Lesson Overview 19.1 The Fossil Record
Lesson Overview The Fossil Record
Lesson Overview 19.1 The Fossil Record
Lesson Overview The Fossil Record
THINK ABOUT IT
Fossils, the preserved remains or traces of ancient life, are priceless treasures. They tell of life-and-death struggles and of mysterious worlds lost in the mists of time.
Taken together, the fossils of ancient organisms make up the history of
life on Earth called the fossil record. How can fossils help us understand life’s history?
Lesson Overview The Fossil Record
Fossils and Ancient Life What do fossils reveal about ancient life?
Lesson Overview The Fossil Record
Fossils 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.
Lesson Overview The Fossil Record
Fossils 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.
Lesson Overview The Fossil Record
Types of Fossils Fossils can be as large and perfectly preserved as
an entire animal, complete with skin, hair, scales, or feathers.
.
Lesson Overview The Fossil Record
They can also be as tiny as bacteria, developing embryos, or pollen
Lesson Overview The Fossil Record
Types of Fossils Many fossils are just fragments of an organism—teeth, pieces of a
jawbone, or bits of leaf.
Lesson Overview The Fossil Record
Types of Fossils Sometimes an organism leaves behind trace fossils—casts of footprints,
burrows, tracks, or even droppings.
Lesson Overview The Fossil Record
Types of Fossils Although most fossils are preserved in sedimentary rocks,
some are preserved in other ways, like in amber.
Lesson Overview The Fossil Record
Fossils 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.
Lesson Overview The Fossil Record
Fossils 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.
Lesson Overview The Fossil Record
Fossils in Sedimentary Rock The preserved remains may later be discovered and studied.
Lesson Overview The Fossil Record
Fossils 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.
Lesson Overview The Fossil Record
Fossils in Sedimentary Rock 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.
Lesson Overview The Fossil Record
What Fossils Can Reveal The fossil record contains an enormous amount of information for
paleontologists, researchers who study fossils to learn about ancient life.
By comparing body structures in fossils to body structures in living
organisms, researchers can infer evolutionary relationships and form hypotheses about how body structures and species have evolved.
Bone structure and trace fossils, like footprints, indicate how animals
moved.
Lesson Overview The Fossil Record
What Fossils Can Reveal Fossilized plant leaves and pollen suggest whether the area was a
swamp, a lake, a forest, or a desert. When different kinds of fossils are found together, researchers can
sometimes reconstruct entire ancient ecosystems.
Lesson Overview The Fossil Record
Dating Earth’s History How do we date events in Earth’s history?
Lesson Overview The Fossil Record
Dating Earth’s History How do we date events in Earth’s 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.
Lesson Overview The Fossil Record
Relative 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.
Lesson Overview The Fossil Record
Relative 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.
Lesson Overview The Fossil Record
Relative Dating A good index fossil species must
be easily recognized and will occur in only a few rock layers (meaning the organism lived only for a short time). These layers, however, will be found in many places (meaning the organism was widely distributed).
Trilobites, a large group of distinctive marine organisms,
are often useful as index fossils.
Lesson Overview The Fossil Record
Geologic Time Scale How was the geologic time scale established, and what are its major
divisions?
Lesson Overview The Fossil Record
Geologic 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.
Lesson Overview The Fossil Record
Geologic Time Scale
Geologists and paleontologists have built a time line of Earth’s history called the geologic time scale.
The basic divisions of the geologic time scale are eons,
eras, and periods.
Lesson Overview The Fossil Record
Establishing the Time Scale By studying rock layers and index fossils, early paleontologists placed
Earth’s rocks and fossils in order according to their relative age. They noticed major changes in the fossil record at boundaries between
certain rock layers.
Lesson Overview The Fossil Record
Lesson Overview The Fossil Record
Establishing 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.
Lesson Overview The Fossil Record
Divisions of the Geologic Time Scale The time scale is based on
events that did not follow a regular pattern.
The Cambrian Period, for
example, began 542 million years ago and continued until 488 million years ago, which makes it 54 million years long.
The Cretaceous Period was 80
million years long.
Lesson Overview The Fossil Record
Divisions of the Geologic Time Scale Geologists now recognize four
eons of unequal length. The Hadean Eon, during
which the first rocks formed, began about 4.6 billion years ago.
The Archean Eon, when life
first appeared, began about 4 billion years ago.
Lesson Overview The Fossil Record
Divisions of the Geologic Time Scale The Proterozoic Eon began
2.5 billion years ago and lasted until 542 million years ago.
The Phanerozoic Eon began
at the end of the Proterozoic and continues to the present.
Lesson Overview The Fossil Record
Divisions of the Geologic Time Scale Eons are divided into eras.
The Phanerozoic Eon, for example, is divided into the Paleozoic, Mesozoic, and Cenozoic Eras.
Eras are subdivided into
periods, which range in length from nearly 100 millions of years to just under 2 million years. The Paleozoic Era, for example, is divided into six periods.
Lesson Overview The Fossil Record
Naming the Divisions Geologists started to name
divisions of the time scale before any rocks older than the Cambrian Period had been identified. For this reason, all of geologic time before the Cambrian is simply called Precambrian Time.
Lesson Overview The Fossil Record
Naming the Divisions The Precambrian actually covers about 90 percent of Earth’s history. In this figure, the history of Earth is depicted as a 24-hour clock. Notice
the relative length of Precambrian Time—almost 22 hours.
Lesson Overview The Fossil Record
Lesson Overview The Fossil Record
Life on a Changing Planet How have our planet’s environment and living things affected each other to
shape the history of life on Earth?
Lesson Overview The Fossil Record
Life on a Changing Planet How have our planet’s 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 Earth’s history. In turn, the actions of living organisms over time have changed conditions in the land, water, and atmosphere of planet Earth.
Lesson Overview The Fossil Record
Life on a Changing Planet Earth and its climate has been constantly
changing, and organisms have evolved in ways that responded to those new conditions.
The fossil record shows evolutionary histories for major groups of organisms
as they have both responded to changes on Earth and how they have changed Earth.
Lesson Overview The Fossil Record
Physical Forces Climate is one of the most important aspects of Earth’s physical
environment. Earth’s climate has undergone dramatic changes over time. Many of
these changes were triggered by fairly small shifts in global temperature.
During the global “heat wave” of the Mesozoic Era, Earth’s average
temperatures were only 6°C to 12°C higher than they were during the twentieth century.
During the ice ages, world temperatures were only about 5°C cooler
than they are now. These relatively small temperature shifts changed the shape of life on
Earth.
Lesson Overview The Fossil Record
Physical Forces Geological forces have transformed life on Earth, producing new
mountain ranges and moving continents. Volcanic forces have altered landscapes and even formed entire
islands. Local climates are shaped by the interaction of wind and ocean currents
with geological features such as mountains and islands.
Lesson Overview The Fossil Record
Physical Forces The theory of plate tectonics explains how solid
continental “plates” move slowly above Earth’s molten core—a process called continental drift.
Over the long term, continents have collided to form
“supercontinents.” Later, these supercontinents have split apart and reformed.
Lesson Overview The Fossil Record
Physical 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.
Lesson Overview The Fossil Record
Geological 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.
Lesson Overview The Fossil Record
Physical 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.
Lesson Overview The Fossil Record
Biological Forces The activities of organisms have affected global environments. For example, Earth’s 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.