The remains, impressions, or traces of organisms (such as
plants and animals) that have been preserved in the rocks
are known as fossils. The process of fossilization generally
takes millions of years, and as special circumstances are
needed for fossils to form, only a tiny proportion of
Earth’s organisms have been fossilized. It is not always
easy to date fossils absolutely, so the palaeontologists
who find, analyze, and identify fossils usually assign
them to named eras and periods. These cover the
major phases in the long history of life on Earth.Fossils
PALAEOZOIC ERA (ANCIENT LIFE)
Life began in the Precambrian era, more than
3.5 billion years ago, but for most of that time
life was restricted to single-celled organisms
like bacteria. The start of the Palaeozoic era 540
million years ago saw an explosion of multicellular
life, such as marine arthropods, molluscs, and
primitive relatives of fish.Familiar animals, like
this dragonfly, were
already common
150 million years agoThis model of a
Deinonychus skull helps
palaeontologists understand
the biology of this predatorFinely detailed fossils
show the feathers of
Archaeopteryx, oneof the first known birdsLobe-finned fish
are the cousins of
all modern animals
with four legsResembling woodlice, trilobites lived on
seabeds throughout the Palaeozoic era172
MESOZOIC ERA (MIDDLE LIFE)
Beginning 252 million years ago, this was the age of
the dinosaurs, when giant reptiles stalked the
land, pterosaurs swooped through the skies,
and marine reptiles such as ichthyosaurs
swam alongside squid-like ammonites.
Flowering plants and small mammals also
appeared on land. The era ended with a mass
extinction 66 million years ago.Sea lilies were plant-
shaped relatives of
starfish, attached to
the seabed by stalksWhole animals such
as insects are preserved
in amber, or fossilized
tree resinFossil ammonites (prehistoric sea
creatures) were recorded in this
book more than 100 years ago172_173_Fossils.indd 172 03/01/19 12:10 PM