KEY CONCEPT The geologic time scale divides Earth’s history based on major past events.

Index fossils are fossils of organisms that exited only during specific spans of time over large areas. Characteristics: common, easy to identify, found all over the world, only existed for a certain period of time. Examples include fusulinids and trilobites. Nonexamples: anything that is rare to find, or is only found in a certain area of the world

The history of Earth is represented in the geologic time scale. The geologic time scale organizes Earth’s history by major changes or events. The history of Earth is represented in the geologic time scale. 100 250 550 1000 2000 PRECAMBRIAN TIME Cyanobacteria This time span makes up the vast majority of Earth’s history. It includes the oldest known rocks and fossils, the origin of eukaryotes, and the oldest animal fossils.

Eras last tens to hundreds of millions of years. consist of two or more period Cenozoic Era Evolution of primates Diversity of mammals Flowering plants Continues today 2. Mesozoic Era Evolution & extinction of dinosaurs Evolution of ferns & mammals 3. Paleozoic Era All existing phyla developed Earliest plants arose

Periods last tens of millions of years. most commonly used units of time on time scale associated with rock systems. Epochs last several million years. Smallest units of time