Transcript Fossils & The Geologic Time Scale

Fossils & The Geologic
Time Scale
Fossils
• Preserved remains or traces of an
organism that lived in the past.
•Fossils are formed when organisms die and are
buried in sediment. Eventually the sediment builds
up and hardens to become sedimentary rock.
•law of superposition – oldest layer is at the
bottom and each higher layer is younger than the
layers below
Kinds of fossils
• Petrified - when minerals replace the
remains and they become rock
• Mold - when the shell remains and the
contents dissolve (hollow)
• Cast - when the mold becomes filled
with minerals that are not a part of the
original organism
PETRIFIED
CAST
MOLD
• Sometimes whole animals become
preserved intact, but this is very rare.
If an organism is surrounded by ice
or tar they might be discovered
looking much the same as they did
when they died.
AETOSAUR
FOUND IN
THE
NATIONAL
PETRIFIED
FOREST
Relative Dating
Relative dating: looks at where the fossil is
located to determine its age relative to other
fossils. This only works if the area has been
undisturbed.
Absolute Dating
• Uses radioactive elements near the fossils
to determine the actual age of the fossils.
•By determining the age of the radioactive
element, scientists can calculate the age of the
fossil buried nearby.
The absolute age
of fossils is
estimated by
dating associated
igneous rock and
lava flows.
Radioactive dating
• Radiometric dating or radioactive dating is
a technique used to date materials such as
rocks or carbon, in which trace radioactive
impurities were selectively incorporated when
they formed
Index Fossils
• Index fossils (also known as guide fossils,
indicator fossils or zone fossils) are fossils
used to define and identify geologic periods
(or faunal stages).
• Help geologist match rocks layers and
represent a type of organism that existed only
briefly
Fossil Record
• The dating of all
fossils is included in
the Geological Time
Scale. This scale
divides the time that
the earth has existed
into 4 eras.
• Eras are then divided
into periods based on
common events in
that time period.
The Geologic Timeline
• a record of the life forms and geologic
events in Earth’s history
PreCambrian
• Began with
the formation
of the Earth
4.6 billion
years ago.
• Bacteria
appeared 3.5
billion years
ago, followed
by algae and
fungi.
Paleozoic Era
• Divided into 5 periods:
• Cambrian period Sponges, snails, clams
and worms evolve
• Ordovician period First fishes evolved and
other species become
extinct
• Silurian period - Land
plants, insects and
spiders appear
• Devonian period Amphibians evolve and
cone-bearing plants
start to appear.
• Carbonferous period Tropical forests appear
and reptiles evolve.
• Permian period - Seed
plants become common
and insects and retiles
become widespread.
Sea animals and some
amphibians begin to
disappear.
Mesozoic Era
• Divided into 3 periods:
• Triassic period - Turtles
and crocodiles evolve and
dinosaurs appear.
• Jurassic period - Large
dinosaurs roam the world.
First mammals and birds
appear.
• Cretaceous period Flowering plants appear,
mammals become more
common, dinosaurs
• become extinct.
Mesozoic
• began about 245 million years ago and
lasted about 180 million years; the Age of
the Dinosaurs and mammals began to
evolve; “meso-“ means middle
Cenozoic Era
• Divided into 2 periods:
• Tertiary period - First
primates appear and
flowering plants
become the most
common.
• Quaternary period Humans evolve and
large mammals like
woolly mammoths
become extinct.
Cenozoic
• most recent era; began about 65 million years ago
and continues to present day; the Age of
Mammals; “ceno-“ means recent
• 3. Periods – these are subdivided eras and are
characterized by what life existed worldwide
• 4. Epochs – further subdivided periods of the
Cenozoic Era because the fossil record of this era
is more complete and there are a lot more events
to place in sequence.