Fossils, Fossils Everywhere

Transcription

1 Fossils, Fossils Everywhere Level: Grades 4 6 Standards: Explain how fossils provide evidence of the history of the Earth. (Michigan E.ST.04.31) Objectives: Materials: Time Consideration: Background: Learners will be able to define fossil Learners will be able to describe processes by which fossils are preserved Plaster (plaster of Paris) water flexible plastic container for mixing plaster (shoebox size) paper cups or bowls hard material (shells, sticks, plastic figurines, blocks) Vaseline or hand soap sponge salt shallow pan or bowl Prep time 10 minutes Lesson time minutes The history of life on Earth is based on the fossil record. The fossil record is a collection of all of the known fossils on Earth. Fossils are the preserved remains of organisms, called body fossils. Fossils can also preserve activities of organisms such as tracks, trails, or burrows, which are called trace fossils. There are a number of different processes by which fossils are preserved. The history of Earth is broken up into different time intervals. These boundaries are based on the fossil record and how it changes at each boundary. For a complete look at the

2 geologic timescale check out the background information tab at information.html. The fossil record is variable throughout the history of Earth. The quality of a fossil is dependent on both the type of organism and the location in which the fossil was preserved. The fossil record improved greatly after the Precambrian (570 million years ago), because organisms began to develop hard parts. When an animal dies its soft parts begin to decay quickly, which leaves behind the hard parts. These parts are commonly in the form of bones or shells. The parts are then buried by layers of sediment which will over time harden into rock. Preservation of the fossil then occurs by one of the following processes. Original material The original hard parts of the organism can be preserved and remain unaltered from their original form. This is very common in organisms with shells or an exoskeleton. An example of this would be a trilobite. Permineralization In this process the minerals that exist in water seeps through the hard parts (i.e., bones) once they are buried and crystallize to help strengthen the material. Replacement new minerals can replace the original minerals in the material. Molds and Casts Prints may be left in the sediment and were preserved as it hardened. A mold is a negative impression in the sediment. Casts are a positive image. Check out the Mold and Cast Animation for a quick visual representation of this. Fossils are better preserved in the ocean. Areas on land have higher rates of erosion, especially in areas with mountains. The erosion leads to the destruction of the hard parts that can otherwise be preserved in the water. The marine fossil record is much better than the nonmarine record. Preparation: Each student will need some hard material that they will create their own fossil with. Shells make a good imprint. Assorted shells are often available at a low price in the decorative section of most stores. Items from around the classroom will work as well (plastic figurines, legos). Letting each student create a fossil using a different hard material gives them the freedom to choose. You may need to experiment with the plaster to make sure it doesn t dry up too quickly. I would recommend using powdered Plaster of Paris that you add water to. Watch this short tutorial video tutorial at for guidance.

3 Plaster is safe to work with. It can dry out your skin, so gloves can be worn while mixing. Individual fossils will be made in paper cups or bowls. The paper cups should be trimmed from the top to be about 1½ inches. The students could also do this while the teacher is prepping the plaster. The day s activities will be recorded in a science journal so students have record of their learning process. This may be a good opportunity to start a journal that could be used for additional science lessons. Instructional Sequence: 1. Discuss what students already know about fossils. Ask students to think about how fossils are formed. Pass around some fossil samples. Brainstorm and create a list of possibilities. Help lead the kids to the definition. Have the students record thoughts in their science journal. 2. Explain to the students that we are going to explore ways that fossils can be preserved. Talk about what being preserved means. 3. Now you can spend some time exploring the different processes. Begin with the concept of original material. See if the students can think of characteristics that would make a good fossil. Think about different materials Ex) bone good fossil skin bad fossil Ultimately help them to understand that hard parts make good fossils. 4. For the mold and cast demonstration have everyone get the material they have collected. At this time the students could trim the paper cups they will use. Paper bowls can also be used if the item will not fit in the cup. A light coat of soap or Vaseline should be put on the outside on the hard material to make removal easier once the plaster sets. The students should be prepared for the teacher to pour the plaster for them. The students could also continue writing in their science journals if all of the above has been completed. The teacher will then need to prepare the plaster. PLASTER PREP: 1) Sift two parts plaster to one part water in a flexible plastic container (shoebox size is generally a good size). Depending on the number of students, you will have to estimate the amount of plaster and water. Generally the amount of water you start with is the amount of plaster you will get as an end product. It is always a possibility that you could mix a few batches.

4 2) Do not mix immediately. Let stand for 2 3 minutes allowing the plaster to wet itself by soaking up the water. 3) Mix the plaster and water with your fingers (a glove can be worn and a whisk could also be used). Make sure you smooth out any clumps. You should have a smooth mixture about the consistency of a milkshake. There are two important variables to recognize in this process. The thicker the plaster starts the faster it dries it is always better to start with a thinner mixture because it will thicken over time. Once the plaster begins to thicken it WILL NOT work to add more water to re thin the mixture. The longer the plaster sits the more it dries. 4) Pour plaster into each student s cup or bowl. Depending on the thickness of the mixture, it may need to set for 5 10 minutes. This step is crucial because there will be instant where the plaster begins to harden quickly. It should be the consistency of soft ice cream. The students will press their fossil into the plaster and leave it. It is always good to leave a portion of the object above the plaster so there is a way to remove it.

5 5) Let the plaster sit overnight with the object still in place. 6) On the following day, students can remove the object. The plaster should be taken out of the cup by tearing the paper from around it. The fossils aren t quite ready. They will need to sit an additional day to make sure they are set completely. At this time students can take them home. The class may even consider painting their fossils. 5. As the teacher conduct one experiment that all students will be able to observe. This experiment will show how permineralization works. Dissolve as much salt as possible in a glass of warm water. Pass around the sponge and tell the students to describe how it feels. Place the sponge in a bowl or shallow pan. Pour enough salt solution over the sponge for it to be saturated. The sponge will need to sit, allowing the water to evaporate (the more water you use the longer it will take). Revisit the sponge when it has completely dried. Pass around the specimen again and have the students describe how it feels. Talk about what has happened to the sponge.

6 Permineralization at Work The sponge in this experiment acts like a bone. It has porous space just like a bone. Water naturally contains minerals. The minerals are represented by the salt. As the water evaporates the salt crystallizes within the sponge. This preserves the sponge and makes it harder. This is similar to the process of material being preserved by permineralization. Conclusion: Discuss the processes observed and have the students write some conclusions in their science journals about fossil preservation. Revisit the plaster fossils as well as the sponge. Have students write their final thoughts. Evaluation: The teacher will know that learners can define what a fossil is when they co construct the definition in step 1 of the instructional sequence. The teacher will know that learners can describe the processes by which fossils are formed when they discuss fossilization processes at the end of the lesson. Assessment: The teacher can assess the students through their notes in their science journal as well as through class discussion.