GEOLOGIC TIME SCALEThe geologic time scale is a record of the major events and diversity of life forms present in Earth’s history.

BEGINS: When the Earth was formedIt is still going on in the present day. GEOLOGIC TIME HAS NOT ENDED!

Geologists divide the time between Precambrian and the present into 3 long units called Eras (Paleozoic, Mesozoic, Cenozoic).

Eras are divided into Periods Periods are divided into Epochs These subdivisions are based on large scale events in Earth’s history that are identified in

the fossil record and rock layers.

We are currently in the Quaternary period in the Holocene epoch. At the end of each era a major mass extinction occurred, many kinds of organisms died out.

Precambrian is the name given to the earliest span of time in Earth history.

Cambrian being the first period is important. The Cambrian Explosion was a time where there was a relatively rapid appearance of most major animal species as evidenced by the fossil record.

If we represent Earth’s history on a 24 hour clock…

The Precambrian era began with the formation of Earth, 4.6 billion years ago and lasted until about 500 million years ago.

= Precambrian era= Precambrian era

Paleozoic Era Mesozoic Era Cenozoic 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.

o Early land plants included simple mosses, ferns, and then cone-bearing plants.

o By the end of the era, seed plants were common.

o The mass extinction that ended the era caused most marine invertebrates as well as amphibians to disappear.

o A major geologic event of the Paleozoic was the formation of the super continent of Pangaea.

Reptiles were the dominant animals of this era, including the various dinosaurs.

o Small mammals and birds also appeared.

o Toward the end of the era, flowering plants appeared and the kinds of mammals increased.

o The mass extinction that ended the era caused the dinosaurs to become extinct.

o A major geologic event of the Mesozoic was the break-up of the super continent of Pangaea into several large continents.

o New mammals appeared while others became extinct.

o The diversity of life forms increased.

o Flowering plants became most common.

o Humans are also part of the most recent period of this era.

o Present day Earth is in this era.

o A major geologic event of the Cenozoic is the further splitting and moving of continents to their current positions.