4600-542 mya

Haden, Archean, and Proterozoic eons

4th geologic eon

covers past 542 million years

Paleozoic, Mesozoic, and Cenozoic eras

• 542-251 mya
• "Cambrian explosion" - rapid diversification of multi-celled organisms
• primitive jaw-less fish, 1st primitive amphibians, 1st vertebrates, fossil vertebrate Tiktaalik documents early fish to tetrapods, 1st primitive reptiles leading to modern day reptiles and therapsids

• 251-65.5 mya
• "Age of Dinosaurs"
• mammals and birds appear
• therapsids decline to extinction but some evolved to 1st "true mammals"

1. Triassic - monotremes (egg-laying mammals)
2. Jurassic - 1st placental mammals, heyday of dinosaurs, birdlike reptiles evolved
3. Cretaceous - mass extinction from asteroid dust blocking sun shifting ecology

• 65.5 mya - present
• "Age of mammals"
• 50 mya primates appear
• 20 mya primitive apes appear
• 6 mya 1st hominins