Extinction and the fossil record
Extinction can be caused by new predators, disease, competition or catastrophic events. The impact 66 million years ago is the best-documented example of a catastrophe, and its evidence is preserved in rocks worldwide.
- List factors that can cause extinction, including catastrophic events.
- Describe the evidence that links the end-Cretaceous extinction to an impact.
- Explain why the fossil record is incomplete.
Links to: GCSE biology, evolution: extinction and its causes, and evidence from the fossil record; working scientifically: evaluating evidence and hypotheses.
Why species go extinct
A species becomes extinct when no individuals remain. Causes include new predators, new diseases, new competitors, changes to the environment over geological time, and single catastrophic events such as massive volcanic eruptions or collisions with asteroids. A species is at greater risk when its environment changes faster than it can adapt by natural selection.
The end-Cretaceous extinction
About 66 million years ago, roughly three quarters of all species disappeared, including all dinosaurs except birds. Rocks worldwide record this at the K–Pg boundary. The boundary clay is enriched in iridium, a metal rare in Earth's crust, and contains shocked quartz and spherules of melted rock, all signs of a large impact.
The Chicxulub crater, about 180 km across and buried beneath Mexico's Yucatán Peninsula, has been dated to the same time. The impactor was roughly 10 km across. Its debris and gases blocked sunlight, cooled the climate and caused food chains based on photosynthesis to collapse.
Reading an incomplete record
The fossil record is incomplete. Most organisms decay before they can fossilise, many fossils have been destroyed by geological processes, and many remain undiscovered. Soft-bodied organisms rarely fossilise at all.
Scientists therefore look at how groups with good fossil records change across the boundary. Microscopic marine plankton, which fossilise in huge numbers, show a sudden collapse exactly at the iridium layer, strong evidence that the extinction was rapid rather than gradual.
Rate of loss
In a sequence of rock layers, 160 species of fossil plankton are found just below the iridium layer, and 20 just above it. What percentage survived, and what percentage became extinct?
- Percentage surviving = (20 ÷ 160) × 100 = 12.5%.
- Percentage extinct = 100 − 12.5 = 87.5%.
Answer: 12.5% survived and 87.5% disappeared. Real plankton records show losses of this size, higher than the roughly 75% across all species.
Write an exam-style answer (6 marks): 'Evaluate the hypothesis that an asteroid impact caused the extinction of the dinosaurs.' Include evidence for the hypothesis, an alternative cause (huge volcanic eruptions in India, the Deccan Traps, at about the same time), and a reasoned conclusion.
Quiz
5 questions. Choose an answer to see whether you are right and why. Finish the quiz to complete the lesson.
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Question 1Which of these is a catastrophic event that can cause extinction?
Asteroid impacts and massive volcanic eruptions are single catastrophic events that can change environments very quickly.
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Question 2Why is iridium in the K–Pg boundary clay important evidence?
A worldwide iridium-rich layer points to material from a large impactor.
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Question 3Why is the fossil record incomplete?
Fossilisation needs rare conditions, and geological processes destroy many fossils.
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Question 4Why are plankton fossils useful for studying the extinction?
Huge numbers of microfossils give a detailed record showing a sudden collapse at the iridium layer.
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Question 5In a rock layer, 40 of 50 species disappear. What percentage became extinct?
(40 ÷ 50) × 100 = 80%.