Geologic Time

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Geologic Time

• P. Lobosco

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The Geologic Time Scale

• Earths history on the geologic time scale is
  divided into four geologic eras Precambrian
  Era, Paleozoic Era, Mesozoic Era and Cenozoic
  Era. Eras are broken down into smaller divisions
  called periods. The Earth formed 4.6 billion
  years ago.

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Precambrian Era

• The Precambrian Era is the longest era. It
  lasted about 4 billion years or 87 of the
  Earths history. Life exists only in the sea.

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Life in the Precambrian Era

• Just beneath the seas surface there bacteria
  clumped together in a tangled mat of threadlike
  fibers. Plants related to modern seaweed are
  also in the seas. They use chemicals from the
  water and energy from the sun to make their own
  food. In the process they produce oxygen. By
  the end of the Precambrian Era, animals such as
  jellyfish, worms, corals and sponges have
  appeared in the seas.

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Paleozoic Era

• The Paleozoic Era lasted about 345 million years.
  In the Paleozoic Era, life comes ashore.

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Life in the Paleozoic Era

• Life is abundant in the seas, including
  trilobites, brachiopods, and fish. The land is
  no longer lifeless. Forests of ferns and cycads
  grow. Amphibians now appear. Late in the period
  reptiles appear.

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Trilobite

• The trilobite is an important index fossil for
  this era since they both evolve and become
  extinct in this era. They have large heads,
  thorny spines and body divisions.

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Brachiopods

• Parts of the ocean floor contain lampshade-
  shaped shells. The shells have two parts that
  close to cover and protect the soft animal
  within. These animals are brachiopods.

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Fish

• Fish are the first vertebrates, or animals with
  backbones. The Devonian Period is often called
  the Age of the Fishes.

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Life on Land in the Paleozoic Era

• Life on land now includes forests of ferns and
  cycads. Amphibians appear. By the end of the
  Paleozoic Era the Amphibians are reduced and new
  animals, reptiles appear on land.

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Amphibians

• Amphibians, such as Eryops, are the first land
  vertebrates. They spend their early lives in
  water and move to land. Eryops is much larger
  than the frogs and toads of the 20th century. It
  is 2 meters long with a large head and a thick
  clumsy body. It searches the forest for
  king-sized roaches. It must keep its skin moist
  so it cannot move far from water.

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Reptiles

• By the end of the era, amphibians have been
  reduced since there is a drought and the climate
  has cooled. Mountains are rising and all the
  landmasses are joined together. The reptiles
  appear. Their tough skin is protected by scales
  or plates. Unlike amphibians, they do not lose
  water through their skins. Their eggs have thick
  shells so they do not dry out.

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Mesozoic Era

• The Mesozoic Era lasted about 160 million years.
  The Mesozoic Era is a period of many changes.
  Pangaea begins to break apart. The expansion
  along the ocean floor causes the continents to
  gradually spread apart. Mountain ranges, such as
  he Sierra Nevadas and the Rockies were formed.
  The Mesozoic is divided into three periods,
  Triassic, Jurassic and Cretaceous.

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The Triassic Period

• The drought that began in the Paleozoic Era has
  not ended. It is even hotter. North and South
  America have begun to separate from Africa. A
  narrow sea forms that will become the North
  Atlantic Ocean. Sea creatures that are reptiles,
  have fingerlike projections, and lungs have
  returned to the sea. Mammals appear. The first
  dinosaurs appear.

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The Jurassic Period

• The Age of Dinosaurs has begun. Volcanoes are
  active in the American West. The mountains of
  the Sierra Nevada and the Rockies are still
  rising. The southern continents are beginning to
  separate. Conifers make up the forest. The
  first bird, Archaeopteryx, appears.

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The Cretaceous Period

• The Cretaceous Period is a time of flooding of
  the continents. The continents continue to move
  apart. By the end of the period the continents
  are almost in their present condition except
  North America and Europe are still joined.
  Dinosaurs rule the world but by the end of the
  period all of the dinosaurs and sea living
  reptiles will have died. Sea levels drop and
  rivers dry up.
• Flowering plants appear.

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Cenozoic Era

• The Cenozoic Era, our present era, has only
  lasted about 65 million years. The Cenozoic Era
  is divided into two periods, the Tertiary Period
  and the Quaternary Period. Great sheets of ice
  will sweep across the land.

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The Tertiary Period

• The Uintatherium was one of the largest mammals
  ever. It existed with the ancestor to the
  rhinoceros. The first humanlike creatures begin
  to walk upright on the African Plains. The
  remains of a small adult female from this period,
  about 1 meter tall, is discovered in 1977. She
  is named Lucy.

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The Quaternary Period

• The climate turns sharply colder. Four ice ages
  occur. Large parts of Europe, North America and
  South America are covered in sheets of ice. The
  Ice age ends about 11,000 years ago. AS the
  world warms, farming becomes widespread and
  modern civilization begins.

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Evolution

• The world evolution comes from Latin and means an
  unfolding or opening out. A scientific
  translation is descent with modification.
  Evolution can be defined as a change in a species
  over time. A species is a group of organisms
  that share similar characteristics and that can
  interbreed with one another to produce fertile
  offspring.

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Mutations

• A change in a gene will produce a change in the
  offspring of an organisms. A change in a gene is
  called a mutation. Usually a mutation produces an
  organism that cannot survive. A change that
  increase the chance of survival is called an
  adaptation. The development of a new species can
  result in the extinction of another.

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The Fossil Record

• The fossil record indicates that species have
  evolved over time. For example, the discovery of
  the skull of a mammal similar to a whale in 1983
  with a structure for hearing that could not have
  worked underwater led scientists to infer that at
  one point the whale lived on land.

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Mass Extinction

• During 1640 and 1859 species began to disappear
  at the rate of one every five years. Between
  1850 and 1900 the rate increased to one lost
  every nine months. At the current rate
  scientists estimate that 1/5 of the species in
  the rainforest will be gone in 50 years.

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