Relative Dating with Fossils: Index Fossils as Indicators of Time
Sequencing the rock layers will show students how paleontologists use fossils to give relative dates to rock strata. Please Choose a Product. Smith would have brought these two arrangements together, overlapping the common scallop layer, to produce a larger succession of three rock strata! This rock layer would be younger as indicated by the appearance of new fossils in the rock stratum. You must create an account to continue watching. Primitive form of chordate; floating form with branched stalks; extinct NAME: Short quiz with each of our 22, videos Printable worksheets and answer keys for each lesson 10, teacher resources to use in your classroom today Create custom courses to save lessons to use later Allow students to create FREE accounts associated with yours.
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Medical Terminology Earth Science: I'm not interested in FTCE. Figure 2-A gives some background information on the individual fossils. Once students begin to grasp "relative" dating, they can extend their knowledge of geologic time by exploring radiometric dating and developing a timeline of Earth's history. Please Choose a Product.
Return to top The study and comparison of exposed rock layers or strata in various parts of the earth led scientists in the early 19th century to propose that the rock layers could be correlated from place to place. Locally, physical characteristics of rocks can be compared and correlated. On a larger scale, even between continents, fossil evidence can help in correlating rock layers. The Law of Superposition, which states that in an undisturbed horizontal sequence of rocks, the oldest rock layers will be on the bottom, with successively younger rocks on top of these, helps geologists correlate rock layers around the world.
This also means that fossils found in the lowest levels in a sequence of layered rocks represent the oldest record of life there. By matching partial sequences, the truly oldest layers with fossils can be worked out.
By correlating fossils from various parts of the world, scientists are able to give relative ages to particular strata. This is called relative dating. Relative dating tells scientists if a rock layer is "older" or "younger" than another. This would also mean that fossils found in the deepest layer of rocks in an area would represent the oldest forms of life in that particular rock formation. In reading earth history, these layers would be "read" from bottom to top or oldest to most recent.
If certain fossils are typically found only in a particular rock unit and are found in many places worldwide, they may be useful as index or guide fossils in determining the age of undated strata. By using this information from rock formations in various parts of the world and correlating the studies, scientists have been able to establish the geologic time scale.
This relative time scale divides the vast amount of earth history into various sections based on geological events sea encroachments, mountain-building, and depositional events , and notable biological events appearance, relative abundance, or extinction of certain life forms.
When you complete this activity, you will be able to: The first card in the sequence has "Card 1, Set A" in the lower left-hand corner and represents the bottom of the sequence.
If the letters "T" and "C" represent fossils in the oldest rock layer, they are the oldest fossils, or the first fossils formed in the past for this sequence of rock layers. Now, look for a card that has either a "T" or "C" written on it. Since this card has a common letter with the first card, it must go on top of the "TC" card. The fossils represented by the letters on this card are "younger" than the "T" or "C" fossils on the "TC" card which represents fossils in the oldest rock layer.
Sequence the remaining cards by using the same process. When you finish, you should have a vertical stack of cards with the top card representing the youngest fossils of this rock sequence and the "TC" card at the bottom of the stack representing the oldest fossils.
Starting with the top card, the letters should be in order from youngest to oldest. Return to top Procedure Set B: Each card represents a particular rock layer with a collection of fossils that are found in that particular rock stratum. All of the fossils represented would be found in sedimentary rocks of marine origin.
Figure 2-A gives some background information on the individual fossils. The letters on the other cards have no significance to the sequencing procedure and should be ignored at this time. Find a rock layer that has at least one of the fossils you found in the oldest rock layer.
This rock layer would be younger as indicated by the appearance of new fossils in the rock stratum. Keep in mind that extinction is forever. Once an organism disappears from the sequence it cannot reappear later. Use this information to sequence the cards in a vertical stack of fossils in rock strata. Arrange them from oldest to youngest with the oldest layer on the bottom and the youngest on top.
This will enable your teacher to quickly check whether you have the correct sequence. Three-lobed body; burrowing, crawling, and swimming forms; extinct NAME: Many were large a few rare species were 5 feet in length ; crawling and swimming forms; extinct NAME: Primitive form of chordate; floating form with branched stalks; extinct NAME: We can't just use any fossil that we find.
Remember that some species of animals and plants lived for a very long time, while others existed only for a short period of time. We don't want to use fossils belonging to species that lived for too long; these fossils would show up in more than one rock layer.
We want fossils of plants and animals that lived for a relatively short amount of time, like a few hundred thousand years or so. I know that doesn't seem like a very short time span, but it is when we're talking about geologic time.
An index fossil is a fossil representing a plant or animal that existed for a relatively short duration of time. These are the fossils that we want to use for relative dating. Index fossils help us to distinguish between rock strata from different time periods, so it's important that they don't cover too much historical ground. We wouldn't want to use a horseshoe crab fossil, because horseshoe crabs have existed for over million years and are still alive today!
We'd want to use a more short-lived fossil, like the dodo bird. We also want our index fossils to be common, widely-distributed species that are easy for scientists to identify. Some of the scientists' favorite index fossils are trilobites, ammonites and scallop shells.
So, how exactly is an index fossil used for relative dating of rocks? Well, let's go back to our surveyor, William Smith. He was often presented with the problem of finding two different rock outcrops from two different periods. Let's say in the first outcrop, he found an upper rock layer containing ammonite fossils and a lower layer containing scallops.
In the second outcrop, miles and miles away, he also found two layers; but these layers were different. The upper layer had scallop fossils, and the lower layer had trilobites. Smith would have brought these two arrangements together, overlapping the common scallop layer, to produce a larger succession of three rock strata! Now we have a more complete piece of geologic history: Index fossils can be used to correlate the relative ages of rocks that are separated by vast distances.
The cool thing is that we can even correlate rocks from different continents! For example, scientists found Barosaurus fossils inside a layer of Tendaguru rocks in East Africa. They also found Hypsilophodon fossils inside a layer of Wealden rocks in Europe. Scientists didn't know how old either of the rocks were, or even which dinosaur was older than the other. But in North America, they found a big chunk of rock which contained both fossils.
Therefore, the Hypsilophodon had to be older than the Barosaurus. And, even though the rock types were different, scientists could assign relative ages to the other rocks based on their fossils. They could safely assume that the Tendaguru rocks in East Africa were older than the Wealden rocks in Europe.
When rocks are made up of distinct strata, we use stratigraphic succession to determine the relative ages of each of the layers in the rock.
However, another form of relative dating is the use of fossil succession: In order to use fossils for relative dating, scientists focus their efforts on index fossils. These fossils represent plants and animals that lived for a relatively short period of time. We use index fossils to identify periods of geologic history and to match up pieces of rock strata that have been separated by large distances.
When one outcrop contains two index fossils from two different time periods, it acts as a 'missing link' between other outcrops that have only one of the two fossils. To unlock this lesson you must be a Study. Did you know… We have over 95 college courses that prepare you to earn credit by exam that is accepted by over 2, colleges and universities.
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To learn more, visit our Earning Credit Page. Not sure what college you want to attend yet? The videos on Study. Students in online learning conditions performed better than those receiving face-to-face instruction. Explore over 4, video courses. Find a degree that fits your goals. Relative Dating with Fossils: Index Fossils as Indicators of Time You may already know how to date a fossil with a rock. But did you know that we can also date a rock with a fossil?
Watch this video to find out how we use index fossils to establish the relative ages of rocks. An error occurred trying to load this video. Try refreshing the page, or contact customer support. You must create an account to continue watching. Register for a free trial Are you a student or a teacher? I am a student I am a teacher. It only takes a few minutes to set up and you can cancel at any time. Atomic Number and Mass Number. Are you still watching? Your next lesson will play in 10 seconds.
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Conditions of Fossil Preservation: Sea Floor Spreading and Polar Reversal. Major Eons, Eras, Periods and Epochs. How to Read Topographic and Geologic Maps. Absolute Time in Geology. Interpreting Tables of Scientific Data: Theories of Geological Evolution: Prentice Hall Earth Science: Remedial High School Physical Science.
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Water Balance on Earth. Your next lesson will play in 10 seconds. Return to top Procedure Set B: He worked in Southern England, and he got to see all kinds of different rock strata that were exposed in outcrops and canals. To learn more, visit our Earning Credit Page Transferring credit to the school of your choice Not sure what college you want to attend fossils and relative dating worksheet I'm not interested in GED.
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