Geologic Time Scale

How’s it divided?  Instead of being divided into months or years, the geologic time scale is divided into eras.  Eras are divided into periods.  4 Eras: Precambrian, Paleozoic, Mesozoic, and Cenozoic  The divisions are based on what lived during that time.

Precambrian Era  Scientists found fossils of bacteria called cyanobacteria  This era accounts for 87% of Earth’s history  Only life forms during this era were unicellular prokaryotes  Towards the end of this era, eukaryotic cells began to form (ex. Sponges & Jellyfish)

Paleozoic Era  Many types of animals & plants appeared  Fossil records show an increase in diversity of life forms (Ex. worms, sea stars, and fishes appeared)  Middle part of the era: ferns & amphibians appeared  Last half of the era, reptiles began to emerge.  In the end, 90% of marine species and 70 % of land species became extinct

Mesozoic Era Three periods to this era: 1. Triassic Period --- beginning – showed that mammals were small 2. Jurassic Period --- Age of Dinosaurs; fossils show that modern birds evolved from dinos 3. Cretaceous Period --- new mammals & flowering plants; mass extinction of dinos ended this period

Changes in Mesozoic Era  Geologic events changed where species lived and distribution on earth  Continental Drift Theory --- Suggests that the continents have moved during the Earth’s history & are still moving today.  Plate tectonics --- explains how the continents move

Cenozoic Era  This is the era that we live in  Mammals began to flourish (grow)  Primates --- we belong to this group  Primate first appeared 30 million years ago  Humans appeared 200,000 years ago