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    how can scientists use fossils in rock layers as evidence that one event happened earlier than another event?


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    Fossils, Rocks, and Time: Fossil Succession


    Three concepts are important in the study and use of fossils: (1) Fossils represent the remains of once-living organisms. (2) Most fossils are the remains of extinct organisms; that is, they belong to species that are no longer living anywhere on Earth. (3) The kinds of fossils found in rocks of different ages differ because life on Earth has changed through time.

    Stratigraphic ranges and origins of some major groups of animals and plants.

    If we begin at the present and examine older and older layers of rock, we will come to a level where no fossils of humans are present. If we continue backwards in time, we will successively come to levels where no fossils of flowering plants are present, no birds, no mammals, no reptiles, no four-footed vertebrates, no land plants, no fishes, no shells, and no animals. The three concepts are summarized in the general principle called the : The kinds of animals and plants found as fossils change through time. When we find the same kinds of fossils in rocks from different places, we know that the rocks are the same age.

    Scientists look for ancestors and descendants through geologic time. The fossil was a Jurassic animal with the skeleton of a reptile, including fingers with claws on the wings (solid arrows), backbone extending into the tail (open arrow), and teeth, but it was covered with feathers. We can see fossils of many other reptiles in rock of the same age and even older, but is the oldest known fossil to have feathers. We conclude that this animal is a link between reptiles and birds and that birds are descended from reptiles. The specimen is about 45 centimeters long.

    How do scientists explain the changes in life forms, which are obvious in the record of fossils in rocks? Early explanations were built around the idea of successive natural disasters or catastrophes that periodically destroyed life. After each catastrophe, life began anew. In the mid-nineteenth century, both Charles Darwin and Alfred Wallace proposed that older species of life give rise to younger ones. According to Darwin, this change or is caused by four processes: variation, over-reproduction, competition, and survival of those best adapted to the environment in which they live. Darwin's theory accounts for all of the diversity of life, both living and fossil. His explanation gave scientific meaning to the observed succession of once-living species seen as fossils in the record of Earth's history preserved in the rocks.

    Scientific theories are continually being corrected and improved, because theory must always account for known facts and observations. Therefore, as new knowledge is gained, a theory may change. Application of theory allows us to develop new plants that resist disease, to transplant kidneys, to find oil, and to establish the age of our Earth. Darwin's theory of evolution has been refined and modified continuously as new information has accumulated. All of the new information has supported Darwin's basic concept--that living beings have changed through time and older species are ancestors of younger ones.

    A species is the most basic unit of classification for living things. This group of fossil clams shows likely ancestor-descendant relationships at the species level. These fossils from the Mid-Atlantic States show the way species can change through time. Notice how the shape of the posterior (rear) end of these clams becomes more rounded in the younger species, and the area where the two shells are held together (ligamental cavity) gets larger. Paleontologists pay particular attention to the shape of the shells and the details of the anatomy preserved as markings on the shells.

    Numbers in the left-hand column refer to the following geologic time segments: 1, Pliocene; 2, Miocene; 3, Oligocene; 4, Eocene; 5, Paleocene; 6, Late Cretaceous.

    The is very important to geologists who need to know the ages of the rocks they are studying. The fossils present in a rock exposure or in a core hole can be used to determine the ages of rocks very precisely. Detailed studies of many rocks from many places reveal that some fossils have a short, well-known time of existence. These useful fossils are called .

    Today the animals and plants that live in the ocean are very different from those that live on land, and the animals and plants that live in one part of the ocean or on one part of the land are very different from those in other parts. Similarly, fossil animals and plants from different environments are different. It becomes a challenge to recognize rocks of the same age when one rock was deposited on land and another was deposited in the deep ocean. Scientists must study the fossils from a variety of environments to build a complete picture of the animals and plants that were living at a particular time in the past.

    The study of fossils and the rocks that contain them occurs both out of doors and in the laboratory. The field work can take place anywhere in the world. In the laboratory, rock saws, dental drills, pneumatic chisels, inorganic and organic acids, and other mechanical and chemical procedures may be used to prepare samples for study. Preparation may take days, weeks, or months--large dinosaurs may take years to prepare. Once the fossils are freed from the rock, they can be studied and interpreted. In addition, the rock itself provides much useful information about the environment in which it and the fossils were formed.

    Source : pubs.usgs.gov

    Rock Layers & Fossils

    Want to learn about rock lays, fossils and earth's ancient landscpaes? Warch our fun video lesson for kids in 3rd, 4th & 5th grade!


    Watch the clip and read more below.

    A simple science lesson & fun video on rock layers & fossils for kids in 3rd, 4th & 5th grade!



    A landscape describes all the visible features of the Earth’s surface now or in the past. We can learn about Earth’s landscapes from the past by studying rock layers and the fossils found in them.

    To better understand Earth’s surface features and landscapes ….


    The oldest rock layers are at the bottom and the newest layers are at the top.

    We can tell about the Earth’s history by looking at the different layers. The upper layers, those that are closest to the surface of the Earth, are the newest layers to be laid down.

    Layers below are older. Since sedimentary rocks form on top of each other, it is very easy to see Earth’s geologic history in areas where this type of rock is found.

    The Grand Canyon is an excellent example. For millions of years, the Earth’s surface was carved by the Colorado River. Many layers of sedimentary rock make up the mile-high walls of the canyon. The walls display a history of the Earth’s surface that dates back about 2 billion years.

    The location of fossils in rock layers provides evidence of Earth’s past landscapes.

    It is hard to guess the age of rock. Scientists have to act like detectives, piecing together a mystery to determine how long ago rocks formed.

    Fossils found in a particular rock layer help scientists determine the age of the rock. Scientists use a technique called radiocarbon dating to find out the age of the fossils. Once they know the age of the fossil in the rock, they also know that rock itself is about the same age!

    Sharktooth Hill in California tells a story of Earth’s past.

    Sharktooth Hill in California is about 100 miles away from the ocean. Millions of years ago, Sharktooth Hill was under the ocean. Here, paleontologists find lots of fossils of marine life, surrounded by silt. The remains of ancient sharks, whales, dolphins, and turtles have been found buried 30 feet underground.

    Scientists believe that this area was once an ocean with a river flowing into it which deposited sediment (sand and clay) at the bottom. When animals died here they were buried in the sediment. One day, the river stopped flowing and lots of animals collected at the bottom. When the river started flowing again, it buried all the bones together.

    Now, millions of years later, this area is no longer an ocean and the bones are preserved as a layer of fossils known as a bone bed.


    Giant seals from over 15 million years ago are found in rock layers at Sharktooth Hill. By knowing the age of the fossils, scientists can also know the age of the rock in which it is found.

    Radiocarbon dating is used to find out how old the fossils are. Scientists compare the carbon in the fossils to carbon today to determine how long ago the fossil formed.

    Millions of year ago, the west coast of the United States was a maze of islands and lagoons. Over time the islands collided to create mountains and valleys that make up the state of California.



    A fossil is evidence of past life on Earth.

    Sharktooth Hill

    A place in California located 100 miles away from the Pacific Ocean. Millions of years ago this region was under the ocean. Today it is on dry land and we find fossils of marine life like whales, sharks, dolphins and turtles in a thin layer 30 feet below the surface.


    The branch of science that studies fossils of animals and plants.


    Fine sand, clay, or other material carried by moving water and deposited at the bottom of a body of water like the ocean.

    Sedimentary Rock

    Sedimentary rocks are made when sand and mud gets laid down in layers. Over time, these layers are squashed under more and more layers. Eventually, the layers turn into rock.

    Bone bed

    A layer in the earth containing large quantities of fossilized animal remains.

    Rock layer

    A layer of rock often formed one on top of the other. We can learn about what the earth’s surface looked like a long time ago by studying the properties of the layers and the different kinds of fossils found in these rock layers.

    Source : www.generationgenius.com

    Evidence of Evolution

    Evidence of Evolution

    Evidence of Evolution The Fossil Record as Evidence for Evolution

    Fossils tell us when organisms lived, as well as provide evidence for the progression and evolution of life on earth over millions of years.


    Synthesize the contributions of the fossil record to our understanding of evolution


    Key Points

    Fossils are the preserved remains or traces of animals, plants, and other organisms from the past.

    Fossils are important evidence for evolution because they show that life on earth was once different from life found on earth today.

    Usually only a portion of an organism is preserved as a fossil, such as body fossils (bones and exoskeletons ), trace fossils (feces and footprints), and chemofossils (biochemical signals).

    Paleontologists can determine the age of fossils using methods like radiometric dating and categorize them to determine the evolutionary relationships between organisms.

    Key Terms

    biomarker: A substance used as an indicator of a biological state, most commonly disease.trace fossil: A type of fossil reflecting the reworking of sediments and hard substrates by organisms including structures like burrows, trails, and impressions.fossil record: All discovered and undiscovered fossils and their placement in rock formations and sedimentary layers.strata: Layers of sedimentary rock.fossiliferous: Containing fossils.

    What Fossils Tell Us

    Fossils are the preserved remains or traces of animals, plants, and other organisms from the past. Fossils range in age from 10,000 to 3.48 billion years old. The observation that certain fossils were associated with certain rock strata led 19th century geologists to recognize a geological timescale. Like extant organisms, fossils vary in size from microscopic, like single-celled bacteria, to gigantic, like dinosaurs and trees.

    “Sue” T-rex skeleton: The bones of this Tyrannosaurus rex were preserved through the process of permineralization, which suggests that this organism was covered by sediment soon after death.


    Permineralization is a process of fossilization that occurs when an organism is buried. The empty spaces within an organism (spaces filled with liquid or gas during life) become filled with mineral-rich groundwater. Minerals precipitate from the groundwater, occupying the empty spaces. This process can occur in very small spaces, such as within the cell wall of a plant cell. Small-scale permineralization can produce very detailed fossils. For permineralization to occur, the organism must be covered by sediment soon after death, or soon after the initial decay process.

    The degree to which the remains are decayed when covered determines the later details of the fossil. Fossils usually consist of the portion of the organisms that was partially mineralized during life, such as the bones and teeth of vertebrates or the chitinous or calcareous exoskeletons of invertebrates. However, other fossils contain traces of skin, feathers or even soft tissues.

    Trace Fossils

    Fossils may also consist of the marks left behind by the organism while it was alive, such as footprints or feces. These types of fossils are called trace fossils, or ichnofossils, as opposed to body fossils. Past life may also leave some markers that cannot be seen but can be detected in the form of biochemical signals; these are known as chemofossils or biomarkers.

    Dinosaur footprints: Footprints are examples of trace fossils, which contribute to the fossil record.

    The Fossil Record

    The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous (fossil-containing) rock formations and sedimentary layers (strata) is known as the fossil record. The fossil record was one of the early sources of data underlying the study of evolution and continues to be relevant to the history of life on Earth. The development of radiometric dating techniques in the early 20th century allowed geologists to determine the numerical or “absolute” age of various strata and their included fossils.

    Evidence for Evolution

    Fossils provide solid evidence that organisms from the past are not the same as those found today; fossils show a progression of evolution. Fossils, along with the comparative anatomy of present-day organisms, constitute the morphological, or anatomical, record. By comparing the anatomies of both modern and extinct species, paleontologists can infer the lineages of those species. This approach is most successful for organisms that had hard body parts, such as shells, bones or teeth. The resulting fossil record tells the story of the past and shows the evolution of form over millions of years.

    Fossil Formation

    Fossils can form under ideal conditions by preservation, permineralization, molding (casting), replacement, or compression.


    Predict the conditions suitable to fossil formation


    Key Points

    Preservation of remains in amber or other substances is the rarest from of fossilization; this mechanism allows scientists to study the skin, hair, and organs of ancient creatures.

    Source : courses.lumenlearning.com

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