Evolution
The Fossil Record
• Fossil – preserved remains or traces of an organism
that lived in the past
• Sedimentary rock – rock formed when layers of
sediments harden over millions of years
• Petrified fossil – a fossil formed when minerals
•
•
•
•
replace all or part of an organism
Mold – a type of fossil formed when a shell or other
hard part of an organism dissolves, leaving an empty
space in the shape of the part
Cast – a type of fossil that forms when a mold
becomes filled in with minerals that then harden
Relative dating – a technique used to determine
which of two fossils is older
Absolute dating – a technique used to determine the
actual age of a fossil
• Radioactive element – an unstable particle that
breaks down into a different element
• Half- life – the time it takes for half of the atoms in a
radioactive element to break down
• Fossil record – the millions of fossils that scientists
have collected
• Extinct – a species that does not have any living
members
• Gradualism – the theory that evolution occurs slowly
but steadily
• Punctuated equilibria – the theory that species evolve
during short periods of rapid change
• Sediments – particles of soil and rock
How fossils form?
• Can be formed from a
bone, tooth, shell, or other
part of an organism
• Can be formed from
traces of an organism,
such as a footprint or
worm burrows left in mud
that later turned to stone
• Formation of a fossil is a rare event
• Most fossils form when organisms that die
become buried in sediments
• Layers of sediments build up and cover
the dead organisms
• Petrified fossils
– Remains that become
buried in sediments
change to rock
– Minerals dissolved in
water soak into
remains
– Minerals replace the
remains changing
them into rock
Technomyrmex hispaniolae
ant entombed in amber
• Molds
– Sometimes shells or
other hard parts buried
by sediments are
gradually
– An empty space
remains in the place
the part once occupied
• Casts
– Mold becomes filled in
with hardened
minerals
1965 pterosaur cast
• Preserved remains
– Organisms preserved in other substances besides
sediment – protects body from decay
• Ice – frozen water
• Tar pits – dark, sticky form of oil
• Amber - sticky sap that hardens
IN THE TAR PIT
• Determining fossil’s age
– Relative dating
• Can only be used when
the rock layers have been
preserved in their original
sequence – top layer
being the youngest layer
• Helps scientists determine
whether one fossil is older
than the other
– Absolute dating
• Helps scientists determine actual age of fossils
• Rocks near fossils contain radioactive elements
– unstable elements that break down into
different elements
• Half-life of a radioactive element is the time it
takes for half of the atoms in a sample to decay
• Scientists compare the amount of radioactive
element in a sample to the amount of the
element into which it breaks down
• Scientists use this info to calculate the age of
the rock, which then tells the age of the fossil
What do fossils reveal?
• Fossil record
– Millions of fossils scientists have collected
– Incomplete: many organisms die without leaving a
fossil behind
– Gives important info about past life on earth
– Provides clues about how and when organisms
evolved
• 540 million years ago – first animals appeared –worms,
sponges, invertebrates
• 500 million years ago – fishes evolved – first vertebrates
• 410 million years ago – first land plants – similar to moss
• Extinct
– No members of the species are still alive
– Scientists use fossils of bones and teeth to build
models of extinct animals
• Geologic Time Scale
– Using absolute dating, scientists have calculated the
ages of fossils and rocks
– Calendar of Earth’s history spanning more than 4.6
billion years
– Divided into units called eras and periods
– Precambrian - largest span of time
• first 4 billion years of history – 4.6 billion years ago
• few fossils – little knowledge –covers 87% of
earth’s history
–PRECAMBRIAN
4.6 billion years ago
– After Precambrian time, scale divided into 3
major blocks of time called eras
• Paleozoic Era – 544million years ago
–
–
–
–
–
–
Cambrian period
Ordovician period
Silurian period
Devonian period
Carboniferous period
Permian period
• Mesozoic Era – 245 million years ago
– Triassic period
– Jurassic period
– Cretaceous period
• Cenzoic Era – 66.4 million years ago
– Tertiary period
– Quaternary period
p. 156
p. 157
How fast does evolution occur?
• Gradualism Theory
– Tiny changes in a species gradually
add up to major changes over very
long periods of time
– Darwin agreed with this theory
– Missing fossil data of intermediate
forms
• Punctuated Equilibria Theory
– Developed by Stephen Jay Gould
and Niles Eldridge
– Agrees with fossil data
– Species evolve during short periods
of rapid behavior separated by long
periods of little or no change
Let us review??
1. Describe how fossils form in sedimentary
rock.
2. Explain the process of absolute dating.
3. What is the fossil record?
4. What does the fossil record reveal about
extinct species?
5. How are the theories of gradualism and
punctuated equilibria similar and different?