Lesson Overview 19.1 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?

Fossils and Ancient Life What do fossils reveal about ancient life?

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.

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.

Types of Fossils Fossils can be as large and perfectly preserved as an entire animal, complete with skin, hair, scales, or feathers. They can also be as tiny as bacteria, developing embryos, or pollen grains.

Types of Fossils Many fossils are just fragments of an organism—teeth, pieces of a jawbone, or bits of leaf.

Types of Fossils Sometimes an organism leaves behind trace fossils—casts of footprints, burrows, tracks, or even droppings.

Types of Fossils Although most fossils are preserved in sedimentary rocks, some are preserved in other ways, like in amber.

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.

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.

Fossils in Sedimentary Rock The preserved remains may later be discovered and studied.

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.

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.

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.

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.

Dating Earth’s History How do we date events in Earth’s history?

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.

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.

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.

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.

Radiometric Dating Relative dating is important, but provides no information about a fossil’s absolute age in years. One way to date rocks and fossils is radiometric dating. 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.

Radiometric Dating 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. After another half-life, another half of the remaining radioactive atoms will have decayed.

Radiometric Dating Different radioactive elements have different half-lives, so they decay at different rates.

Radiometric Dating The half-life of potassium-40 is 1.26 billion years.

Radiometric Dating Carbon-14, which has a short half-life, can be used to directly date very young fossils. Elements with long half-lives can be used to indirectly date older fossils by dating nearby rock layers, or the rock layers in which they are found.

Radiometric 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. 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 it’s only useful for dating fossils no older than about 60,000 years.

Radiometric 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).

Geologic Time Scale How was the geologic time scale established, and what are its major divisions?

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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?

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.

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.

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.

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.

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.

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.

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.

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.

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.