Geologic TimeGeologic Time

Trilobite eye position Trilobite eye position changed as the species changed as the species

adapted to various adapted to various environments.environments.

Trilobite bodies and tails Trilobite bodies and tails changed in response to changed in response to changing environments.changing environments.

Continental collisions Continental collisions formed the giant landmass formed the giant landmass

Pangaea near the end of the Pangaea near the end of the Paleozoic Era. These Paleozoic Era. These collisions may have collisions may have

dropped sea levels causing dropped sea levels causing the extinction of trilobites.the extinction of trilobites.

Precambrian time: from 4 Precambrian time: from 4 billion to ~544 million billion to ~544 million

years ago.years ago. Very few fossils remain from this Very few fossils remain from this

time.time. Many Precambrain rocks were Many Precambrain rocks were

deeply buried, causing the fossils in deeply buried, causing the fossils in them to be changed by heat and them to be changed by heat and pressure.pressure.

Most Precambrain organisms lacked Most Precambrain organisms lacked hard parts.hard parts.

Cyanobacteria are blue-green algae.Cyanobacteria are blue-green algae. One of the earliest life forms to appearOne of the earliest life forms to appear Added oxygen to the atmosphere through Added oxygen to the atmosphere through

photosynthesis.photosynthesis.

Invertebrates and Ediacaran animals Invertebrates and Ediacaran animals appeared late in Precambrian time.appeared late in Precambrian time.

The Paleozoic Era: 544 The Paleozoic Era: 544 million years ago to ~245 million years ago to ~245

million years ago.million years ago. Many organisms with shells and Many organisms with shells and

vertebrates evolved in the warm vertebrates evolved in the warm shallow seas.shallow seas.

Amphibians evolved to survive in Amphibians evolved to survive in water and on land.water and on land. Might have evolved from fishMight have evolved from fish Could obtain oxygen from gills or from Could obtain oxygen from gills or from

lungslungs

Reptiles evolved from amphibians to Reptiles evolved from amphibians to survive farther from water.survive farther from water.

Several mountain-building episodes Several mountain-building episodes occurred during the Paleozoic Era occurred during the Paleozoic Era because of plate collisions.because of plate collisions.

Most marine and land species Most marine and land species became extinct at the end of the became extinct at the end of the Paleozoic.Paleozoic.

Mesozoic Era: 245-65 million Mesozoic Era: 245-65 million years ago.years ago.

Pangaea separated into continents Pangaea separated into continents and the climate became drier.and the climate became drier.

Dinosaurs evolved; they might have Dinosaurs evolved; they might have been warm-blooded, traveled in been warm-blooded, traveled in herds and nurtured their young.herds and nurtured their young.

Birds, which probably evolved from Birds, which probably evolved from small, meat-eating dinosaurs, small, meat-eating dinosaurs, appeared during the Jurassic Period.appeared during the Jurassic Period.

Small, mouse-like mammals which Small, mouse-like mammals which are warm-blooded vertebrates with are warm-blooded vertebrates with hair and milk to feed their young, hair and milk to feed their young, appeared in the Triassic Period.appeared in the Triassic Period.

Gymnosperms, plants that produce Gymnosperms, plants that produce seeds but not flowers, appeared in seeds but not flowers, appeared in the Paleozoic Era.the Paleozoic Era.

Flowering plants or angiosperms Flowering plants or angiosperms appeared during the Cretaceous appeared during the Cretaceous Period.Period.

A great extinction, perhaps caused A great extinction, perhaps caused by a comet or an asteroid collision, by a comet or an asteroid collision, occurred about 65 million years ago, occurred about 65 million years ago, marking the end of the Mesozoic marking the end of the Mesozoic Era.Era.

The Cenozoic Era: 65 The Cenozoic Era: 65 million years ago – today.million years ago – today.

Many mountain ranges formed, Many mountain ranges formed, perhaps creating cooler climates perhaps creating cooler climates worldwide.worldwide.

Mammals continued to evolveMammals continued to evolve Many species became isolated as the Many species became isolated as the

continents continued to separate.continents continued to separate. Homo sapiens or humans appeared Homo sapiens or humans appeared

about 400,000 years ago.about 400,000 years ago.