Unit 2 part2
Geologic time scale.
MESOZOIC
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PROTEROZOIC
Divisions of Geologic Time
Eon,
Largest span of time
Era,
Period,
Epoch
Smallest span of time
Scientist and their contribution to the Geologic Time Scale.
Nicolaus Steno
 Principle of Original Horizontality.
 Principle of Superposition.
James Hutton and Charles Lyell
 Principle of Uniformitarianism
William Smith
 Principle of Faunal Succession
Life and Geologic Time
Majority of life in the history of Earth, 4.6 billion years, is confined to the past 600 million years.
This life as outline by fossil evidence in the geologic time scale is grouped according to the three eras of
time.
1) Paleozoic
2) Mesozoic
3) Cenozoic
Little direct evidence of life is seen from Precambrian time. Precambrian time represents 87% of Earth’s
history and most evidence of life during this time exist as trace fossils.
Precambrian (4600 – 570 million years ago)
Rocks from this time make up the core of all major land masses, called Precambrian shields.
Earliest evidence of life focuses on bacteria and algae seen in Precambrian rocks dating back to 3.5
billion years. This main fossil is called Stromatolites. Plant fossils are dated to the mid-Precambrian and
animal fossil to the late Precambrian, mainly as trace fossils.
As the Precambrian came to a close, the fossil record showed multi-cellular organisms which made it
possible for the evolution of more complex plants and animals in the Paleozoic era.
There was an explosion of life at end of Precambrian (start of the Phanerozoic
eon, Paleozoic era, Cambrian period)
Paleozoic (570 – 245 million years ago)
The Paleozoic era represents more than one dominant form of life. For example, the early part of the
Paleozoic era is referred to as the Age of Invertebrates, the middle part of the era is referred to as the
Age of Fishes, and the late (most recent) part of the Paleozoic Era is referred to as the Age of
Amphibians.
The beginning of the Paleozoic era is marked by the first appearance of life-forms which had hard body
parts. Thus our knowledge of the diversity of life improves as throughout Paleozoic time.
Abundant fossils since the early Paleozoic allowed scientist to construct a detailed time scale for the last
13% of geologic time.
To study the vast amount and continually changing life, the Paleozoic time can be divided as Early
Paleozoic (Cambrian, Ordovician, Silurian) and Late Paleozoic (Devonian, Mississippian, Pennsylvanian,
Permian).
Early Paleozoic (570 – 408 million years ago)
Life during this time was restricted to the seas. Consisted of several invertebrate groups.
Organisms started to excrete material which formed hard outer coverings, such as shells. This may have
been for protection.
Cambrian
- Trilobite evolved.
Ordovician
- Brachiopods and Cephalopods evolved.
- First fish (vertebrate) evolved.
Silurian
- First land plants appear.
Late Paleozoic (408 - 245 million years ago)
Plant life at the waters edge moved in land and formed forests by the end of the Devonian.
In the oceans the first fish (armor-plated fish) evolved to become more mobile.
Devonian
Marked the evolution of; sharks and first bony fish. This period is referred to as the “age of fishes.”
Lung fish and lobe-finned fish adapt to land environments. It is believed that these organisms evolved
into amphibians by the end of the Devonian period. Plant life at the waters edge moved in land and
formed forests by the end of the Devonian.
In the oceans the first fish (armor-plated fish) evolved to become more mobile.
Mississippian and Pennsylvanian
Insects and amphibians abundant on land.
Large coal swamps present.
It is believed that the first retile evolved during this time. Possibly from one species of amphibians.
Permian
Reptile increase in numbers.
Mass extinctions of approx 95% of all life. Possible explanation involve the formation of the
supercontinent Pangaea. Extinction of trilobite is most common example.
Mesozoic (245 - 70 million years ago)
Organisms that survived the extinction began to diversify . On land the dinosaur became dominant and
this era is called the “age of reptiles – dinosaur.”
Drier climates dominated during this era and life had to adapt. Gymnosperms (seed bearing plants) and
reptiles dominated land environments.
Reptiles have shell covered eggs that could be laid on land. This eliminated the water stage of
reproduction as seen with amphibians. This one evolution break through enabled reptiles to dominate
Earth for the next 160 million years until the end of the Mesozoic, 66 million years ago.
One form of reptile made a remarkable step in evolution, it was the first to take to the sky and fly.
These were referred to as dragons of the sky, and were called the pterosaurs. Jurassic time marked the
first evidence of birds in the fossil record.
First flowering plants evolved at the end of the Jurassic period.
At the end of the Mesozoic many reptile groups became extinct. Many dinosaur groups are only known
through the fossil record
Cenozoic (66 million years ago to Present)
Life during this time evolved into the modern life-forms.
Mammals replaced the reptiles as the dominant animals on Earth. This era is called the “age of
mammals.” Flowering plants became the more dominant plant life.
The development and specialization of mammals throughout the Cenozoic took four main directions;
Increase in size.
Increase in brain capacity
Specialization of teeth.
Specialization of limbs.
Time frames that correlates with the dominant life form on Earth.
(i) Cenozoic -Age of Mammals
(ii) Mesozoic -Age of Reptiles
(iii) Paleozoic (late) -Age of Amphibians
(iv) Paleozoic (middle) -Age of Fishes
(v) Paleozoic (early) -Age of Invertebrates
Summary of the dominant life forms present at each era.
1.
2.
3.
4.
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6.
7.
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Single-celled and other simple life forms (Precambrian)
Invertebrates (early Paleozoic)
Fishes (middle Paleozoic)
First land plants (between early and middle Paleozoic)
Amphibians (late Paleozoic)
Reptiles (Mesozoic)
Birds (Mesozoic)
Flowering plants (Mesozoic)
Mammals (Cenozoic)
You may use the following mnemonic device to remember the dominant life forms present at each Era:
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“Evolution through Geologic Time Student Worksheet” (Topic 1 - Unit 4).
Major events in earth’s history and how they fit into the geologic time scale:
Pleistocene Glaciation A great Ice Age occurred between 1.6 million and 10,000 years ago, causing
widespread glaciation on northern continents.
With study, scientists have become convinced that ice had advanced and retreated over the continents
several times in the recent geological past. The Ice Age (Pleistocene Epoch) can be divided into four
major stages of glaciation in North America, named for the states where deposits of a particular period
were first studied or where they are well exposed: Nebraskan, Kansan, Illinoian ,Wisconsinan, - actually
multiple glaciation events.
About 27% of the land surface was covered by ice during the Wisconsin age, and the glaciers were up to
3,000 meters thick. In North America, ice reached as far south as New Jersey in the east, St. Louis in the
mid-west, and to southcentral New Mexico in the western mountains. Greenland, Scandinavia, Great
Britain, Ireland, and part of northern Russia were also covered with ice.
The Appalachian Orogeny is an excellent example of a mountain building event resulting from tectonics.
The Appalachian Orogeny resulted from three separate continental collisions formed the Appalachian
Mountains and Allegheny Mountains at the end of the Paleozoic( about 248 mya.
The combined continents of Europe and Africa (Gondwana) collided with North America to form the
supercontinent of Pangaea approximately 350 million to 300 million years ago during the
Carboniferous period.
The collision exerted enormous stress on what today is eastern North America, an causing an enormous
upward thrusting of the entire region. These stresses concurrently caused faults (mostly thrust faults
and some strike-slip faults) as well as folding. It is believed that the Appalachians once could have risen
as high as or even higher than the Himalaya (i.e., more than 4 ½ miles high). These mountains, while still
impressive in places, are mere remnants of their glory days during the Paleozoic. They have been worm
away by the relentless forces of erosion, forming vast amount of sediments in the process.
The Acadian orogeny is a middle Paleozoic mountain building event (orogeny), especially in the
northern Appalachians, between New York and Newfoundland.
The Acadian orogeny most greatly affected the Northern Appalachian region (New England
northeastward into the Gaspé region of Canada). The Acadian orogeny should not be regarded as a
single tectonic event, but rather as an orogenic era. It spanned a period of about 50 million years, from
375 to 325 million years ago.
The cause of this great period of deformation is a result of the plate-docking of a small continental
landmass called Avalonia (named after the Avalon Peninsula of Newfoundland). The docking of Avalonia
onto the composite margin of Ganderia and Laurentia resulted in the closing of a portion of the Rheic
Ocean.