Extinction and Fossils
Most species that have ever lived are now gone. Their disappearance, called extinction, and the fossils they left behind are two sides of the same story: how life on Earth has changed and sometimes vanished. This topic connects the history of life to the physical clues buried in rock.
This lesson explains extinction and how fossils record it.
Extinction is the permanent loss of a species when the last of its members die. Fossils are preserved remains or traces of past organisms, found mostly in sedimentary rock. The fossil record shows that extinction is a normal part of life’s history and reveals mass extinctions when many species disappeared in a short time.
What causes extinction?
Extinction happens when a species cannot survive the conditions it faces. Common causes include a changing climate, loss of habitat, new competitors or predators, disease, and sudden catastrophes like a major volcanic eruption or an asteroid impact. When the environment changes faster than a species can adapt, that species can die out completely.
How do fossils form?
Fossils usually form when an organism is buried quickly in sediment before it can decay. Over long periods, minerals replace the hard parts, turning them to rock, while the surrounding sediment hardens around them. Because sediment builds up in layers, fossils in lower layers are generally older than those above. This layering lets scientists read the fossil record like a history book of life.
| Idea | Meaning |
|---|---|
| Extinction | A species dies out permanently |
| Fossil | Preserved remains or trace of an organism |
| Mass extinction | Many species lost in a short time |
What are mass extinctions?
A mass extinction is an event in which a large fraction of Earth’s species die out in a relatively short time. The most famous ended the age of the dinosaurs about 66 million years ago, likely triggered by an asteroid impact and its aftermath. Mass extinctions reshape life on Earth, clearing the way for surviving groups to expand, as mammals did after the dinosaurs. The fossil record is how scientists detect these events at all.
Watch: A Short Video Lesson
Paleontological Research Institution walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:
A routine for extinction and fossil questions
- Define extinction: the permanent loss of a species.
- List possible causes: climate change, habitat loss, competition, catastrophe.
- Recall that fossils form mostly in sedimentary rock, in layers.
- Deeper layers are older, so fossil age follows position.
- A mass extinction is many species lost in a short time.
Practice questions
- What does extinction mean?
- Name two causes of extinction.
- In what type of rock are most fossils found?
- Why are fossils in lower rock layers usually older?
- What likely caused the extinction of the dinosaurs?
- True or false: mass extinctions can open opportunities for surviving groups.
Answers:
- The permanent loss of a species when its last members die.
- Any two of: climate change, habitat loss, new competitors or predators, disease, catastrophe.
- Sedimentary rock.
- Because sediment builds up in layers over time, so lower layers formed earlier.
- An asteroid impact and its effects, about 66 million years ago.
- True. Survivors, like mammals after the dinosaurs, can expand.
Where this fits
Extinction and fossils provide part of the evidence for evolution and connect to how species change through natural selection. Fossils sit in rock layers you study in earth science, such as rocks and the rock cycle. Find every topic on the ASVAB General Science Learning Hub.
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