Geologic Time Scale Unit 6, Part 3 Our Changing Past
I. Proof Organisms change over time
A. ______________: prove that types of organisms on Earth have changed throughout
history (ex. _____________)
B. ___________: proof that the Earth’s plates have moved over time (example: there are
______________ fossils found in the rocks on Mt. Everest)
C. ____________: when scientists drill into polar ice caps, there is evidence that the
climate has changed throughout Earth’s history (ex. Ocean temperature being raised
as a result of _______________ impacts the organisms living in the ocean.)
II. How has the Earth’s surface changed?
A. ______________
B. ______________
C. ______________
D. ______________
E. ______________
III. What is the geologic time scale?
A. The _____________ or ______________ of types of organisms throughout Earth’s
history marks important occurrences in geologic time.
B. ______________________ have been able to divide Earth’s history into time units
based on the ______________ that lived only during certain periods.
C. This division of Earth’s history makes up the ______________________.
A. The oldest rocks on Earth contain ___________________.
B. Later in Earth’s history came an explosion in the _______________ and
_____________ of organisms.
C. These organisms left a _____________________.
V. Four Major Subdivisions
A. There are four major subdivisions of geologic time:
1. ______: the longest subdivision based on the abundance of certain fossils
2. ____: marked by major, striking, and worldwide changes in the types of fossils
present
3. _______: divide eras up into smaller units characterized by the types of life
existing worldwide
4. _______: divide periods up into smaller units characterized by the type of life
existing from continent to continent
VI. Geologic Time Scale (see diagram)
VII. Precambrian time
A. Precambrian time is the _______________ of Earth’s history.
B. It includes the ____________, ____________, and ______________ Eons.
C. It last from 4.5 billion years ago to about 544 million years ago.
D. The oldest rocks on Earth have been dated to about _____________ years ago, but
they are rare.
E. There is relatively little known about the _______________ that lived during this
F. The rocks are so deeply buried that they have been changed by ____________ and
____________________ and fossils cannot stand these conditions.
VIII. Paleozoic Era
A. Known as the “_____________________”
B. Began just after the Precambrian Era, about 544 million years ago to about 248
million years ago
C. Many of the life forms discovered in this time were ___________, living in the vast
seas that covered the Earth.
D. _____________ were common as well as other hard-shelled _______________
(animals without a backbone).
E. _________________ (animals with backbones) were discovered in this era as well.
F. The first vertebrates to be discovered were _______________________________.
G. Some fish discovered in this time had _____________________, which indicates
they could live on ______ as well as water.
H. Paleontologists believed ___________ might have evolved from fish with leg-like
fins and lungs, and later in this era evolved into __________ who no longer needed
bodies of water to live and survive.
IX. Paleozoic Era (con’t.)
A. The __________________________ formed during this time.
B. The formation of mountains occurred in several stages:
1. ____________ Islands formed in the Atlantic were ________ against the ______
of North America, while at the same time Africa _______________ North
2. The African plate ______________ with the North American plate, forming
mountains on ____________________.
3. About 200 million years ago the ______________________ opened, separating
the two continents.
4. The end of this era is marked by ____ of all _____ species and _____ of all land
species having died off.
5. This ____________________ occurred because of changes in climate, a lowering
of the sea level, or a catastrophic event.
6. The _____________________, at the end of this era, is when it is suspected that
the continental plates came together to form the supercontinent ____________.
X. Pangaea
A. It is believed that glaciers covered the southern part of Pangaea.
B. The gradual collision of the continents caused mountains to form.
C. The seas closed and deserts spread over much of North America and Europe.
D. Many species could not adapt to these changes which led to the mass extinction and
the end of the era.
XI. Mesozoic Era
A. Also called the “era of middle life”
B. All the continents were still joined together as Pangaea
C. During the Triassic Period Pangaea separated into two landmasses called Laurasia
and Gondwanaland.
E. The animals that survived the mass extinction at the end of the Paleozoic Era adapted
to the changing environments, believed to much drier in the Mesozoic Era.
XII. Mesozoic Era: Dinosaurs
A. The Mesozoic Era is the age of the dinosaurs, specifically beginning in the Triassic
Period with small dinosaurs.
B. The Jurassic and Cretaceous Periods saw larger dinosaurs.
C. The dinosaurs continued to evolve, adapt and some became extinct during this era.
D. Paleontologists study fossilized foot prints to calculate how fast dinosaurs
walked/ran.
E. They also study their bone structure and some evidence suggests some species of
dinosaurs were actually warm-blooded like modern day mammals.
XIII. Mesozoic Era: Mammals
A. The first mammals appeared during the Triassic Period.
B. The earliest mammals are believed to be small mouse-like creatures.
C. Remember a mammal is a warm-blooded animal that has hair covering its body, feed
their young with milk produced by the mother’s body, and are vertebrates.
XIV. Mesozoic Era: Birds
A. Birds first appeared in the Jurassic Period.
B. Some paleontologists believed birds evolved from small, meat-eating dinosaurs.
C. The earliest bird, Archaeopteryx, had wings and feathers.
D. BUT, paleontologists cannot confirm this dinosaur bird is a direct descendent of
modern day birds because it also had features not associated with today’s birds.
A. Angiosperms are flowering plants.
B. They first evolved during the Cretaceous Period.
C. Angiosperms produce seeds with hard outer coverings which protect them allowing
the angiosperms to live in many environments.
D. Today angiosperms are the most abundant and diverse plants alive.
E. Modern day angiosperms include magnolia and oak trees.
XVI. Mesozoic Era: end
A. The Mesozoic Era ended approximately 65 million years ago with a mass extinction
of land and marine animals.
B. Many groups of animals, including the dinosaurs, disappeared suddenly.
C. Paleontologists believe a comet or asteroid collided with the Earth, creating a huge
cloud of dust and smoke that blocked the sunlight from reaching the Earth’s surface.
D. Without sunlight, all the animals and plants that rely on it died.
E. There is a crater in Mexico and worldwide deposits of ash and iridium (element found
in asteroids) that support this hypothesis.
XVII. Cenozoic Era
A. Also called the “era of recent life”
B. This is the era we are currently living in and spans back 65 million years ago.
C. Many mountain ranges in North and South America and Europe began to form.
D. The ice ages occurred during this era.
E. Homo sapiens (or humans) appeared approximately 140,000 years ago and may have
caused the extinction of other mammals because early evidence suggests early
