Fossil Hunters
Dig into deep time: the three ways fossils form, why the record has gaps we can never fully fill, and what wipes a whole species off the Earth.
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Fossil Hunters 🦴
Fossils are messages from millions of years ago — the closest thing we have to a photo album of life on Earth. This module digs into how those messages survive, why so many are missing, and what makes an entire species vanish. (All of 4.6.3 is **Biology-only**.)
Evidence in the rocks 🔍
A **fossil** is the remains or traces of an organism from millions of years ago, preserved in rock. Fossils are a major piece of **evidence for evolution**: by comparing fossils of different ages, scientists can see **how organisms have gradually changed** over enormous spans of time.
Three ways to make a fossil 🪨
Fossils form in a few different ways: 1. **No decay** — from parts of an organism that have **not decayed**, because one or more conditions needed for decay (warmth, moisture, oxygen) are **absent** — e.g. an insect in amber, a mammoth in ice. 2. **Replacement** — hard parts are slowly **replaced by minerals** as they decay, forming a rock-like copy (e.g. petrified wood or bone). 3. **Traces** — preserved **casts, impressions and traces**: footprints, burrows and rootlet traces.
Match how it formed
- A mosquito trapped in amber
- A tree trunk turned to stone
- A dinosaur footprint in rock
- Preserved because it did not decay
- Hard parts replaced by minerals
- A trace or impression left behind
Why amber preserves
An insect sealed in tree resin (amber) does not rot away. Why not?
- The resin seals out the conditions decay microbes need, such as oxygen and moisture
- Its body is quickly replaced by minerals
- Amber is always frozen solid
- It only leaves an impression, not the body
The missing pages 📉
The **fossil record has big gaps**, especially for the earliest life. Two reasons: • Many early organisms were **soft-bodied**, so they decayed before they could fossilise, leaving few traces. • Many fossils that did form have since been **destroyed** by geological activity (heat, pressure, movement of rock). Because of this, scientists **cannot be certain** exactly how life on Earth first began.
Why the gaps?
Why do we have so few fossils of the very earliest life forms?
- They were mostly soft-bodied and decayed before fossilising
- There was no life at all back then
- Early organisms were too big to fossilise
- Scientists simply have not looked hard enough
When a species ends 💀
**Extinction** is when there are **no individuals of a species left alive** anywhere. It can be caused by: • a **new disease** • **new predators**, or **new, more successful competitors** • a **change to the environment** over geological time (e.g. climate change) • a single **catastrophic event** — an asteroid strike or huge volcanic eruptions.
Spot the cause
A species of bird thrives on one island until humans introduce cats, which hunt the birds and their chicks. What kind of extinction cause is this?
- A new predator
- A new disease
- Environmental (climate) change
- A catastrophic event
Trace an extinction
An interactive activity.
In the exam 🎓
Dig complete. Grade-9 habits for fossils and extinction: • Know the **three** fossil routes: **no decay** (conditions absent), **mineral replacement**, and **casts/impressions/traces**. • Explain the incomplete record by **soft-bodied** early life + fossils **destroyed** over time — so we can't be sure how life began. • Extinction = **no individuals left**; causes include new disease, new predators/competitors, environmental change, and catastrophic events.