Geologic Time Scale
8.E.6A.1: Develop and use models to organize Earth’s history (including era, period, and
epoch) according to the geologic time scale using evidence from rock layers.
Geologic Time Scale
Definition:
The geologic time scale is a
record of the major events and
diversity of life forms present in
Earth’s history.
Length:
The geologic time scale began
when Earth was formed and
goes on until the present.
It divides Earth’s long history into units of time.
Precambrian is the name given to the earliest span of time in Earth
history.
Geologists divide the time between Precambrian and the present
into three long units called eras (Paleozoic, Mesozoic, Cenozoic).
o eras are divided into periods
o periods can be further divided into epochs
o these subdivisions are based on large scale events in
Earth’s history that are identified in the fossil record
and rock layers
At the end of each era a major mass extinction occurred, many
kinds of organisms died out, although there were other extinctions
going on during each period of geologic time.
Evidence: The layers of rock on Earth serve as evidence when
identifying the geologic time scale. Using the fossil record,
paleontologists have created a picture of the different types of
common organisms in each geologic period.
Cambrian Importance: Cambrian being the first period is
important. The Cambrian Explosion was a time where there was a
relatively rapid appearance of most major animal phyla as
evidenced by the fossil record.
Paleozoic Era
 Began with the early invertebrates, such as trilobites and
brachiopods; continued to develop early vertebrate fish,
then arachnids and insects; later came the first amphibians,
and near the era’s end the reptiles became dominant.
 Early land plants included simple mosses, ferns, and then
cone-bearing plants.
 By the end of the era, seed plants were common.
 The mass extinction that ended the era caused most marine
invertebrates as well as amphibians to disappear.
 A major geologic event of the Paleozoic was the formation of
the super continent of Pangaea.
Mesozoic Era
 Reptiles were the dominant animals of this era, including the
various dinosaurs.
 Small mammals and birds also appeared.
 Toward the end of the era, flowering plants appeared and
the kinds of mammals increased.
 The mass extinction that ended the era caused the dinosaurs
to become extinct.
 A major geologic event of the Mesozoic was the break-up of
the super continent of Pangaea into several large continents.
Cenozoic Era
 New mammals appeared while others became extinct.
 The diversity of life forms increased.
 Flowering plants became most common.
 Humans are also part of the most recent period of this era.
 Present day Earth is in this era.
 A major geologic event of the Cenozoic is the further splitting
and moving of continents to their current positions