The History of Life
The Fossil Record
Types of Fossils
Permineralized fossils
Natural casts
Trace fossils
Amber-preserved fossils
Preserved remains
Permineralization
Minerals carried by water are deposited around
a hard structure or may replace hard structure.
Most common form of fossil
Natural casts
Flowing water removes all of original tissue or
bone, leaving an impression
Minerals fill mold to recreate shape of organism
Trace fossils
Record activity of an organism
Include nests, burrows, imprints
of leaves or footprints
Amber-preserved fossils
Organisms trapped in tree resin that hardens into
amber
Preserved remains
Entire organism becomes encased in
material such as ice, volcanic ash or
immersed in bogs.
Fossils
You and your partner will be given a
fossil. You job is to:
Describe the type of fossil it is
What living thing was fossilized
When you are finished, trade with
another group until you have described
three different fossils.
How are some fossils
formed?
Mainly form in sedimentary rock
Many layers of small rock particles (clay, silt, sand)
1.) After organism dies, must be buried by sand, sediment,
mud or tar
2.) Water fills in spaces of organism and minerals are
deposited
Tissue is replaced by hard minerals
3.) More and more layers continue to build up and pressure
compresses bodies
Journal Question
Is the fossil record complete?
In other words, is there fossil
evidence for every living thing
that has existed? Why or why
not?
Relative Dating
Estimation of time which organism
existed
Compares location of fossils in different
layers
Law of Superposition
Allows scientists to determine the order
(sequence) of species groups that
existed
NOT ACTUAL AGES OF FOSSILS
Radiometric Dating
Absolute age
Use natural decay (break down) of
radioactive isotopes found in dead materials
When nucleus is unstable, it decays (breaks
down) over time – known as “half-life”
Half-life
Amount of time it takes for half of the
isotope to decay into another isotope
Example: carbon-14 decays into nitrogen-
14 with half life of about 5,700 years
Radiocarbon dating
While organism is alive, it breathes in carbon-
14.
When organism dies, carbon-14 continues to
decay
If carbon-14 has half-life of 5,700 years, a 5,700
year old fossil will have half of its C-14
decayed into N-14.
After 11,400 years (two half-lives) ____ percent
will have decayed?
75%
25% of the original remains
Radioisotopes used to
determine Earth’s Age
Radiometric dating used to
determine the age of Earth
Based on radioisotopes from
meteorites, Earth is about 4.6 billion
years old.
Geologic Time Scale
Index Fossils
Fossils of organisms that existed
only during specific spans of
time over large geographic
areas.
Helps to determine the age of
rock layers
Geologic Time Scale
Shows history of Earth, and is divided into
eras and periods
Era:
Tens to hundreds of millions of years
Consist of two or more periods
Periods:
Last tens of millions of years.
Associated with particular type of rock
Paleozoic Era
Paleo = ancient
Cambrian Period
544-505 mya
Variety of animals developed very
quickly
All life was found in water
“Cambrian Explosion”
Ordovician Period
505-440 mya
Marine invertebrates evolved
First vertebrates evolved
Glaciers formed sea level
dropped MASS EXTINCTION to
marine life
MASS EXTINCTION
Mass extinction:
When a large number of organisms
become extinct over a relatively short
period of geologic time
Glaciers caused global cooling
Killed organisms that are adapted to
warmer climates
Silurian Period
440-410 mya
Land plants arose
Glaciers melted seas formed
Jawless and freshwater fish
evolved
Devonian Period
410-360 mya
Fish became more diverse
Sharks, amphibians and insects
appeared
Trees and forests arose
MASS EXTINCTION
Global cooling due to glaciers
Another 50% of organisms die
Carboniferous Period
360-286 mya
Coal forming sediments in swamps
Fish became more diverse
Amphibians, winged insects,
conifers (ex. Pine trees) and small
reptiles
Permian Period
286-248 mya
Modern pine trees appeared
Pangaea supercontinent was
formed as land masses joined
MASS EXTINCTION
Largest mass extinction in the history of life
on Earth
“Permo-Triassic Extinction”
95 % of species die
Less water available for life
Sea level changed
Mesozoic Era
Meso = middle
Triassic Period
248-213 mya
Dinosaurs evolved
Fern plants and cycads evolved
Mammals and flying reptiles arose
Jurassic Period
213-145 mya
Dinosaurs diversified
Trees became more similar to trees
today
Oceans were very diverse with fish
and squid
Birds arose
Cretaceous Period
145-65 mya
Dinosaurs peaked, then went
extinct
Birds survived
Flowering plants arose
MASS EXTINCTION
Large meteorite (10km in diameter) hit
Yucatan Peninsula
Adding dust into the atmosphere
Blocking UV light needed for life
Massive volcanic activity also added to the
dust in atmosphere
Cenozoic Era
Ceno = recent
Tertiary Period
65-1.8 mya
Mammals
Flowering plants
Grasslands
Insects
Fish
Birds
Primates
Quaternary Period
1.8 mya-PRESENT
Continues today
Includes all modern forms of life
Plate Tectonics
Plate Tectonics
Study of the formation and
movement of the plates
Plates: rigid, moving pieces of
Earth’s surface
Plates are either moving past each
other, moving apart, or moving
together (colliding)
Evidence for Plate Tectonics
Diverging Boundaries
Also known as spreading centers
Places where two plates are
moving apart
Sliding Boundaries
Plates are sliding past each other
San Andreas Fault in California
Fault: break or crack in Earth’s crust
where movement has occurred
Likely places for earthquakes
Converging Boundaries:
Collision
Two plates are coming together
(converge)
Collision refers to the plates being
pushed upward
Mountain range
Himalayan Mountains
Converging Boundaries:
Subduction
Plates come together
One plate plunges down under
another plate
Deep-sea trenches usually found in
oceans