Geologic Time Scale

• The geologic time scale is divided into eons, eras, periods, and epochs.
• The geologic time scale is a record of life forms and geologic events in Earth’s history.
• The divisions of the geologic time scale are based on changes or events recorded in rocks and fossils.
• Precambrian eon is most of Earth’s history.
• During this period, no plants or animals existed. Multicellular organisms develop late in the Precambrian.
• At the beginning of the Paleozoic era, all life lived in the oceans.
• Mesozoic era is known as the age of the reptiles/dinosaurs
• The first humans appeared in the later part of the Cenozoic era, which continues today.
• Mass extinctions occurred during the Ordovician, Devonian, Permian, Triassic, and K-T.
• Formal geologic time begins at the start of the Archean Eon (4.0 billion to 2.5 billion years ago)
• The oldest and by far the longest is called the Precambrian
• Humans evolved in the Cenozoic era
• Non-bird dinosaurs lived between about 245 and 66 million years ago, in a time known as the Mesozoic Era.