Start the investigation by asking students to think about this question: Based on what you learned from the last investigation, what kinds of things become fossils? Ask them the following: Describe the changes that seem to be happening to the decomposing fruit. What do you think is causing these changes? How long will the fruit continue to decompose? What will eventually happen to the fruit? What do you think is the likelihood of the fruit becoming a fossil, either all of it or part of it?
When would the fruit be more likely to become a fossil — if it decomposed quickly, decomposed slowly, or did not decompose at all? Presenting the Investigation Question After the scene is set, introduce your students to the investigation question: Why do some things become fossils, but others do not?
Have your students brainstorm ideas about how this investigation question could be investigated. How would you design an experiment that could be used to test the investigation question? What materials would be needed? What would you have to do? What would be measured? How long would the experiment take? Here are some initial questions that your students can discuss, in pairs, then in groups: What happens when a living thing rots or decays?
Why do some things, like fruit, rot or decay? Do all living things decay? Why or why not? Do all parts of living things decay? How could the decay of a living thing be slowed down or stopped? What impact does decay have on how a fossil forms? Have your students share their ideas with the class and record them as a list on a flipchart. Ask students the following: What would you like to learn about why some things become fossils, but others do not?
Exploring the Concept Have students cover their desks with newspaper or newsprint. Explain to students that they will be exploring different ways to preserve a piece of plant material, in this case, slices of banana. Tell them that they will be placing the banana slices in paper cups and covering them with different materials. They will let them sit overnight.
The next day, they will look for any changes that may have occurred. Present to students the different materials for preserving the banana slices: soil, sand, gravel, and plaster of Paris.
Have students predict how the slices will change in each of the materials. Record predictions on the whiteboard or flipchart. Ask students to write their names on each of the cups.
Ask students to partly fill the cups with each of the materials. They should then place banana slices in each cup and cover them with more of each material. Have students place one slice of banana in an empty cup to be used as a control to compare with the other samples. Have students place the slices on a counter where they can be collected the next day. The next day, have students remove the slices from each of the materials.
The slices in the Plaster of Paris will have to be cracked open with a hammer. Be sure that students wear impact goggles when doing this, or you should do this yourself. Ask groups to complete the following: How does each slice compare to the slice left unprotected?
Humans and many other organisms have skeletons that are already mineralized, so when it comes to fossilization that gives us bony animals a built-in advantage over plants, jellyfish and mushrooms—to name a few of our soft-bodied, easily recycled fellow Earthlings.
Think of all of the shells you have seen on the beach, the rocky coral reefs, the white chalk cliffs of Dover in England. These are all formed of biominerals—meaning that organisms built them while they were alive, usually for strength and protection, and then left them behind when they died.
These examples are all made of calcium carbonate—note that they contain carbon—and their billions of skeletons were responsible for removing vast amounts of carbon from the atmosphere in times past.
Dinosaur skeletons may get all the glory, but the most common fossils on Earth are the tiny skeletons of micro-organisms that live in the water. Untold numbers can be found in the uplifted and exposed ancient rocks that can now be found on land or are still buried deep under the oceans. Micro-skeletons rain down to form new sediment layers on the ocean floor today, just as they have for millions of years.
Acidic water, or even just cold water, can dissolve the tiny carbonate skeletons before they hit bottom. After burial, the minute shells may recrystallize or dissolve unless they are protected by mud that blocks the flow of water, and the ones that survive as fossils are highly valuable to paleontologists because of their unaltered biominerals.
This is a different process from what happens with petrified wood, which is mostly turned to stone. We know that many buried micro-shells are pristine, meaning that their biominerals remained unchanged over millions of years, so geochemists can use them to reconstruct water chemistry and global temperature at the time when the micro-organisms died. A whole lot of careful science has gone into chemical tests that show which tiny shells are unchanged and therefore okay for inferring past climate, and which are not.
Though we call them fossils because they are old and buried deeply in rock, many of these micro-skeletons were not changed when they were preserved underground. Instead, they were encased within muddy sediment, which was turned into stone around them. The tiny inside hollow parts of the shells are filled with mud as well, keeping them from being crushed by the heavy rock layers that seal their graves. Microbes, as well as insects, quickly infest dead animals and plants, and we humans consider this quite disgusting.
This process — which is called carbonization, or distillation — yields a detailed carbon impression of the dead organism in sedimentary rock. The most common method of fossilization is called permineralization, or petrification. After an organism's soft tissues decay in sediment, the hard parts — particularly the bones — are left behind.
Water seeps into the remains, and minerals dissolved in the water seep into the spaces within the remains, where they form crystals. These crystallized minerals cause the remains to harden along with the encasing sedimentary rock. In another fossilization process, called replacement, the minerals in groundwater replace the minerals that make up the bodily remains after the water completely dissolves the original hard parts of the organism.
Fossils also form from molds and casts. If an organism completely dissolves in sedimentary rock, it can leave an impression of its exterior in the rock, called an external mold.
If that mold gets filled with other minerals, it becomes a cast.
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