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FIND OUT MORE. Dinosaurs 78–79 • Evolution 74–75

76 Nature

HOW ARE FOSSILS FORMED?

When an animal dies it is rapidly covered by sediment,

such as mud or sand. Its soft parts decompose (rot),

but its hard parts are gradually replaced by minerals,

or fossilized. Rather than a buried bone or an insect

preserved in amber, for example, a fossil can also

dissolve and then appear as a mould on a rock

millions of years after the organism has died.

WHAT DO FOSSILS SHOW US?

Fossils prove that ancient life forms were different

from those alive today. They are laid down in layers

of rocks, each layer older than the one above. Since

evolution is the gradual change in living things over

time, fossil layers provide an historical record of the

different stages in the evolution of life.

WHAT CAN PALAEONTOLOGISTS LEARN

FROM FOSSILS?

Fossil evidence shows how features of today’s

organisms have evolved over time. Palaeontologists

can identify organisms that may be ancient relatives

of those living today. By looking at fossils from the

same layer of rocks, they can also suggest how these

organisms lived together in their ancient habitat.

WHAT ARE FOSSIL INTERMEDIATES?

Fossil intermediates are species that provide a link

between a new group of organisms and an older

group. They also indicate how one group evolved

from the other. Archaeopteryx lived 150 million years

ago (mya). Its fossils show how birds evolved from

theropod (“beast-footed”) dinosaurs – which includes

Tyrannosarus Rex – because its skeleton was like a

dinosaur, but it had feathers similar to a bird.

This science studies the evolution, the way

of life, and the extinction of organisms

that existed in time periods before the

present. Palaeontologists are scientists who

study the fossil remains of species that

lived a long time ago.

MODEL OF AN ARCHAEOPTERYX

1 EARLY BIRD

This fossil of Archaeopteryx

shows feather imprints on

its body. Scientists studied

the fossil and made a model

of Archaeopteryx.

Fossils

Fossils are the ancient remains of

living things preserved in rocks.

They are usually formed from hard

animal or plant parts, such as bone,

shell, or wood. Studying fossils is

a part of. PALAEONTOLOGY.

Coiled shell is divided into

gas-filled chambers that keep

the animal buoyant (afloat)

PALAEONTOLOGY

FOSSIL AMMONITES 1

These are the fossilized shells of

ammonites (molluscs related to present-day

squids). Ammonites were marine predators

that lived between 500 and 65 million years

ago (mya), when they became extinct.

4.5 BYA Earth forms

3.8 BYA First living things

500 MYA First vertebrates

(fish)

440 MYA First land plants

360 MYA First land

vertebrates

(amphibians)

248 MYA First mammals

appear

150 MYA Earliest birds

65 MYA Dinosaurs extinct

60 MYA Mammals

become diverse

2 MYA Ice ages

160,000 Modern humans

YA appear

FOSSIL RECORD

fossils