Fossil evidence
What fossils are and how they form
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A fossil is any preserved trace of a once-living organism.
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Fossilisation requires a specific and uncommon set of conditions:
- Rapid burial in sediment, before scavengers or decomposers destroy the remains.
- Absence of oxygen, which slows decay.
- Hard parts — bone, shell, teeth, wood — which resist decay far better than soft tissue.
- Minerals replacing the original material over time, and no later destruction by heat, pressure or erosion.
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Because all of these must coincide, fossilisation is rare, and this shapes everything on this page.
What fossils show
- Change over time. Fossils in deeper rock layers are older than those above — the principle of superposition. Comparing layers shows how organisms changed through time.
- Transitional forms. Some fossils show combinations of features found in two different groups, indicating a lineage in the process of changing.
- Extinction and replacement. Many fossil species have no living representatives. The record shows repeated extinctions and subsequent radiations.
- Absolute age. Radiometric dating of volcanic layers gives numerical ages, so change can be placed on a real timescale rather than only in sequence.
Why the fossil record is incomplete
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This is examinable, and it is the point most answers miss. The record is biased, not simply patchy:
- Hard-bodied organisms are massively over-represented. Soft-bodied organisms — worms, jellyfish, most invertebrates — rarely fossilise at all.
- Aquatic and coastal environments are over-represented, because sediment accumulates there. Upland and forest environments erode instead.
- Abundant, widespread species are far more likely to be preserved than rare, localised ones.
- Later erosion, heat and pressure destroy fossils that did form.
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The last two biases matter directly for speciation, and connect to punctuated equilibrium: the small, isolated populations in which speciation often begins are exactly the populations least likely to be fossilised.
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So gaps in the record are expected, and an absence of fossils is weak evidence of absence.
Fossil evidence in New Zealand
- Moa are known from abundant skeletal remains, and from preserved soft tissue and feathers in dry caves and rock shelters. Coprolites — fossil droppings — show what they ate, which is direct evidence about their ecological role and about co-evolution with divaricating plants.
- The Saint Bathans fauna in Central Otago preserves an early Miocene assemblage roughly 16–19 million years old, including a terrestrial mammal — evidence that New Zealand was not always mammal-free, which revised a long-standing assumption.
- Tuatara are the only survivors of the Rhynchocephalia, a group known mainly from fossils and once far more diverse worldwide. The tuatara is not a lizard, and the fossil record is what establishes that.
Selective advantage
- Fossils are evidence, not responses, so they have no selective advantage. But they let selection be inferred:
- A trend in fossil form over successive layers — increasing body size, changing tooth shape — combined with independent evidence of environmental change, allows a directional selection hypothesis to be tested against a real timescale.
- Long stasis in the record indicates stabilising selection rather than an absence of selection.
Worked Example
Worked Example
A continuous rock sequence contains fossils of one mammal lineage over 20 million years.
- The oldest layers contain small animals with low-crowned teeth suited to soft leaves.
- Younger layers show progressively larger body size and higher-crowned teeth with more enamel ridges.
- Independent evidence from pollen in the same layers shows a shift from forest to open grassland across the same period.
- Grasses contain silica, which is highly abrasive to teeth.
- No fossils of this lineage are found in three separate 200,000-year intervals within the sequence.
Explain what this evidence shows, and evaluate how strong the conclusion is.
Answer:
What the evidence shows.
The fossils show a directional change in two traits — increasing body size and increasing tooth crown height — across 20 million years, in a continuous sequence, so the change is a real trend rather than an artefact of comparing unrelated layers.
The pollen evidence is what makes this interpretable. It shows the environment changed from forest to open grassland over the same period, and grasses are abrasive because they contain silica.
This supports a directional selection hypothesis:
- Individuals varied heritably in tooth crown height.
- As grass replaced leaves in the diet, animals with low-crowned teeth wore them down before the end of their lifespan. Once teeth are worn out the animal cannot feed, so it dies or fails to reproduce further.
- Animals with higher-crowned teeth retained functional grinding surfaces for longer, so they continued feeding, survived longer and left more offspring.
- The alleles for higher crowns therefore increased in frequency, and mean crown height rose across generations.
Larger body size is consistent with the same shift: grass is lower in nutrients than leaves, so a larger gut and longer retention time extract more energy from it, and open habitat also favours larger size for predator defence and for travelling between patches.
Evaluating the strength of the conclusion.
The conclusion is well supported, and the reason is worth stating precisely: it rests on two independent lines of evidence that converge. The fossils alone would show a trend but not explain it, and the pollen alone would show environmental change but say nothing about the animals. Together they supply both the change and a plausible mechanism linking them, and the mechanism makes a prediction — teeth should get taller as grass increases — that the data match.
But three limitations must be acknowledged:
- Correlation is not causation. The tooth change coincides with the vegetation change, but something else changing at the same time — a new predator, a climate shift affecting body size directly — could contribute. The silica mechanism makes the causal link plausible, but the data alone cannot exclude other factors.
- The gaps. Three intervals of 200,000 years contain no fossils of this lineage. These may mean the lineage was locally absent, or simply that conditions for fossilisation were unsuitable then, or that the population was too small or localised to be sampled. Absence of fossils is weak evidence of absence, so nothing should be concluded about those intervals.
- We cannot be certain this is one lineage. The fossils may represent a succession of related species rather than continuous descent within one, and later forms could be side branches rather than direct descendants. Fossils establish sequence and timing well but relationship only weakly.
How the conclusion could be strengthened. Additional independent evidence would help — for example microscopic wear patterns on the teeth showing directly whether the animals were eating grass, or molecular evidence from any preserved material to confirm the fossils form a single lineage.
Why this matters for the Nature of Science. The strength here comes not from any single line but from independent lines agreeing, and from the willingness to state what the evidence cannot show. That is the standard scientific claims are held to in peer review — not whether a story fits, but whether alternatives have been excluded.