Fossils, Mass Extinctions & Earth's Changing Life - Study Guide

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...rocks tell Earth's story by combining FOUR tools: (1) the rock cycle (how rocks form and reform), (2) superposition (deeper = older), (3) index fossils (snapshots of life), and (4) radiometric dating (numeric ages from isotope decay). Together they turn a cliff face into a calibrated history book.

WRITE AFTER HOOK FORM Reflect before continuing

I learned that... every concept in Cycle 7 builds on the rest. Igneous, sedimentary, and metamorphic rocks each preserve different evidence; fossils give relative ages; radiometric dating adds absolute years; plate tectonics explains why matching rocks appear on continents that are now thousands of miles apart.

WRITE Station 1 — Reading Life Through Strata

I observed that the same index fossils appear in rock layers at sites that are: the rock cycle (W1) explains where each rock came from — igneous from cooled magma, sedimentary from compacted sediments, metamorphic from heat + pressure. The cycle has no 'start' — any rock can become any other rock over time.

This shows that index fossils let geologists: rocks are records of past environments. Sedimentary rocks contain fossils (life evidence); igneous rocks can be radiometrically dated (timing evidence); metamorphic rocks record tectonic events (force evidence). Each type contributes a different chapter.

The key concept of relative dating with index fossils is: Earth's history is preserved in the rock record. Reading it requires multiple lines of evidence — no single technique tells the whole story.

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.

Rocks tell Earth's complete story when we combine four tools: the rock cycle (W1) tells us how each rock formed; superposition + index fossils (W2) give relative order and identify time periods; radiometric dating (W3) adds numeric ages from isotope decay; and plate tectonics (W4) explains why rocks and fossils show up on continents that have moved over hundreds of millions of years. Each tool covers what the others miss — together they let scientists reconstruct environments, climates, and life forms back to ~4 billion years ago.

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: from W2 we used superposition (deeper = older) and index fossils to place rocks in RELATIVE order. From W3 we used radiometric dating (parent-to-daughter ratio + half-life) to put NUMERIC ages on volcanic layers. Combined, we can give a fossil a precise age window even when we can't date the fossil itself.

This connects to W3 (radiometric dating) because the AGE of each extinction is established by: the 4.6-billion-year scale of Earth's history. Meteorites date the solar system at ~4.56 billion years; Earth's oldest zircon crystals are ~4.4 billion years old; multicellular life only appears in the last ~600 million years; humans only in the last ~300,000 years (the last 0.007% of Earth's history).

WRITE Station 3 — Predict the Sixth Extinction

The main idea of the modern extinction-rate comparison is...plate tectonics ties the whole cycle together. Continents drift centimeters per year, but over hundreds of millions of years that adds up to ocean-sized movement. That's why identical fossils show up on now-separated continents (South America + Africa = once-joined Pangaea).

This connects to past Big-Five extinctions because...plate motion explains mountains (continental collisions), earthquakes (sliding plates), volcanoes (subduction), and the deep-time mismatch — why a Missouri trilobite has identical relatives in Morocco. The continents weren't always where they are now.

I can apply this pattern by...reading any geologic map by looking for (1) rock type, (2) fossils, (3) layer order, (4) radiometric dates, and (5) plate-boundary evidence. From those five clues, I can reconstruct what an area was like 100 million or 1 billion years ago.

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

Central timeline labeled '4.6 billion years' running left to right. Stacked rock layers on the left, modern Earth on the right. Above the timeline: tools used to read it — 'Rock Cycle (W1)' over the leftmost layers, 'Superposition + Index Fossils (W2)' over middle layers, 'Radiometric Dating (W3)' with parent/daughter atom inset, 'Plate Tectonics (W4)' showing two continents drifting apart with matching fossils. Bottom: tiny human stick figure in the last 0.007% of the timeline labeled 'humans appear'.

≥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... no single rock or fossil tells the whole story — you need all four tools (rock cycle, superposition, radiometric dating, plate tectonics) overlapping to read Earth's history confidently.

Remaining Question

I still wonder... If plate tectonics has been moving continents for 4 billion years, how do scientists know what Earth's continents looked like 3 billion years ago — when almost no rocks that old survive?

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

CLAIM: Rocks tell Earth's complete history when we combine the rock cycle, superposition, index fossils, radiometric dating, and plate tectonics — each tool fills in what the others can't.

EVIDENCE: Earth is ~4.6 billion years old (radiometric dating of zircons and meteorites). Trilobites date Paleozoic layers, dinosaurs date Mesozoic, mammals date Cenozoic. Volcanic ash above and below a fossil brackets its age in years. Matching fossils on now-separated continents (S. America + Africa) prove plate motion.

REASONING: Each technique answers a different question. The rock cycle says WHAT a rock is and HOW it formed; superposition says WHICH is older; index fossils say WHEN (relatively); radiometric dating says HOW MANY YEARS; plate tectonics says WHERE it formed. Together, they turn the rock record into a calibrated, location-aware timeline of Earth's 4.6-billion-year history.