Precambrian History 13.1 Precambrian Time: Vast and Puzzling The Precambrian encompasses immense geological time, from Earth’s distant beginnings 4.56

Precambrian History

13.1 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


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


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.

Early Paleozoic

13.2 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


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


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

seas.• Animals developed hard parts, such as shells,

for the first time.• This provided organisms with protection and an

“environment” for body organs to function more efficiently.

Life in the Ordovician Period

Cephalopods, trilobites, brachiopods, snails, and corals inhabited the waters of the Ordovician period.

snail

squid

corals

trilobites

Late Paleozoic


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


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

Armor-plated fish were common during the Devonian period. These armor-plates are similar to present day fish scales.

Model of a Pennsylvanian Period Coal Swamp

Large tropical swamps extended across North America, Europe, and Siberia. Trees approached 98 feet tall. The coal deposits that we use today for fuel originated in these swamps.

The Great Paleozoic Extinction


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.

Continental Positions

Plant Life Animal Life

Early Paleozoic

Late Paleozoic

Gondwana at the South Pole and other landmasses near the equator

Water Plants

Restricted to the seas (invertebrates)

Pangaea Land Plants Fish and amphibians

13.2 Assessment

1. What are the 7 periods that make up the Paleozoic era?

2. Which life forms dominated the early and late parts of the Paleozoic era?

3. What allowed amphibians to flourish on land?

4. Compare and contrast the life of Early Paleozoic and Late Paleozoic.

Writing in Science

• Imagine you are uncovering rocks and fossils from a site that formed during the Paleozoic era. Write a paragraph (5-7 sentences) describing what kinds of fossils you would expect to find as you dug from the surface and moved downward.

** Remember the Law of Superposition

Mesozoic Era

• Mesozoic era spanned about 183 million years

• It is divided into 3 periods: Triassic, Jurassic and Cretaceous

Mesozoic Era

13.3 Mesozoic Era: Age of Reptiles

Dinosaurs were land-dwelling reptiles that thrived during the Mesozoic era.

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

breakup of Pangaea.• Plate tectonic activity began

Mesozoic Era Mesozoic Life


• Gymnosperms are seed-bearing plants that do not depend on free-standing water for fertilization. (Example: Pinecone)

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

•When the Mesozoic era began, its life forms were the survivors of the Paleozoic extinction•On land, conditions favored life that could adapt to drier climates

Canadian Rockies Were Formed Throughout the Cretaceous Period

Mesozoic Era The Shelled Egg


• 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


• 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.

•Only few types of reptiles survived to recent time, including the turtles, snakes, crocodiles and lizards.

• Most scientists believe that extinction of these reptiles was caused by a large meteorite that collided with Earth. They believe this collision created huge quantities of dust that blocked out the sun, causing plants to die b/c they could not turn sunlight into food

• Without plants the huge herbivores could not find enough to eat and eventually could not survive and then carnivores could no longer find food.

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.

13.4 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.


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

Cenozoic Life Mammals Replace Reptiles


• 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


• 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.

Geologic Time Scale Review

Plant Life Animal Life Continental Position

Early Paleozoic

Late Paleozoic

Mesozoic

Cenozoic

Water Plants Restricted to seas

Gondwana

Land Plants Amphibians Pangaea

ReptilesGymnosperms(seeds)

Pangaea breaks up

Angiosperms(flowers) Mammals

Mtn. building & earthquakes

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Precambrian History 13.1 Precambrian Time: Vast and Puzzling The Precambrian encompasses immense geological time, from Earth’s distant beginnings 4.56