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The module is an integrated unit which addresses the following National Science Education Standards: Fossils indicate that many organisms that lived long ago are extinct. Extinction of species is common; most of the species that have lived on the earth no longer exist. Fossils provide important evidence of how life and environmental conditions have changed. The complete "Paleontology and Dinosaurs" module takes approximately four weeks to teach.
The "Who's On First? Scientific measurements such as radiometric dating use the natural radioactivity of certain elements found in rocks to help determine their age. Scientists also use direct evidence from observations of the rock layers themselves to help determine the relative age of rock layers. Specific rock formations are indicative of a particular type of environment existing when the rock was being formed. For example, most limestones represent marine environments, whereas, sandstones with ripple marks might indicate a shoreline habitat or a riverbed.
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. Login here for access. Log in or sign up to add this lesson to a Custom Course. Login or Sign up. In previous lessons, we talked about the Geologic Time Scale and how scientists use it to piece together the history of the earth.
We talked about relative dating of rocks and how scientists use stratigraphic succession to compare the ages of different rock layers. You should already understand that the lower rock strata are generally older than the strata found higher up in the rock. When a scientist finds a section of rock that has lots of different strata, he assumes that the bottom-most layer is the oldest, and the top-most layer is the youngest.
But sometimes, a scientist finds a couple of rock outcrops that are separated by a wide distance. One outcrop shows layers from one geologic time period, while the other outcrop represents a different time. What can a scientist do with these two outcrops? Can he put the pieces together to make the story more complete?
Can he match one set of strata to the other? Let's find out how scientists deal with this common problem by using the fossils inside the rocks. Back in , there lived a land surveyor named William Smith. He worked in Southern England, and he got to see all kinds of different rock strata that were exposed in outcrops and canals. William Smith collected fossils from his work sites and, over time, he learned to recognize which fossils tended to show up in which rock strata.
He began to identify rock layers by the fossils they contained, and he even noticed that the general order of strata was identical over many different parts of the country. Smith was the first person to understand the principle of fossil succession.
Fossil succession is based on the observation that certain assemblages, or groups, of animals and plants have lived during certain time periods over geologic history. For example, human beings and modern elephants are part of the same assemblage because we live in the same time period. Stegosaurus and Triceratops were not part of the same assemblage because they lived at different times. Obviously, the fossil assemblages change from period to period.
They follow an ordered progression that is very clear and predictable. Therefore, we can use the succession of fossil assemblages to establish the relative ages of rocks. Now, when we use fossils to date rocks, we have to be careful.
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. Get FREE access for 5 days, just create an account. 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 college courses that prepare you to earn credit by exam that is accepted by over 1, colleges and universities.
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By creating an account, you agree to Study. 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. Start Your Free Trial Today. An error occurred trying to load this video. Try refreshing the page, or contact customer support. 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. What teachers are saying about Study.
Atomic Number and Mass Number. Are you still watching? Your next lesson will play in 10 seconds. Add to Add to Add to. Want to watch this again later? Principles of Radiometric Dating. What is Relative Dating? What is Relative Age? Methods of Geological Dating: Numerical and Relative Dating. Conditions of Fossil Preservation: Sea Floor Spreading and Polar Reversal.
Major Eons, Eras, Periods and Epochs. Absolute Time in Geology. How to Read Topographic and Geologic Maps.