environmental indicators.

Result: Use graptolites to date the mudstone (widespread, fast-evolving zone fossils) and corals to show the limestone formed in a warm shallow sea.

Typical mistakes

Confusing brachiopods (two unequal valves, each symmetrical about the midline) with bivalves (two equal valves, mirror images of each other).
Treating corals as good zone fossils; they are restricted to warm shallow seas and are better environmental than time indicators.

Active revision

Explain why graptolites are used to zone the Ordovician and Silurian, while corals are used mainly to interpret environment.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — fossil groups (British Geological Survey)

Mass extinctions#

StandardWJEC/Eduqas A level Geology (Component 3) — Past life and past climates

The 'big five' mass extinctions

Fig. 4The five major mass extinctions of the Phanerozoic; the end-Permian (252 Ma) was the most severe and the end-Cretaceous (66 Ma) ended the dinosaurs.

Against the background of the continuous, low-level extinction of species, the fossil record shows several mass extinctions — short intervals in which a large fraction of all species died out worldwide. Five are conventionally recognised in the Phanerozoic (the 'big five'), at the end of the Ordovician, in the Late Devonian, at the end of the Permian, at the end of the Triassic, and at the end of the Cretaceous. Each marks a sudden drop in diversity in the record and, because they are global and abrupt, they make natural boundaries in the time scale — two of them define the boundaries between the great eras.

The end-Permian extinction, about 252 million years ago, was the most severe of all, eliminating an estimated majority of marine species and marking the end of the Palaeozoic; it is often called 'the Great Dying'. The end-Cretaceous extinction, about 66 million years ago, ended the Mesozoic and famously wiped out the non-bird dinosaurs along with the ammonites and much else. These two events are the most examined, both for their severity and for the way they cleared the way for new groups — the dinosaurs after the end-Permian, the mammals after the end-Cretaceous.

The causes are read from the geological evidence and are usually multiple. A large meteorite impact is strongly implicated in the end-Cretaceous event by a worldwide iridium-rich layer (iridium being rare in the crust but common in meteorites) and an impact crater of the right age; massive volcanism, forming vast flood basalts, coincides with the end-Permian and other events, releasing gases that changed climate and ocean chemistry; and changes in sea level, climate and ocean oxygenation contributed. Often several stresses acted together, which is why untangling the cause of an extinction is a genuine scientific problem.

Mass extinctions are pivotal because they reset the course of evolution. By removing dominant groups they open ecological space into which the survivors radiate rapidly, so the recovery from an extinction is often a burst of evolutionary innovation. They also demonstrate the value of the fossil record for understanding present-day biodiversity loss, and they are a favourite context for AO3 questions that ask students to weigh competing lines of evidence for a cause and to evaluate how securely a cause can be inferred from the rocks.

Worked example

Evaluating evidence for an extinction cause

A worldwide clay layer at the Cretaceous-Paleogene boundary is enriched in iridium and contains shocked quartz. Explain what these suggest and how confidently a single cause can be claimed.

Interpret the iridium
Iridium is rare in the crust but abundant in meteorites, so a global iridium spike points to a large meteorite impact spreading debris worldwide.
Interpret the shocked quartz
Shocked quartz forms under the extreme pressures of an impact, supporting the impact interpretation.
Add balance
However, massive volcanism was also occurring near the boundary and would have stressed the climate, so the extinction may have resulted from combined stresses rather than the impact alone.

Result: The iridium and shocked quartz strongly support a major impact, but coincident volcanism means a single cause cannot be claimed with certainty.

Typical mistakes

Assuming the end-Cretaceous impact is the only cause considered; large-scale volcanism and environmental change are implicated in several extinctions.
Thinking extinctions only destroy; they also trigger adaptive radiations of the survivors.

Active revision

Outline the evidence that a meteorite impact contributed to the end-Cretaceous mass extinction, and explain why other causes are also considered.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — mass extinctions (British Geological Survey)

Reconstructing past climates#

AdvancedWJEC/Eduqas A level Geology (Component 3) — Past life and past climates

Climate proxies and what they indicate

Fig. 5Sedimentary and fossil proxies each indicate a climate: coal (humid), evaporites (arid), tillites (glacial), corals (warm shallow sea), oxygen isotopes (ice volume).

Because we cannot measure ancient temperatures directly, past climates are reconstructed from proxies — features of rocks and fossils that formed under, and so record, particular climatic conditions. The reasoning is uniformitarian: an organism or sediment that today is confined to a certain climate indicates that climate wherever it is found in the past. A single proxy can be ambiguous, so, exactly as with environmental interpretation, the securest reconstructions combine several independent proxies pointing to the same conclusion.

Several sedimentary rocks are strong climate indicators. Coal forms from the lush plant growth of warm, humid, tropical to temperate swamps, so a coal seam indicates a wet, well-vegetated climate. Evaporites such as rock salt and gypsum form where evaporation exceeds water supply, so they indicate a hot, arid climate. Tillites — lithified glacial tills — and striated pavements indicate cold, glacial conditions; finding tillites in rocks now near the Equator was historically powerful evidence for continental drift as well as for past ice ages. Red beds, oxidised in warm, well-drained settings, and desert dune sandstones likewise indicate warm, dry climates.

Fossils are equally informative. Reef corals indicate warm, clear, shallow tropical seas; particular plants indicate particular climate belts, and fossil pollen assemblages track vegetation and hence climate change, especially through the Quaternary. The most quantitative fossil proxy is the ratio of oxygen isotopes (oxygen-18 to oxygen-16) in the calcite shells of marine microfossils such as foraminifera: because the lighter isotope is preferentially locked into ice during glacials, the isotope ratio in shells records global ice volume and ocean temperature, giving a detailed record of the ice ages.

Reconstructing a climate history therefore means reading a vertical sequence as a succession of climates: a change from coral limestone up into evaporites and then coal records a shift from a warm shallow sea, through arid conditions, to a humid swamp, whether because the climate changed or because the region drifted across climate belts on its moving plate. Distinguishing genuine climate change from the effects of plate movement, and weighing several proxies together, is the sophisticated interpretation this section develops, tying the fossil record to the plate tectonics and Quaternary chapters.

An oxygen-isotope climate record

Fig. 6An oxygen-isotope proxy record (illustrative): higher values indicate more global ice (colder), tracing an alternation of glacials and interglacials.

Worked example

Reading a climate history

A vertical sequence shows, from bottom to top: reef limestone with corals, then rock salt and gypsum, then a coal seam. Reconstruct the climate history and evaluate the interpretation.

Read the base
Reef limestone with corals indicates a warm, clear, shallow tropical sea.
Read the middle
Rock salt and gypsum are evaporites, indicating a hot, arid climate where evaporation exceeded water supply (a drying or restricted basin).
Read the top
A coal seam indicates a warm, humid, well-vegetated swamp.
Evaluate
The succession records warm sea, then arid, then humid conditions; this could be genuine climate change or the region drifting across climate belts on its plate, so the two should be weighed.

Result: The sequence records a warm shallow sea, then arid, then humid climates; the change could be true climate change or plate drift across climate belts.

Typical mistakes

Relying on a single proxy; a secure reconstruction combines several independent indicators.
Forgetting that a change of climate up a sequence may reflect the plate drifting across climate belts rather than a global climate change.

Active revision

A sequence passes upward from coral limestone, through gypsum, into a coal seam. Reconstruct the succession of climates and give one alternative explanation for the change.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — past climates (British Geological Survey)

References & sources

Sources

WJEC / Eduqas

WJEC/Eduqas A level Geology specification

British Geological Survey

Past life and past climates

5 sections~17 min reading time3 competenciesLevel Standard 4 · Advanced 1

Past life and past climates

Key points

Explaining a mode of preservation

Key points

Interpreting a fossil succession

Key points

Choosing a fossil for a purpose

Key points

Evaluating evidence for an extinction cause

Key points

Reading a climate history

Sources

Past life and past climates

Key points

Explaining a mode of preservation

Key points

Interpreting a fossil succession

Key points

Choosing a fossil for a purpose

Key points

Evaluating evidence for an extinction cause

Key points

Reading a climate history

Sources