How It Works-Book Of Dinosaurs

Dependent on climate and ground

conditions, deceased animals can

be fossilised in many ways

Typ e s of

fossilisation

The origin of life on Earth is irrevocably

trapped in deep time. The epic, fl uid and

countless beginnings, evolutions and

extinctions are immeasurable to humankind; our

chronology is fractured, the picture is incomplete.

For while the diversit y of life on Earth today is

awe-inspiring, with animals living within the most

extreme environments imaginable – environments

we as humans brave ever y day in a effort to chart

and understand where life begins and ends – it is but

only a fraction of the total life Earth has seen inhabit

it over geological time. Driven by the harsh realities

of an ever-changing environment, Armageddon-

level extinction events and the perpetual, ever-

present force of natural selection, wondrous

creatures with fi ve eyes, fi erce predators w ith

12-inch fangs and massive creatures t w ice the size of

a double-decker bus have long since ceased to exist.

They are forgotten, buried by not just millions, but

billions of years. Still, all is not lost to us. By

exploiting Earth’s natural processes and modern

technolog y over the last t wo hundred years,

scientists and palaeontologists have begun to

unravel Earth’s tree of life and, through the

discover y and excavation of fossils – preser ved

remains and traces of past life in Earth’s crust – piece

the jigsaw back together.

The fossilisation of an animal can occur in a

variety of ways (see ‘Types of fossilisation’ boxout)

but, in general, it occurs when a recently deceased

creature is rapidly buried by sediment or subsumed

in an oxygen-defi cient liquid. This has the effect of

preser v ing parts of the creature – usually the harder,

solid parts like its skeleton – often in the original,

liv ing form w ithin the Earth’s crust. The softer parts

of fossilised creatures tend not to sur v ive due to the

speed of decay and their replacement by minerals

contained in their sediment or liquid casing, a

process that can leave casings and impressions of

the animal that once lived, but not its remains.

Importantly, however, creature fossilisation tends to

Permineralisation

A process in which mineral deposits form

internal casts of organisms,

permineralisation works when a deceased

animal dies and then is rapidly submerged

with groundwater. The water fills the

creature’s lungs and empt y spaces, before

draining away leav ing a mineral cast

Mold

A type of fossilisation process

similar to permineralisation,

molds occur when an animal is

completely dissolved or destroyed,

leaving only an organism-shaped

hole in the rock. Molds can turn

into casts if they are then filled

with minerals

Recrystallisation

When a shelled creature’s shell,

bone or tissue maintains its

original form but is replaced with

a cr ystal – such as aragonite

and calcite – then it is said to

be recrystallised

Bioimmuration

Bioimmuration is a type of fossil

that in its formation subsumes

another organism, leav ing an

impression of it within the fossil.

This type of fossilisation usually

occurs between sessile skeletal

organisms, such as oysters

Resin

Referred to as amber, fossil resin is a natural

polymer excreted by trees and plants. As it is stick y

and soft when produced, small invertebrates such as

insects and spiders are often trapped and sealed

within resin, preserving their form

Adpression

A form of fossilisation caused by

compression within

sedimentar y rock. This t y pe of

fossilisation occurs mainly

where fine sediment is

deposited frequently, such as

along rivers. Many fossilised

plants are formed this way

Carbon dating

A crucial tool for palaeontologists,

carbon dating allows ancient

fossils to be accurately dated

Carbon dating is a method of radioactive dating

used by palaeontologists that utilises the

radioactive isotope carbon-14 to determine the

time since it died and was fossilised. When an

organism dies it stops replacing carbon-14,

which is present in ever y carbonaceous

organism on Earth, leav ing the existing

carbon-14 to decay. Carbon-14 has a half-life

(the time it takes a decaying object to decrease

in radioactiv it y by 50 per cent) of 5,730 years, so

by measuring the decayed levels of carbon-14

in a fossil, its time of death can be extrapolated

and its geological age determined.

This scientist is dating

archaeological specimens in a

Tandetron particle accelerator © Science Photo Librar y

© Michael S. Engel

© Slade Winstone

“ The softer parts of fossilised creatures tend

not to survive due to the rapidity of decay”

DID YOU KNOW? (^) Fossils are useful in targeting mineral fuels, indicating the stratigraphic position of coal streams