EARTH SCIENCE
LESSON: GEOLOGIC TIME SCALE
Index Fossils • (2.5 billion to 542 million years ago)
Index or Guide Fossils • The first continents appeared in this eon. This
was also the time when continests merged into a
• Organisms that existed for a short period or single land mass called Rodinia.
time, then went extinct.
• Fossils of bacteria, archea, and eukaryotic cells
• E.g., Branchiopods, Ammonite, Trilobite. were found in this eon.
To be considered an index fossil, it must meet 3 criteria: • Levels of oxygen in the atmosphere fluctuates,
1. The fossilized organism must be easily- but continuous biological activity allowed
recognizable. It must be easy to ID and look atmosphere oxygen levels to increase.
unique. • Marked by the appearance of multicelluar
2. The fossils must be geographically widespread organisms and their mass extinction.
or found over large areas so that we can use
them to match layers separated by huge
Paleozoic Era
distances.
3. The fossil must have lived for only a short time, • (541 to 252 million years ago)
so that it appears in only horizontal layer of • Ancient life
sedimentary rocks. • The supercontinent Pangea was formed.
• Divided by the following periods: Cambrian,
Ordovician, Silurian, Devonian, Mississippian,
Geologic Time Scale Pennsylvanian, and Permian.
• The study of the Earth from the beginning of time Ordovician Period
to the present has been the task of geologists
who attempt to unfold the secrets of the events • A rich diversity of marine life.
that have shaped the planet. • Diversification of ammonites and belemnites.
• Through rocks and fossil correlations, they were • First primitive land plants appeared.
able to establish the geologic time scale which • Rapid seafloor spreading.
shows the sequence of events that happened in • First mass extinction
the past up to the recent time. Silurian Period
Periods • Recovery of biodiversity.
• A division of geologic history that spans no more • Dominance of invertebrates still continued.
than one hundred million years. • Diversification of euryptids.
• Fish with movable jaws (placoderms) appeared.
• Bony fish appeared.
Hadean Eon
• Diversification and peak of Ostracoderms.
• (4.5 to 4 billion years ago)
Devonian Period
• “Rockless eon”
• During this time, the earth was molten and • Also known as Age of Fishes.
would only solidify as it cooled. • Fishes became abundant and diverse.
• Earth’s continental and oceanic crusts began to • Emergence of first amphibians.
solidify, marking the beginning of the Earth’s • Terrestrial plants became abundant.
geologic history. • Second mass extinction.
Carboniferous Period
Archean Eon • Formation of Pangea.
• (4 to 2.5 billion years ago) • Pennsylvanian and Mississippian subperiods are
part of the Carboniferous.
• Single-celled organisms, such as blue green
algae, archeans, and bacteria, first appeared. • Known for huge swathes of rainforests across the
globe.
• Marks the beginning of the free oxygen in the
atmosphere. • Climate change is extreme
Permian Period
Proterozoic Eon • Also known as the Age of Amphibians
EARTH SCIENCE
LESSON: GEOLOGIC TIME SCALE
• Colonization of land vertebrates • Age of insects.
• Diversification of sharks • This era is divided into these periods: Paleogene,
• Extinction of trilobites Neogene, and Quaternary.
• Phytoplankton and plants supplied oxygen to • These periods are also divided into epochs due
the Earth’s atmosphere which was close to the to the vast amounts of observable evidences in
present level. this era as compared with other eras of the
• Rise of synapsids. geologic time scale.
• Ended with the largest and third mass extinction Paleocene Period
recorded on Earth. • (65.5 to 23.03 million years ago)
o AKA The Great Dying or; • Paleocene epoch
o Permian-Triassic Extinction Event
• With the absence of major groups of dinosaurs,
the mammals slowly took over every niche in the
Mesozoic Era Earth.
• (225 to 66 million years ago) • Diversification of birds
• “Middle life” • First primates begin to appear.
• Age of Reptiles • Ancestors of elephants appear.
• Breakout of Pangea opened for hot and humid Neogene Period
greenhouse climate. • (23.03 to 2.6 million years ago)
• Divided by the following periods: Triassic, • Divided into tow epochs: Miocene and Pliocene.
Jurassic, and Cretaceous. • Miocene epoch
Triassic Period o Characterized by a warmer climate than
• The first dinosaur appeared. the Oligocene epoch.
• Emergence of plesiosaurs o Arabian Peninsula collided with Eurasia,
• Emergence of pterosaurs which allowed faunal interchange across
• Emergence of lineages of modern-day lizards, Eurasia and Africa.
snakes, crocodiles, and chelonians. o Entry of proboscideans to Eurasia.
• Emergence of the first mammals. o Horses continued to evolve.
o Climate became warmer.
Jurassic Period
• Pliocene epoch
• Dinosaurs diversified during the Jurassic period. o Marked by cooler and more arid climate
• Cone bearing plants are still dominant. compared to Miocene epoch.
• Emergence of the first bird—Archaeopteryx. o Ice accumulated at the poles, resulting in
Cretaceous Period the extinction of the species there.
o Australopithecus was thought to
• Except for birds, dinosaurs became extinct
evolved in this epoch.
during the Cretaceous period.
• Fourth mass extinction Quaternary Period
• Earliest angiosperms started to emerge. • (2.6 million years ago to present)
• Divided into two epochs: Pleistocene and
Holocene.
Cenozoic Era
• Many species of living things today were already
• (66 million years ago to the present) existing.
• “New Life” • Many large animals like mammoths went
• Most recent of the three major subdivisions of extinct.
Phanerozoic eon. • Homo sapiens spread out worldwide.
• Sometimes called the Age of Mammals, though • Pleistocene epoch
the history of mammals began long before this o Characterized by the global cooling—Ice
era and the diversity in this era is not only limited Age
to mammals.
• Age of Flowering Plants
