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How do scientists know that
different kinds of plants &
animals lived in the past?
Fossils
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Preserved remains/traces of living things
Clues to the past
Usually found in sedimentary rocks
Bones, shells, teeth, seeds, woody stems
become fossilized
• It is rare for the soft part of an organism to
become a fossil
• Fossils provide information for:
– What the environment looked like
– Climate
– Shape of Earth’s surface
How do fossils form?
• Living things are buried by sediments
• Slowly harden into rock
• The shape of the organism is preserved
Who studies fossils?
• Paleontologists!
Kinds of Fossils:
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Petrified fossils
Molds
Casts
Carbon Films
Trace fossils
Other
Petrified Fossils
• Petrified means “turning into stone”
• Minerals replace all/part of an organism
Molds & Casts
• Mold is a hollow area in sediment in the
shape of an organism/part of an organism
• Cast is a copy of the shape of an organism
when a mold is filled with mud, ash, lava
• Most common types of fossils
Carbon Films
• Extremely thin coating of carbon on rock
• Carbon comes from living things buried
underground
• Carbon can preserve delicate parts of
plants & insects
Trace Fossils
• Provide evidence of the activities of
ancient organisms
• Ex: footprint, burrows, trails
Other
• Some processes preserve the remains of
organisms with little/no change
• Ex: tar, amber resin, freezing
How are fossils classified?
• Paleontologists classify fossils in order in
which they lived from earliest to latest into
a fossil record
• Fossil records show how organisms have
changed over time
How do geologists use fossils to
decide how old the rocks are?
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Relative age
Absolute age
Law of Superposition
Index fossils
Relative Age
• The rock’s age compared to the ages of
other rocks
Absolute Age
• The number of years since the rock
formed
• Geologists use radioactive dating to
determine this
– Carbon-14 dating
– Potassium-Argon dating
Law of Superposition
• In horizontal sedimentary rock layers, the
oldest layer is at the bottom and each
layer above it is younger
Index fossils
• A fossil found in many different areas
• Represents a type of organism that
existed only briefly
• Geologists look for index fossils in rock
layers
• Index fossils are useful because they tell
the relative age of the rock layers they are
in
Index Fossil Example
• Trilobite: hard-shelled animal with 3
distinct body sections
• Many types of trilobites existed over 500
million years ago
• Geologists must find one trilobite that is
different in some way than the others to
date the rock layer in which it was found
– Ex: Large eyes
What if a small piece of time is
missing?
• Unconformity is a gap in the geologic
record
• Some rock records are lost due to erosion
What is the
geologic time scale?
• A record of the life forms and geologic
events in Earth’s history
• First developed by studying rock layers &
index fossils
• Later radioactive dating helped to
determine the absolute age of each
divisions of the time scale
Different units
of the time scale:
• Eras
• Periods
• Epochs
Eras
• Long units of time
– Precambrian
– Paleozoic
– Mesozoic
– Cenozoic
Periods
• Sub-divided units of an era
• Range in length from 2-10million years
• Names came from where geologists first
discovered the rocks/fossils of that period
– Ex: Jurassic
Epochs
• Sub-divided units of a period
Precambrian Era
• 4.6billion-544million years
• Earliest life forms similar to bacteria of
today called cyanobacteria
• Made food through the use of
photosynthesis
Paleozoic Era
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544million-245million years
Explosion of life forms
Animals began coming onto land
Evolving:
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Hard-shelled organisms
Invertebrates (jellyfish)
Vertebrates (jawless fish)
Amphibians
Small reptiles
• Mass extinction ended this era
Mesozoic Era
• 245million-65million years ago
• Called the “Age of Reptiles”
• Evolving:
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Dinosaurs
Mammals
Birds
Flowering plants
• Jurassic Period-dinosaurs ruled Earth for about
150million years
• Mass extinction occurred when an object from space hit
Earth’s surface causing a dust cloud which blocked the
Sun
– No dinosaurs survived!
Cenozoic Era
• 65million-Present
• Evolving:
– Marine mammals (whales)
– Grass
– Mammals, birds, & insects continue to flourish
– Fossil records show that humans appeared
about 3.5million years ago