Fossils, Mass Extinctions & Earth's Changing Life

REF STUDY GUIDE: Big Ideas Use to answer form questions

1.The fossil record is read by combining tools you already learned this cycle. Superposition (W2) tells you which rock layers are older. Radiometric dating of volcanic ash above and below a fossil (W3) gives a numeric age window. Index fossils — species that lived only during a narrow time window — let you match the SAME slice of time across distant continents.

2.A mass extinction is a short interval of geologic time when a large fraction of Earth's species disappear from the rock record together. Five mass extinctions stand out across the last 542 million years (the Phanerozoic eon). The most famous — the K-Pg boundary 66 million years ago — is marked by a thin iridium-rich clay layer found on every continent and ends the age of the non-bird dinosaurs.

3.Between mass extinctions, species die out at a slow background extinction rate of about one species per million per year. Today's measured rate is 100 to 1000 times higher, driven by habitat loss, climate change, and invasive species. Scientists test for a sixth mass extinction using three patterns from the Big Five: rate spike, broad taxonomic reach, and a planetary driver.

DRAW KEY TERMS Google each → Images tab → sketch what you see

index fossil

Fossil of a species that lived in a narrow time window — used to match rock ages across sites

mass extinction

Short interval when a large fraction of Earth's species vanish from the fossil record together

K-Pg boundary

Iridium-rich clay layer marking the dinosaur extinction 66 million years ago

background extinction rate

Normal slow rate of species loss between mass-extinction events

OTHER VOCAB: fossil record · biodiversity · iridium · Phanerozoic · stratigraphic column

WRITE FOCUS QUESTION Your initial thinking

66 million years ago, every non-bird dinosaur disappears from the fossil record at one thin iridium-rich rock layer — the same layer on every continent. Above the line, mammals diversify and take over. Geologists call this the K-Pg boundary; it is one of FIVE great mass extinctions in the last 542 million years.

What does it mean when an entire group of organisms disappears from the rock record at the same time?

I think...

WRITE AFTER HOOK FORM Reflect before continuing

I learned that...

WRITE Station 1 — Reading Life Through Strata

I observed that the same index fossils appear in rock layers at sites that are:

This shows that index fossils let geologists:

The key concept of relative dating with index fossils is:

CHECK DAY 1 CHECK-IN Before you leave

Q1: Based on today's work with index fossils and the K-Pg boundary, explain one key finding using evidence. Connect it to this week's focus question on disappearing groups in the rock record.

Confidence:

Still building Getting clearer Ready to explain

WRITE Station 2 — The Big Five Mass Extinctions

The diversity-vs-time graph shows that five sharp drops occurred at:

This connects to W3 (radiometric dating) because the AGE of each extinction is established by:

WRITE Station 3 — Predict the Sixth Extinction

The main idea of the modern extinction-rate comparison is...

This connects to past Big-Five extinctions because...

I can apply this pattern by...

DRAW CONCEPT MODEL Annotated geologic timeline of life

Draw a horizontal timeline of the last 542 million years (the Phanerozoic). Mark the FIVE mass extinctions as downward spikes, label the K-Pg with "iridium / 66 Mya / dinosaurs out", and add a small modern-day arrow labeled "sixth extinction?" with the 100–1000× rate. Label ≥3 word-bank terms.

Word Bank: fossil record · index fossil · mass extinction · K-Pg boundary · iridium · Phanerozoic · background extinction rate · biodiversity

≥3 word bank terms labeled

Big Five spikes located on timeline

Modern rate arrow with 100–1000× annotation

CHECK TEST YOUR THINKING Rate each: FALSE ← 1-2-3-4-5 → TRUE · 4 new + 1 last week + 1 spiral

1. Carbon-14 dating can be used directly on a 65-million-year-old dinosaur bone.

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2. Radiometric dating measures the age of the fossil bone itself, not the volcanic ash around it.

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3. "Half-life" means half the age of the rock.

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4. Independent radiometric methods agree on Earth's age of about 4.6 billion years.

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5. [Last week] Matching fossils on continents that are now separated by oceans are best explained by past plate motion connecting those landmasses.

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6. [Spiral · C7W2 Geologic Time] In an undisturbed stack of sedimentary layers, the deeper rocks are always younger than the rocks above them.

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Pick any TWO statements -- especially any you rated 2, 3, or 4. Justify your rating with evidence:

Statement #___ -- I rated it ___ because:

THINK SCIENTIST CIRCLE DEBRIEF Reflect after your Scientist Circle discussion

AHA! Moment

The one I changed my thinking most about is...

Remaining Question

I still wonder...

WRITE FOCUS QUESTION REVISIT Final written response

What does it mean when an entire group of organisms disappears from the rock record at the same time?

Word Bank: fossil record · index fossil · mass extinction · background extinction rate · biodiversity · K-Pg boundary · iridium · Phanerozoic · stratigraphic column