Download History of the Earth

The Fossil Record
 Scientists who study fossils are called paleontologists.
 The fossil record is an order of fossils that give
information about the past.
 The fossil record gives evidence of earth’s history and
how organisms changed over time.
 Species that died out are said to be extinct.
 About 99% of all species that have ever lived on earth
have become extinct.
 Most organisms which die do not leave fossils.
 Fossils can include footprints, eggs, or other traces of
organisms.
 Most fossils form in sedimentary rock.
 The quality of fossil preservation varies.
 After an organism dies, it is buried over and over by
sediments. As time passes, the sediments compact
and press the organism’s remains into the rock layer.
 Two techniques paleontologists use to determine the
age of fossils are:
 Relative dating
 Radioactive dating
 Determines the age of a fossil by comparing its
placement with that of fossils in other layers of rock.
 Uses index fossils.
 Provides no information about absolute age.
 Older rock layers are usually further from the earth’s surface than
more recent rocks.
 Law of superposition
 Scientists use radioactive decay to assign absolute ages
to rocks.
 The length of time required for half of the radioactive
atoms in a sample to decay is called a half-life.
 The use of half-life to determine the age of a sample is
called radioactive dating.
 One way scientists determine the age of fossils is by
comparing the amounts of Carbon-12 to Carbon-14 in a
rock or fossil. The more Carbon-12 compared to
Carbon-14, the older the sample is.
 Not all radioactive elements have the same half-life.
Time (MYA)
Period
Era
1.8-Present
Quaternary
Cenozoic
65-1.8
Tertiary
145-65
Cretaceous
208-145
Jurassic
245-208
Triassic
290-245
Permian
360-290
Devonian
410-360
Devonian
440-410
Silurian
505-440
Ordovician
544-505
Cambrian
650-544
Vendian
Mesozoic
Paleozoic
Precambrian
 Cambrian explosion: Occurred the beginning of the
Paleozoic era (about 544 mya). This is when
conditions on earth were at their best so that many
organisms originated then. Diversity took off.
 Mass extinction occurred at the end of the Paleozoic
era (about 245 mya). Many organisms died out and
became extinct. As much as 95% of living things
became extinct.