Precambrian History

18.2 Precambrian Time: Vast and Puzzling

The Precambrian encompasses immense geological time, from Earth’s distant beginnings 4.56 billion years ago until the start of the Cambrian period, over 4 billion years later.

Precambrian Rocks• Shields are large, relatively flat expanses of

ancient metamorphic rock within the stable continental interior.

• Much of what we know about Precambrian rocks comes from ores mined from shields.

Geologic Time Scale

Remnants of Precambrian Rocks

Precambrian History

18.2 Precambrian Time: Vast and Puzzling

Earth’s Atmosphere Evolves• Earth’s original atmosphere was made up of

gases similar to those released in volcanic eruptions today—water vapor, carbon dioxide, nitrogen, and several trace gases, but no oxygen.

• Later, primary plants evolved that used photosynthesis and released oxygen.

• Oxygen began to accumulate in the atmosphere about 2.5 billion years ago.

Precambrian History

18.2 Precambrian Time: Vast and Puzzling

Precambrian Fossils• The most common Precambrian fossils are

stromatolites.

• Stromatolites are distinctively layered mounds or columns of calcium carbonate. They are not the remains of actual organisms but are the material deposited by algae.

• Many of these ancient fossils are preserved in chert—a hard dense chemical sedimentary rock.

Stromatolites

• Stromatolites of Hamelin Pool

Early Paleozoic

18.2 Paleozoic Era: Life Explodes

Following the long Precambrian, the most recent 540 million years of Earth’s history are divided into three eras: Paleozoic, Mesozoic, and Cenozoic.

Early Paleozoic

18.2 Paleozoic Era: Life Explodes

Early Paleozoic History• During the Cambrian, Ordovician, and Silurian

periods, the vast southern continent of Gondwana encompassed five continents (South America, Africa, Australia, Antarctica, and part of Asia).

Gondwana and the Continental Landmasses

Early Paleozoic

18.2 Paleozoic Era: Life Explodes

Early Paleozoic Life• Life in early Paleozoic time was restricted to the

seas.

Life in the Ordovician Period

Late Paleozoic

18.2 Paleozoic Era: Life Explodes

Late Paleozoic History• Laurasia is the continental mass that formed the

northern portion of Pangaea, consisting of present-day North America and Eurasia.

• By the end of the Paleozoic, all the continents had fused into the supercontinent of Pangaea.

Late Paleozoic Plate Movements

Late Paleozoic

18.2 Paleozoic Era: Life Explodes

Late Paleozoic Life• Some 400 million years ago, plants that had

adapted to survive at the water’s edge began to move inland, becoming land plants.

• The amphibians rapidly diversified because they had minimal competition from other land dwellers.

Armor-Plated Fish

Model of a Pennsylvanian Coal Swamp

The Great Paleozoic Extinction

18.2 Paleozoic Era: Life Explodes

The world’s climate became very seasonal, probably causing the dramatic extinction of many species.

The late Paleozoic extinction was the greatest of at least five mass extinctions to occur over the past 500 million years.

Mesozoic Era

18.2 Mesozoic Era: Age of Reptiles

Dinosaurs were land-dwelling reptiles that thrived during the Mesozoic era. (Jurassic Period)

Mesozoic History • A major event of the Mesozoic era was the

breakup of Pangaea.

Mesozoic Era Mesozoic Life

18.2 Mesozoic Era: Age of Reptiles

• Gymnosperms are seed-bearing plants that do not depend on free-standing water for fertilization.

• The gymnosperms quickly became the dominant plants of the Mesozoic era.

Canadian Rockies Were Formed Throughout the Cretaceous Period

Mesozoic Era The Shelled Egg

18.2 Mesozoic Era: Age of Reptiles

• Unlike amphibians, reptiles have shell-covered eggs that can be laid on the land.

• The elimination of a water-dwelling stage (like the tadpole stage in frogs) was an important evolutionary step.

Mesozoic Era Reptiles Dominate

18.2 Mesozoic Era: Age of Reptiles

• With the perfection of the shelled egg, reptiles quickly became the dominant land animals.

• At the end of the Mesozoic era, many reptile groups became extinct. (end of the Cretaceous)

The Flying Reptile Pteranodon

Fossil Skull of an Extinct Crocodile

Cenozoic North America The Cenozoic era is divided into two

periods of very unequal duration, the Tertiary period and the Quaternary period.

18.2 Cenozoic Era: Age of Mammals

Plate interactions during the Cenozoic era caused many events of mountain building, volcanism, and earthquakes in the West.

Cenozoic Life Mammals—animals that bear live young

and maintain a steady body temperature— replaced reptiles as the dominant land animals in the Cenozoic era.

18.2 Cenozoic Era: Age of Mammals

Angiosperms—flowering plants with covered seeds—replaced gymnosperms as the dominant land plants.

Cenozoic Life Mammals Replace Reptiles

18.2 Cenozoic Era: Age of Mammals

• Adaptations like being warm blooded, developing insulating body hair, and having more efficient heart and lungs allow mammals to lead more active lives than reptiles.

Fossils from La Brea Tar Pits

Cenozoic Life Large Mammals and Extinction

18.2 Cenozoic Era: Age of Mammals

• In North America, the mastodon and mammoth, both huge relatives of the elephant, became extinct. In addition, saber-toothed cats, giant beavers, large ground sloths, horses, camels, giant bison, and others died out on the North American continent.

• The reason for this recent wave of extinctions puzzles scientists.

The Geologic Time Scale

Paleozoic Era (six periods) Means “Ancient Life”

Cambrian Period (marine invertebrates) (540 million years ago)

Trilobites

Brachiopods

Ordovician Period (primitive fish) vertebrates (490 million yrs. ago)

Ostracoderms (bony plated fish)

No plant life on land

Silurian Period (marine vertebrates and invertebrates) (443 mya)

Eurypterids

Earliest land plants and animals such as spiders and

Millipedes

The Geologic Time Scale

Devonian Period (Age of Fishes)(417 mya)

Lung fish

Ichthyostega (first true amphibian)

Land plants such as giant horsetails, ferns and cone bearing plants

Carboniferous Period(Mississippian 354 mya,Pennsylvanian 323 mya)

Forests and swamps cover much of the land

Coal formation begins

Amphibians and fish thrive

Crinoids (relative of modern sea star) thrive

Reptiles appear at the end of this period

Permian Period (290 mya)

Mass extinction of numerous life forms

Appalachian Mountains form

Trilobites become extinct

Pangaea comes together

The Geologic Time Scale

The Mesozoic Era (three periods)

Triassic Period (248 mya)

Dinosaurs (Terrible Lizards)

Ranged in size from small squirrel to 30 meters long

Ichthyosaurs

Ammonite (shellfish similar to the nautilus)

1st mammals appear

Jurassic Period (206 mya)

Apatosaurus, Stegosaurus, pterosaurs(flying reptiles)

First birds appear

Cretaceous Period (144 mya)

Tyrannasaurus Rex

Angiosperms (flowering plants)

Mass extinction of the dinosaurs

The Geologic Time Scale

The Cenozoic Era (The Age of Mammals)

Teritiary Period

Paleocene and Eocene Epochs (65 mya – 54.8 mya)

Lemuroids

Hyracotherium (ancestor of the horse)

Flying squirrels, bats, whales

Oligocene and Miocene Epochs (golden age of Mammals)

Largest known land animals existed at this time

Raccoons, wolves, foxes, saber toothed cat

Modern polar ice caps began to form

Pliocene Epoch

Bear, dog, cat became fully evolved

Continental ice sheets began to spread

Bering Land bridge appeared as sea levels fell

The Geologic Time Scale

Quaternary Period

Pleistocene Epoch

Glaciation over Eurasia and North America

Large mammal extinctions as humans entered the

Holocene Epoch

Ice age ends, sea levels rise 140 meters

Great Lakes form

Humans developed agriculture

Questions

1. Why are fossils rare in Precambrian rocks? Because possible fossils have been destroyed by weathering, erosion, volcanic

activity and metamorphism and because early life forms lacked the hard parts that normally fossilize well

2. How did the formation of Pangaea affect Paleozoic life-forms?

The shallow inland seas disappeared, causing many species of marine invertebrates to die out

3. How did the ice ages affect animal life during the Cenozoic Era?

Warm-blooded, fur-covered mammals survived and became the predominant life-form

4. Compare the Permian extinction with the Cretaceous extinction. Both extinctions allowed a new group of animals to become the dominant life-form in

the following era Permian extinction=reptiles became dominant

Cretaceous extinction=mammals became dominant