Page 25 - York Global Science 4 - Earth & Space

Earth and Space Science

Let’s Learn
Let’s Learn

A fossil is from a living thing that died long ago. The soft parts of dead
plants and animals break down. Hard parts like bones and shells last

longer. They can be buried in sand or mud. After a long time, the sand or
mud changes into sedimentary rock. The bones or shell stay in the rock
and form a fossil.

Fossils can tell us a lot about Earth’s history. They show how living things
changed with time. They show what the Earth was like long ago. But we
need to know how old the fossils are. Scientists have different ways to find

out how old a fossil is.
Sedimentary rocks are made in layers. The oldest layers form at the bottom.
Newer layers are made on top. So, scientists know that usually older fossils

will be further down than younger ones. However, scientists need to be
careful. Something like an earthquake can mix the layers. Scientists need to
look at the rocks carefully to see what they can show about this.

Scientists also look at other fossils in the same rock layer. We know some
living things were on Earth only at some times. For example, the fossils in
this picture are ammonites. Scientists find lots of these fossils.

The ammonites all died 65 million
years ago. So if you find a fossil in the
same layer as an ammonite, you

know this fossil is at least 65 million
years old. Fossils that help us find out
the age of rocks and other fossils are
called index fossils.

Scientists also use radiometric
dating. This can tell us the age of a
rock and any fossil inside it. We know

that any fossil in the rock is the same age as that rock.

25