Geologic Timeline
Fossils
From the fossil record,
paleontologists learn:
the structure of ancient
organisms
their environment
the ways in which the
organisms lived.
Fossils and Extinct Species
Fossils are the most
important source of
information about extinct
species
Species that have died out
The fossil record is
incomplete so there are
many organisms that have
died out that we do not know
about.
Types of Fossils
Many fossils are
just fragments of
an organism—
teeth, pieces of a
jawbone, or bits
of leaf.
Types of Fossils
 Most fossils are preserved in sedimentary rock.
 Sedimentary rock usually forms when small particles of
sand, silt, clay, or lime muds settle to the bottom of a body
of water.
 As sediments build up, they bury dead organisms that
have sunk to the bottom.
Types of Fossils
 As layers of sediment continue to build up over time, the
remains are buried deeper and deeper.
 Over many years, water pressure gradually compresses
the lower layers and turns the sediments into rock.
Paleontologists
Researchers who
study fossils to
learn about ancient
life.
What Fossils Can Reveal
By comparing body
structures in fossils to
body structures in living
organisms, researchers
can infer evolutionary
relationships and form
hypotheses about how
body structures and
species have evolved.
What Fossils Can Reveal
Bone structure
and trace fossils,
like footprints,
indicate how
animals moved.
What Fossils Can Reveal
Fossilized plant leaves
and pollen suggest
whether the area was a
swamp, a lake, a forest, or
a desert.
When different kinds of
fossils are found together,
researchers can
sometimes reconstruct
entire ancient ecosystems.
Geologic Time Scale
Geologists and
paleontologists
have built a time
line of Earth’s
history called the
geologic time
scale.
Hadean Eon
About 4600 mya
(million years ago)
– 4000 mya
Mya – million years
ago
Archean Eon
About 4000
mya – 2500
mya
Proterozoic Eon
About 2500
mya – 542 mya
Cambrian Period
Inside the
Paleozoic Era
About 542 mya
– 488 mya
Ordovician Period
Inside the
Paleozoic Era
488 mya – 444
mya
Silurian Period
Inside the
Paleozoic Era
444 mya – 414
mya
Devonian Period
Inside Paleozoic
Era
416 mya – 359
mya
Carboniferous Period
Inside Paleozoic
Era
359 mya – 299
mya
Permian Period
Inside Paleozoic
Era
299 mya – 251 mya
Triassic Period
Inside Mesozoic
Era
251 mya – 200
mya
Jurassic Period
Inside Mesozoic
Era
200 mya – 146 mya
Cretaceous Period
Inside Mesozoic Era
146 mya – 65.5
mya
Paleogene Era
Inside Cenozoic
Era
65.5 mya – 23 mya
Neogene Period
Inside Cenozoic
Era
23 mya – 1.8 mya
Quaternary Period
Inside Cenozoic
Era
1.8 mya - present
Life on a Changing Planet
Building mountains, opening
coastlines, changing
climates, and geological
forces have altered habitats
of living organisms
repeatedly throughout
Earth’s history.
In turn, the actions of living
organisms over time have
changed conditions in the
land, water, and atmosphere
of planet Earth.
Life on a Changing Planet
Earth and its climate has
been constantly changing,
and organisms have
evolved in ways that
responded to those new
conditions.
The fossil record shows
evolutionary histories for
major groups of organisms
as they have both
responded to changes on
Earth and how they have
changed Earth.
Physical Forces
Climate
Temperatures
Geologic Forces
Volcanic activities
Wind and ocean
currents
Physical Forces
 The theory of plate tectonics explains how solid
continental “plates” move slowly above Earth’s
molten core—a process called continental drift.
Biological Forces
The activities of
organisms have
affected global
environments.
For example,
Earth’s early
oceans contained
large amounts of
soluble iron and
little oxygen.
Biological Forces
During the
Proterozoic Eon,
however,
photosynthetic
organisms produced
oxygen gas and also
removed large
amounts of carbon
dioxide from the
atmosphere.