Index Fossils and
Geologic Time
“The Past is the Key to the
Present”
Determining
Relative Age
 Rock layers
(strata) detail
sequences of
Earth’s events
 the oldest rock
layers are on the
bottom and
youngest are on top
 Relative Age
(Approximate)
 Fossils
Determining
Absolute Age
 Absolute Age =
Exact Age
 Radioactive
Decay
 Half-Life
 Carbon Dating
 Fossils
The Fossil Record
 Remains or traces
of animals or
plants from a
previous geologic
time
 Quick burial;
Sedimentary Rock
 Used for absolute
and relative ages
 Fossil succession
(fossils succeed
one another in a
definite order)
 Kinds of Fossils
 Hard Parts
 Trace (footprint)
 Imprints, molds and
casts
 Coprolites (dino
doo-doo)
 Gastroliths
(Digestive stones)
Index Fossils
 Fossils found
exclusively in rock
layers of a
particular
geologic age
 Present over a wide
area
 Easily distinguished
 Relatively short
geologic time span
 Large numbers
within rock layers
More on Index Fossils
 Used to establish the relative
ages of the rock layers in which
the fossils are found (trilobite)
 Used to date rock layers found
in widely separated areas
 Used to locate oil and natural
gas deposits
Geologic Time
 Fossil record (worldwide) used to
construct a “geologic column”
which shows the ordered
arrangement of rock layers based
upon ages
 Distinguished primarily by kinds of
fossils they contain
 Fossils in the lower, older layers are
very different than those within the
younger layers
Divisions of Geologic
Time
 Earth’s geologic history is
punctuated by major changes in the
earth’s surface or climate and by the
extinction of various species
 Geologists use these events as the
basis for dividing the geologic time
scale (Eons) into smaller units
 Eras
 Periods
 Epochs
Precambrian Time
 4.6 bya to 570 mya
 88% of Earth’s
history
 Atmosphere forms
 Fossils rare
 Soft-bodied
 Deformed rock
 Bacterial reefs
 Worms, jellyfish
and one-celled
organisms in late
precambrian
Paleozoic Era
 570 mya to 245
mya
 Dramatic
increase in plant
and animal
species
 Rich marine and
land fossil
record
More on the
Paleozoic Era
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First vertebrates (fishes) appear
First land plants and animals appear
First amphibians appear
First reptiles appear
Pangaea comes together
Mass extinctions mark the end of
this era
 Marine invertebrates (trilobites,
eurypterids) due to retreat of the seas
Mesozoic Era
 245 mya to 65 mya
 Surface of the
earth changed
dramatically
(break-up of
Pangaea)
 Conditions favored
the survival of the
reptiles
More on the
Mesozoic Era
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Dinosaurs first appear
First mammals appear
Flying reptiles and first birds appear
First flowering plants appear
Mass extinctions (dinosaurs, marine
animals) mark the end of this era
 Cretaceous-Tertiary Boundary (K-T)
 Meteorite-Impact hypothesis
Cenozoic Era
 65 mya to
present
 Time of
increased
tectonic activity
 Dramatic
changes in
climate (ice
ages)
More on the
Cenozoic Era
 “Age of Mammals”
 Divided into two periods:
 Tertiary: time before the last
major ice age
 Whales, Horses, deer, grazing herds,
wolves, raccoons
 Quaternary: Last ice age
(Pleistocene) to present
 Wooly mammoths, rhinos, humans