Identify evidence of extinction and appearance of species...

SECTION 1

Historical Context & Motivation

How Did We Learn That Species Come and Go?

Imagine finding a giant tooth buried in the ground that does not match any animal alive today. For centuries, people found strange bones and shells in rocks but had no idea what they were. Some thought they were the remains of dragons or sea monsters!

Over time, scientists realized these objects were fossils (the preserved remains or traces of organisms that lived long ago). Studying fossils showed that life on Earth has changed dramatically. Some species that once existed are completely gone, while new species appeared over time.

This is our anchoring phenomenon: When scientists dig through layers of rock, they find different fossils at different depths. Some fossils suddenly disappear in certain layers, while brand-new ones appear above them. Why do fossils change as you move through rock layers?

1796

Cuvier Proposes Extinction

Georges Cuvier compared fossil elephant bones to living elephants. He proved that some species, like the mastodon, no longer exist. He was the first scientist to present strong evidence for extinction (when a species completely dies out).

1815

William Smith Maps Rock Layers

William Smith created the first geologic map of England. He noticed that certain fossils always appeared in certain rock layers. This showed that fossils could be used to tell the age of rocks.

1859

Darwin Publishes On the Origin of Species

Charles Darwin explained that species change over time through natural selection. He used the fossil record as key evidence that new species appear and old ones go extinct.

1980

Alvarez Hypothesis for Dinosaur Extinction

Luis and Walter Alvarez proposed that a massive asteroid impact caused the extinction of the dinosaurs about 66 million years ago. They found a layer of rare metal called iridium in rocks from that time period around the world.

These discoveries led to a big question: How can we use the fossil record to identify when species appeared and when they went extinct? That is exactly what you will learn in this lesson.

SECTION 2

Core Principles & Definitions

Key Ideas About Fossils, Extinction, and Appearance

To understand how the fossil record shows extinction and appearance, you need to know a few core ideas. These ideas connect a Disciplinary Core Idea (DCI) about biological evolution with the Crosscutting Concept of Patterns and the Science Practice of analyzing and interpreting data.

The Fossil Record

The fossil record is the collection of all known fossils and their placement in rock layers. It acts like a timeline of life on Earth. Deeper layers are generally older, and shallower layers are younger.

Extinction

Extinction happens when every individual of a species dies and none are left. In the fossil record, evidence of extinction appears when a species' fossils stop showing up in younger rock layers above a certain point.

Appearance of New Species

The first appearance of a species is shown when its fossils first show up in a rock layer but are absent in older layers below. New species arise through evolution over long periods of time.

Superposition

The law of superposition says that in undisturbed rock, the bottom layers formed first and are the oldest. Layers on top formed later and are younger. This is how we know the order of fossil appearances.

Mass Extinction Events

A mass extinction is when a huge number of different species go extinct in a short period of geologic time. After mass extinctions, many new species often appear. Scientists have identified at least five major mass extinctions.

✦ KEY TAKEAWAY

Think of the fossil record like a stack of old photo albums in your attic. Each album (rock layer) holds pictures (fossils) of who was alive at that time. If Aunt Martha appears in the 1990 album but not in any album after 2005, you know she is no longer around. If a new baby shows up in the 2010 album but not in earlier ones, that is a first appearance. Fossils work the same way — their presence or absence in layers tells us who appeared and who went extinct.

SECTION 3

Visual Explanation — Reading Rock Layers

A Cross-Section of Rock Layers and Their Fossils

The diagram below shows a cross-section of rock layers, from the oldest at the bottom to the youngest at the top. Each layer contains fossils of different species. Notice which species appear, which continue through several layers, and which disappear.

This diagram shows five rock layers. Trilobites appear only in Layer A (oldest) — they went extinct. Dinosaurs appear in Layer C but are gone by Layer D — evidence of extinction. Horses first appear in Layer D — evidence of a new species appearing. Ferns span Layers A through D, showing they survived for a long time.

Look carefully at the diagram. The pattern is clear: different species appear at different times, and many eventually disappear. When a fossil is found in a lower layer but not in any layer above it, that is evidence the species went extinct. When a fossil first shows up in a layer but is not in any layer below it, that is evidence of a new species appearing. This pattern — fossils appearing and disappearing — is one of the strongest lines of evidence for evolution.

SECTION 4

How Fossils Form and What They Tell Us

The Process of Fossilization

Not every organism becomes a fossil. Fossilization (the process of becoming a fossil) requires special conditions. An organism usually needs to be buried quickly in sediment like mud, sand, or volcanic ash. Over millions of years, minerals replace the original bone or shell material, turning it into rock.

Because fossilization is rare, the fossil record is incomplete. It is like a book with many pages ripped out. We do not have fossils of every species that ever lived. Still, the fossils we do have show powerful patterns of cause and effect.

What Causes Extinction?

Environmental change: Climate shifts, volcanic eruptions, or asteroid impacts can change habitats so quickly that species cannot adapt fast enough.
Competition: A new species may out-compete an existing one for food, water, or space.
Disease: A widespread disease can wipe out a species if it cannot fight off the infection.
Loss of food sources: If a species' main food goes extinct, that species may follow.

What Leads to New Species Appearing?

New species develop through evolution (the process by which populations change over generations). Small changes build up over very long periods. After a mass extinction, many ecological niches (roles in an ecosystem) become empty. Surviving species can evolve to fill those roles. This is why the fossil record often shows a burst of new species after a mass extinction.

🔗 NGSS Connection

Crosscutting Concept — Cause and Effect: Environmental changes (cause) lead to extinction of some species and appearance of new ones (effect). Scientists use the fossil record to identify these cause-and-effect relationships across Earth's history.

SECTION 5

The Big Five Mass Extinctions

Earth's Five Major Mass Extinction Events

Scientists have identified five major mass extinctions in Earth's history. Each one wiped out a large percentage of all species. After each event, new species appeared to fill the empty roles. The table below summarizes these events.

The Big Five Mass Extinctions in Earth's History (MYA = Million Years Ago)

Extinction Event	When (MYA)	% Species Lost	Possible Cause
End-Ordovician	~445	~85%	Ice age, dropping sea levels
Late Devonian	~375	~75%	Ocean oxygen loss, volcanic activity
End-Permian	~252	~96%	Massive volcanic eruptions, climate change
End-Triassic	~201	~80%	Volcanic activity, climate change
End-Cretaceous	~66	~76%	Asteroid impact, volcanic eruptions

This graph shows how the total number of species on Earth changed over the last 500 million years. The green line shows species diversity rising over time. The red dashed lines mark each mass extinction where diversity dropped sharply. Notice that after every drop, diversity eventually rises even higher as new species evolve to fill empty roles.

The graph above is a simplified version of what scientists see in the real fossil record. Notice the pattern: diversity tends to increase over time, but it crashes during mass extinction events. After each crash, many new species eventually appear. The End-Permian extinction was the worst — it wiped out about 96% of all species. Yet life recovered and eventually became even more diverse.

SECTION 6

Worked Example — Reading a Fossil Record

Analyzing Fossil Data from Rock Layers

Let's practice the Science and Engineering Practice of analyzing and interpreting data. A scientist collects fossils from four rock layers. Here is their data:

Fossil data from four rock layers at a dig site

Rock Layer	Age (MYA)	Fossils Found
Layer 4 (Top)	5	Deer, Oak tree, Rabbit
Layer 3	30	Small horse, Oak tree, Saber-toothed cat
Layer 2	70	Dinosaur, Oak tree, Ammonite
Layer 1 (Bottom)	250	Trilobite, Ammonite, Ancient fern

Identifying Extinction and Appearance

Step 1 — Identify species that disappearLook for fossils that appear in lower (older) layers but are gone in upper (younger) layers. Trilobites appear only in Layer 1. They are not in Layers 2, 3, or 4. This means trilobites went extinct after the time of Layer 1.

Trilobite: EXTINCT after Layer 1 (~250 MYA)

Step 2 — Find more extinctionsAmmonites appear in Layers 1 and 2 but are gone in Layers 3 and 4. This means they went extinct between 70 and 30 MYA. Dinosaurs appear only in Layer 2 and are gone by Layer 3. They also went extinct. Saber-toothed cats appear in Layer 3 but not Layer 4.

Ammonite: EXTINCT between Layers 2 and 3. Dinosaur: EXTINCT after Layer 2. Saber-toothed cat: EXTINCT after Layer 3.

Step 3 — Identify species that appearNow look for fossils that are absent in lower layers but first show up in an upper layer. Dinosaurs first appear in Layer 2 (not in Layer 1). Small horses and saber-toothed cats first appear in Layer 3. Deer and rabbits first appear in Layer 4.

New species appeared at each layer, replacing extinct ones.

Step 4 — Identify species that persistedSome species show up in multiple layers. Oak trees appear in Layers 2, 3, and 4. This means oak trees survived for a long time — at least 70 million years! Ammonites persisted from Layer 1 to Layer 2 before going extinct.

Oak tree: PERSISTED across Layers 2–4 (70 MYA to present)

Step 5 — Construct an explanationPutting it all together: The data shows that life on Earth has changed over time. Species appear, live for a period, and many eventually go extinct. New species replace them. The pattern of appearance and extinction is strong evidence that species evolve and that environmental changes drive both extinction and the rise of new organisms.

The fossil record provides evidence that species change over time through evolution.

SECTION 7

Types of Fossil Evidence

Comparing Different Types of Fossil Evidence

Not all fossils are the same. Scientists use different types of fossils to piece together the story of life. Each type has strengths and limitations. Understanding these helps you evaluate evidence like a real scientist.

Types of fossil evidence and their strengths and limitations

Fossil Type	What It Is	Strengths	Limitations
Body Fossils	Preserved bones, teeth, shells, or whole organisms	Show body structure and size; can compare to living species	Soft tissues rarely preserved; only hard parts usually fossilize
Trace Fossils	Footprints, burrows, nests, or bite marks	Show behavior (how an animal moved, what it ate)	Hard to identify which species made them
Mold & Cast Fossils	Impressions left in rock after the original material dissolves	Show the shape and texture of organisms	No original material remains for chemical analysis
Index Fossils	Fossils of species that lived for a short time but were widespread	Excellent for dating rock layers; narrow time range	Only useful if the species was common and widespread
Preserved Remains	Organisms trapped in amber, ice, or tar	Can preserve soft tissues, DNA, hair, feathers	Extremely rare; only found in special conditions

✦ KEY TAKEAWAY

Think of it this way: body fossils are like finding someone's old shoes in the attic — you know they existed and what size they were. Trace fossils are like finding muddy footprints on the floor — you know someone walked there, but you might not know exactly who. Index fossils are especially useful because they are like a date stamp on a letter — they tell you exactly when that rock layer formed.

SECTION 8

Connecting to Modern Science & Advanced Ideas

From Fossils to DNA — The Bigger Picture

The fossil record is not the only evidence for extinction and the appearance of new species. Modern scientists also use DNA analysis and comparative anatomy (comparing body structures of different organisms) to study how species are related. These tools work together with the fossil record to paint a more complete picture.

Comparing fossil record evidence with modern DNA evidence

Feature	Fossil Record Evidence	Modern DNA Evidence
What it shows	Physical appearance and location of organisms across time	Genetic relationships between species, including those with no fossil evidence
Time range	Billions of years (but incomplete)	Only living species and very recent extinct ones
Evidence for extinction	Fossils disappear from the record	"Missing branches" on family trees of life
Evidence for new species	Fossils appear for the first time in a layer	DNA differences show when species split apart

In high school and college biology, you will learn more about how scientists build phylogenetic trees (branching diagrams that show how species are related through evolution). These trees combine fossil evidence with DNA data. Scientists also study radiometric dating to determine the exact age of fossils using the decay of radioactive elements. For now, the key idea is that the fossil record provides direct, physical evidence of how life has changed on Earth.

🔭 Looking Ahead

Some scientists worry we are currently in a sixth mass extinction caused by human activities like habitat destruction and climate change. Understanding how past extinctions happened helps us predict and possibly prevent future ones. This connects to the NGSS Crosscutting Concept of Stability and Change.

SECTION 9

Practice Problems

Test Your Understanding

PROBLEM 1 — CONCEPTUAL

A scientist finds fossils of Species X in rock layers that are 300, 250, and 200 million years old. No fossils of Species X are found in any layers younger than 200 million years. What does this evidence suggest? A) Species X is still alive but hides well. B) Species X went extinct sometime after 200 million years ago. C) Species X first appeared 200 million years ago. D) Species X moved to another continent.

PROBLEM 2 — BASIC

A student examines three undisturbed rock layers. Layer 1 (bottom) contains trilobites. Layer 2 (middle) contains trilobites and fish. Layer 3 (top) contains only fish. Which statement is best supported by this data? A) Fish evolved from trilobites. B) Trilobites and fish never lived at the same time. C) Trilobites went extinct between the time of Layer 2 and Layer 3. D) Layer 1 is the youngest layer.

PROBLEM 3 — INTERMEDIATE

Scientists studying a cliff face find that many different species' fossils all disappear at the same thin rock layer. Above that layer, completely different species appear. Which explanation best fits this evidence? A) The different species all migrated away at the same time. B) A mass extinction event occurred, followed by evolution of new species. C) The fossils were placed there by humans. D) The rock layers are in the wrong order.

PROBLEM 4 — APPLIED

A paleontologist discovers a new fossil site with four layers. She finds the following: • Layer 4 (top, ~2 MYA): Mammal A, Mammal B, Plant X • Layer 3 (~20 MYA): Mammal A, Reptile C, Plant X • Layer 2 (~65 MYA): Reptile C, Reptile D, Plant X • Layer 1 (bottom, ~100 MYA): Reptile C, Reptile D, Plant Y Which species appeared most recently? A) Reptile C B) Plant X C) Mammal B D) Plant Y

PROBLEM 5 — CRITICAL THINKING

A student argues: "If we don't find a fossil of a species in a certain rock layer, that proves the species did not exist at that time." Evaluate this claim using your knowledge of the fossil record. Is the student correct? Why or why not? A) The student is correct — absence of fossils always means the species did not exist. B) The student is incorrect — the fossil record is complete, so other evidence must be wrong. C) The student is incorrect — the fossil record is incomplete, so absence of a fossil does not prove a species did not exist at that time. D) The student is correct — only hard-shelled organisms can be proven to have existed.

SUMMARY

Lesson Summary

The fossil record provides powerful evidence that species have appeared and gone extinct throughout Earth's history. Using the law of superposition, scientists read rock layers from bottom (oldest) to top (youngest). When a species' fossils first appear in a layer, that is evidence the species evolved around that time. When a species' fossils disappear from the record, that is evidence of extinction. Scientists look for patterns in the data — the Crosscutting Concept of Patterns helps us see that life has constantly changed.

Earth has experienced at least five mass extinction events where a huge percentage of species died out. After each event, new species evolved to fill the empty ecological niches. Different types of fossil evidence — body fossils, trace fossils, index fossils, and preserved remains — each provide different clues. The fossil record is incomplete, but it remains one of the strongest lines of evidence for biological evolution and the cause-and-effect relationship between environmental change and the diversity of life.