Geologic History
Fossils and the Past
• A fossil is the preserved remains or evidence of
life.
In megalodon
sharks, only the
teeth are found as
fossils, dating 10 to
70 millions years
old.
Wooly Mammoth Teeth
Wooly Mammoth Tusk
Soft-bodied organisms, such as jellyfish, are
usually not preserved very well as fossils.
Most fossils form when organisms are buried
in sediments.
• Sediments often harden and change into
rock. When this happens, organisms may be
trapped in the rock. Most fossils are found
in sedimentary rocks.
Burgess Shale
• Mud and creatures from the shallow sea floors at the top of the cliff
was periodically swept over the edge, falling to deeper water where
the creatures were buried and preserved as fossils. Unusually some
of the fossils are preserved as carbon films which means that the
fossils are formed from the original creatures themselves, rather
than the more usual replacement by minerals.
Marine fossils in
Limestone (sedimentary)
Fossils in Sandstone
(Sedimentary)
Crinoid
Fossils are almost never found in
igneous rocks because magma is found
deep within Earth where no living
things exist, and lava at the surface of
Earth burns organisms before fossils
can form.
This site in Nebraska is
home to hundreds of
skeletons of extinct
rhinos, camels, threetoed horses, and other
vertebrates that were
killed and buried by ash
from a huge volcanic
eruption some 12
million years ago.
Pompeii, Italy
Fossils are rarely found in metamorphic
rocks because the heat, pressure and/or
chemical activity that causes a rock to
change, also detroys or damages the
fossils.
A distorted and
stretched trilobite
fossil in
metamorphic slate.
Interpreting Fossils
• Fossils indicate that many different kinds of
life forms have existed at different times in
Earth’s history.
Trilobites lived from about 520 to 250 million
years ago, and are now extinct.
Eurypterids are the NYS fossil, and
lived 400 million years ago.
When fossils are arranged according to age,
they show that certain living things have
changed or evolved over time.
488 – 444 mya
444 – 416 mya
444 – 416 mya
416 – 359 mya
The Geologic Time Scale
• The geologic time scale divides earth’s history into
sections of time.
• Divided into:
– Eons
– Eras
– Periods
– Epochs
•
The boundaries between geologic time intervals
represent major changes on earth which include
major extinctions
•
Geologic time began when earth first formed about
4.6 billion years ago
Pages 8 and 9 ESRT
The Geologic Time Scale
• Precambrian: a “super” eon composed of the earliest
Archean and Proterozoic eons
• 88% of all geologic time
• Fossils in precambrian rocks are rare
Flora and fauna of the Precambrian sea
Precambrianaged rocks in
North America
Uniformity of Process
• “The present is the key to the past.”
• Present observations help geologists interpret the
past AND make predictions about the future.