Childrens Illustrated Encyclopedia

218

Ichthyosaur

sometimes the outline of an animal’s

skin is preserved as well as its bones.

this happened to the ichthyosaur

shown above—a sea reptile from

about 150 million years ago. the

ichthyosaur looked like a dolphin,

so it probably led a life similar

to that of dolphins. the

outline of this fossil shows

a fin on the back and a

two-lobed tail. the dozens

of sharp teeth in the long

jaws tell us that this animal

grabbed fish and other

slippery prey.

types

of fossIls

When rock-forming

minerals slowly replace

the original parts of a dead

creature or plant, they

make a mineralized fossil.

sometimes the parts of a creature

or plant rot away after being

buried, leaving a hole in the rock;

this is called a mold fossil. If the

hole fills up with rock minerals,

it becomes a cast fossil. the

fossilized signs of animals, such

as footprints, droppings, and

tracks, are called trace fossils.

the fIrst plaNts, the earliest animals, the

beginnings of human life—we know about prehistoric

times because of fossils. preserved or mineralized for

thousands or even millions of years, fossils may

comprise, for example, parts of animals, molds,

footprints, and burrows. By studying fossils, we can

learn what ancient creatures and plants looked like

and how they lived. Most fossils are of plants and

animals that lived in water. When the living plant or

animal died, its soft parts rotted away, leaving the hard

pieces such as bones or leaf veins. Gradually, layers of

mud piled up and squeezed the remains of the plant

or animal at great pressure. slowly, the mud, bones,

and other remains fossilized, or turned to rock, in the

place where they lay underground. over many thousands of years, the

movements of earth twisted and buckled the rocks, lifting the fossils

closer to the surface of the soil. sun, rain, and wind wore away the rocks

and exposed the fossil.

Fin on back

for steering

and stability

Find out more

Dinosaurs

evolution

prehistoric life

rocks and minerals

aMMoNIte

some of the most common fossils

are the shells of sea creatures called

ammonites. ammonites were related

to squid and octopuses. they were

very widespread about 250 million

years ago. the smallest ammonites

measured less than 1 in (2 cm) across;

the largest measured about 8 ft (2.5 m)

across. ammonites died out with the

dinosaurs about 66 million years ago.

Fossil of a fish called Sparnodus—an

ancestor of the sea bream

Fossils

The word fossil literally means

“dug up.” People who study fossils

are called paleontologists.

Large eye socket

Long jaws and

short, sharp teeth

Ribs

Powerful two-

lobed tail

Rear paddles

Front paddle

for steering

Mineralized

fossil of a

poplar leaf,

25 million

years old

Cast fossil of a creature

called a trilobite, which

lived in the sea

Backbone

Fossil collecting is a hobby

that anyone can enjoy. You

can find fossils in rocks, on

beaches, and in quarries.

US_218_Fossils.indd 218 21/01/16 4:59 pm