Title: Lesson Overview
1Lesson Overview
2Fossils and Ancient Life
- What do fossils reveal about ancient life?
- From the fossil record, paleontologists learn
about the structure of ancient organisms, their
environment, and the ways in which they lived.
3Fossils and Ancient Life
- Fossils are the most important source of
information about extinct species, ones that have
died out. - Fossils vary enormously in size, type, and
degree of preservation. They form only under
certain conditions. - For every organism preserved as a fossil, many
died without leaving a trace, so the fossil
record is not complete.
4Fossils in Sedimentary Rock
- Most fossils are preserved in sedimentary rock.
- Sedimentary rock usually forms when small
particles of sand, silt, clay, or lime muds
settle to the bottom of a body of water. - As sediments build up, they bury dead organisms
that have sunk to the bottom.
5Fossils in Sedimentary Rock
- As layers of sediment continue to build up over
time, the remains are buried deeper and deeper. - Over many years, water pressure gradually
compresses the lower layers and turns the
sediments into rock.
6Fossils in Sedimentary Rock
- Usually, soft body structures decay quickly
after death, so usually only hard parts like
wood, shells, bones, or teeth remain. These hard
structures can be preserved if they are saturated
or replaced with mineral compounds. -
Sometimes, however, organisms are buried so
quickly that soft tissues are protected from
aerobic decay. When this happens, fossils may
preserve imprints of soft-bodied animals and
structures like skin or feathers. This fish
fossil was formed in sedimentary rock.
7What Fossils Can Reveal
- The fossil record contains an enormous amount of
information for paleontologists, researchers who
study fossils to learn about ancient life. - 1) Evolutionary relationships comparing body
structures in fossils to body structures in
living organisms. - 2) Movement Bone structure and trace fossils,
like footprints.
3) Type of environment Fossilized plant leaves
and pollen suggest whether the area was a swamp,
a lake, a forest, or a desert. 4)
Reconstruction of an ecosystem when different
kinds of fossils are found together.
8Dating Earths History
- How do we date events in Earths history?
- Relative dating allows paleontologists to
determine whether a fossil is older or younger
than other fossils. - Radiometric dating uses the proportion of
radioactive to nonreactive isotopes to calculate
the age of a sample.
9Relative Dating
- Lower layers of sedimentary rock, and fossils
they contain, are generally older than upper
layers. - Relative dating places rock layers and their
fossils into a temporal sequence.
10Relative Dating
- To help establish the relative ages of rock
layers and their fossils, scientists use index
fossils. Index fossils are distinctive fossils
used to establish and compare the relative ages
of rock layers and the fossils they contain. - If the same index fossil is found in two widely
separated rock layers, the rock layers are
probably similar in age.
11Radiometric Dating (Absolute Age)
- Radiometric dating relies on radioactive
isotopes, which decay, or break down, into
nonradioactive isotopes at a steady rate. - Radiometric dating compares the amount of
radioactive to nonreactive isotopes in a sample
to determine its age.
A half-life is the time required for half of the
radioactive atoms in a sample to decay. After
one half-life, half of the original radioactive
atoms have decayed.
12Radiometric Dating
- Carbon-14 is a radioactive form of carbon
naturally found in the atmosphere. It is taken up
by living organisms along with regular carbon,
so it can be used to date material that was once
alive, such as bones or wood. - After an organism dies, carbon-14 in its body
begins to decay to nitrogen-14, which escapes
into the air. - Half life 5,730 years
- Researchers compare the amount of carbon-14 in a
fossil to the amount of carbon-14 in the
atmosphere, which is generally constant. This
comparison reveals how long ago the organism
lived. - Carbon-14 has a half-life of only about 5730
years, so its only useful for dating fossils no
older than about 60,000 years.
13Radiometric Dating
- For fossils older than 60,00 years, researchers
estimate the age of rock layers close to
fossil-bearing layers and infer that the fossils
are roughly same age as the dated rock layers. - A number of elements with long half-lives are
used for dating very old fossils, but the most
common are potassium-40 (half-life 1.26 billion
years) and uranium-238 (half-life 4.5 billion
years).
14Geologic Time Scale
- How was the geologic time scale established,
and what are its major divisions? - The geologic time scale is based on both
relative and absolute dating. The major divisions
of the geologic time scale are eons, eras, and
periods.
15Geologic Time Scale
- Geologists and paleontologists have built a time
line of Earths history called the geologic time
scale. - The basic divisions of the geologic time scale
are eons, eras, and periods.
16Establishing the Time Scale
- By studying rock layers and index fossils, early
paleontologists placed Earths rocks and fossils
in order according to their relative age. - They noticed major changes in the fossil record
at boundaries between certain rock layers.
17Establishing the Time Scale
- Geologists used these boundaries to determine
where one division of geologic time ended and the
next began. - Years later, radiometric dating techniques were
used to assign specific ages to the various rock
layers.
18Life on a Changing Planet
- How have our planets environment and living
things affected each other to shape the history
of life on Earth? - Building mountains, opening coastlines,
changing climates, and geological forces have
altered habitats of living organisms repeatedly
throughout Earths history. In turn, the actions
of living organisms over time have changed
conditions in the land, water, and atmosphere of
planet Earth.
19Physical Forces
- The theory of plate tectonics explains how solid
continental plates move slowly above Earths
molten corea process called continental drift. - Over the long term, continents have collided to
form supercontinents. Later, these
supercontinents have split apart and reformed.
20Physical Forces
- Where landmasses collide, mountain ranges often
rise. - When continents change position, major ocean
currents change course. - All of these changes affect both local and
global climate.
21Geological Cycles and Events
- Continental drift has affected the distribution
of fossils and living organisms worldwide. As
continents drifted apart, they carried organisms
with them. - For example, the continents of South America and
Africa are now widely separated. But fossils of
Mesosaurus, a semiaquatic reptile, have been
found in both South America and Africa. - The presence of these fossils on both
continents, along with other evidence, indicates
that South America and Africa were joined at one
time.
22Physical Forces
- Evidence indicates that over millions of years,
giant asteroids have crashed into Earth. - Many scientists agree that these kinds of
collisions would toss up so much dust that it
would blanket Earth, possibly blocking out enough
sunlight to cause global cooling. This could have
contributed to, or even caused, worldwide
extinctions.
23Biological Forces
- The activities of organisms have affected global
environments. - For example, Earths early oceans contained
large amounts of soluble iron and little oxygen. - During the Proterozoic Eon, however,
photosynthetic organisms produced oxygen gas and
also removed large amounts of carbon dioxide from
the atmosphere. - The removal of carbon dioxide reduced the
greenhouse effect and cooled the globe. The iron
content of the oceans fell as iron ions reacted
with oxygen to form solid deposits. - Organisms today shape the landscape by building
soil from rock, and sand and cycle nutrients
through the biosphere.