Geologic Time Periods
Divisions of Time
Eon: Longest time unit, measured in billions of years.
•Archean: 4600 MYBP
•Proterozoic: 2500 MYBP
•Phanerozoic: present
The Three Eras
Era: measured in hundreds of millions to billions of years, defined by differences in life forms
Precambrian Time lasted 4 billions years/87% of the earth’s history, not considered an era because it is lacking
life
The
three geologic time eras are
lasted about 345 million years
Mesozoic lasted about 160 million years
Cenozoic is the era in which we live today, and has lasted for only 65 millions years to date.
Paleozoic
The Periods
Period: measured in tens of million to hundreds of millions of years, defined by life-forms that were abundant
or became extinct
Cambrian, Ordovician, Silurian, Devonian, Carboniferous (Mississippian & Pennsylvanian), Permian, Triassic,
Jurassic, Cretaceous, Tertiary, Quaternary
Epochs
Epochs: measured in millions of years, defined by:
groups of organisms (marine fossils, terrestrial plant fossils)
In The Beginning…
4.6 billion years ago, the Earth is a planet of molten rock. The atmosphere contains mainly poisonous gases.
There are no records of rocks from this time because of the widespread volcanic eruptions.
1.5 billion years later, the Earth is cooler. Continents and oceans have formed. Little oxygen exists. Most of
the air is sulfur dioxide gas from the volcanoes. Most of the water is highly acidic with little CO2 to support
life) This is the time that life begins.
Precambrian: The Dawn of Life
Imagine rocky shorelines with no plants or animals. The only life in the seas are patches of millions of bluegreen bacteria (cyanobacteria) clumped together called Stromatolites. They produce small amounts of oxygen
and are the oldest fossils ever found. At the end of this Era there is sea weed, jellyfish, worms, coral, and
sponges appearing in the seas.
Varangian Glaciation: Major Ice Age that extended down to the equator, earth resembled a gigantic snowball
The Paleozoic Era: Life Comes Ashore
570 mya…Land is lifeless but life is now abundant in the seas. Trilobites become the index fossils for this time
because they developed and became extinct during this era.
Cambrian Period
“Big Bang” of life
Trilobites, brachiopods, marine invertebrates were abundant
Burgess Shale very important fossil find for this period
Brachiopods are bottom dwelling, marine, bivalves. They are considered living fossils, with 3 orders present in
today’s oceans. They are rare today but during the Paleozoic Era they dominated the sea floors.
Ordovician Period
First
Fish Appear
Appalachians form for the first time
Silurian Period
First land plants appeared
First Insects they were non-flying… think beetle like
Coral Reefs form in tropical oceans
Air breathing animals
Devonian Period
There are brachiopods on the sea floor. Fish can be found almost everywhere. They are the first vertebrates
(with backbones). This is known as the Age of the Fishes.
First sharks and amphibians
Carboniferous Period
The Carboniferous Period is broken into two parts; the Mississippian and the Pennsylvanian Periods
During the Mississippian Period the Amphibians appear and are called Eryops. They are the dominant creature
Vast limestone deposits (MS) were created by inland seas and then overlain by coal deposits (PN) created by
swamps
Winged insects, swamps that later became the coal deposits found.
Seed Ferns are among the first seed plants. By the end of the Era, the climate has cooled and mountains are
rising in what will become Norway, Scotland, and Greenland. There is one single landmass that will later be
named Pangaea.
Permian Period
Reptiles flourish
Coal Swamps form, cone bearing plants
Shallow seas begin to withdraw
Extinction of the Trilobites and sea invertebrates (90% of all living organisms) This extinction ended the
Paleozoic Era
Paleozoic Review
The Paleozoic era lasted from about 540 to 250 million years ago, and is divided into six periods The 320-odd
million years of the Paleozoic era saw many important events: development of most invertebrate groups, life's
conquest of land, the evolution of fish, reptiles, insects, and vascular plants, the formation of the supercontinent
of Pangea, and no less than two distinct ice ages. The earth rotated faster than it does today so days were
shorter, and the nearer moon meant stronger tides.
Era ended with a long period of drought, volcanism and acid rain introduced into the environment or a large
comet/meteor… either way 95% of species (marine and terrestrial) die out
The Mesozoic Era
Mammals develop 225 mya. Pangaea begins to break apart. Expansion of the ocean floor begins, earthquakes
and volcanoes form many mountains including the Rocky Mts. and the Sierra Nevada.
The Triassic Period
There is a drought, climate is hotter.
The North Atlantic Ocean is formed,
Reptiles abundant, Mammals appear,
Ferns are still common and the cycads appear
The trees from the Petrified Forest in Arizona are uprooted by floods.
The first dinosaurs appear but are barely bigger than chickens.
Modern insects, fish and coral develop
The Jurassic Period
The Age of the Dinosaurs has begun. Volcanoes are active, mountains are still rising, animals and plants are
similar throughout the “super continent”. There are modern-looking evergreens in the forests but no flowers yet.
First birds appear.
Cretaceous Period
Widespread flooding of continents by seas.
Continents
continue to move apart.
Dinosaurs still dominate the world… tyrannosaurus, triceratops
By the end of this period all the dinosaurs will and the sea living reptiles will die out. Only crocodiles, turtles,
lizards, and snakes will survive.
First flowering plants
End of the Mesozoic Era
2 most recognized theories
A severe change in the climate
An asteroid crashed into the earth and blocked out the sun for days and days.
The Cenozoic Era
Tertiary Period
Tertiary Period – 65 mya - birds, redwoods, oaks, and cedars grow.
The climate is mild worldwide.
Humanlike creatures begin walking upright. In Africa the oldest skeleton is found, named Lucy, her frame is
only 1 meter tall (~3 feet) is found in 1977.
Mammals take on present day features
Alps, Himalayas, Andes form
Early Life….
Modern Humans: The Neanderthals met up with a new group of people, the Cro-Magnons, during the
Neanderthal movie. The fossil record shows a gap (unconformity in species)… the Neanderthal fossils
disappear and the Cro-Magnon's appear. What happened to the Neanderthal? Will we ever know?
The current theory states that the Neanderthals were driven to extinction through competition with the CroMagnon. Greater linguistic competence (language) and cultural changes are suggested for tilting the
competitive balance in favor of Cro-Magnon’s.
They made stone and bone tools that were much lighter and stronger. Shell and ivory jewelry, and paintings
found on cave walls all testify to the cultural advancement of Cro-Magnon man.
Quaternary Period… our time
Quaternary Period – the climate turns sharply cold. Great sheets of ice advance from the poles.
The ice age is like a winter that lasts for centuries. It ends about 11,000 years ago, Great Lakes form
Mammoths die out… why?
Modern humans appear
Everything from this point on is considered modern history.
Evolution
Evolution is a change in a species over time
Organisms evolve because they can no longer compete with other organisms or live in their environment any
longer
A change in the genetic make-up of the organism (mutation) increases its chances of survival, this is called
adaptation
Over a long period of time (1,000’s of years) there are many small adaptations. The new species may not
resemble its ancestors due to these changes.
The fossil record shows that organisms do evolve over time