• Assess student understanding through the class/group discussions and activities.

• Provide feedback to students throughout the lesson to assess student progress.

• Observe students during the composition of fossil explanations and their fossil histories, providing feedback.

• Collect worksheets and fossil histories for individual assessment.

“Today we’re going to explore an important tool Earth scientists use in reconstructing Earth’s history—fossils. What do we know about fossils?” Facilitate a short class discussion, posting student comments on the board. “So, a fossil is . . .” Facilitate a class discussion to construct a definition of the term “fossil” based on the posted student input. Post the definition on the board. For example, fossils are pieces of old bones, shells, and “stuff” from living things that lived a long time ago.

Recent Seashell and Seashell Molds. Group students and distribute a recent seashell to each group and the Seashell worksheet (S-7-2-1_Seashell Worksheet.doc). “Examine and discuss your shells. Answer the questions on your worksheet. Be prepared to contribute to a class discussion of your findings.” Facilitate a short class discussion, summarizing group comments on the board:

1. Answers will vary.

2. A clam.

3. No. The soft parts rotted away. The other shell is no longer connected so we don’t have it.

4. Along a seashore.

5. Recent; clams in the oceans today make them.

“Now I’m going to demonstrate for you, modeling how natural impressions or molds of your shells may form in rocks.”

To make the casts, first press the outside of each shell into modeling clay to make a mold, or an impression, of the outside of the shell (S-7-2-1_Making Fossil Casts Instructions.doc). Pull the shell carefully back out again. If the shell sticks to the modeling clay, it may be necessary to apply a thin layer of petroleum jelly to the outside of the shell before pressing the shell into the clay. Mix the plaster of Paris according to instructions on its container and pour the mixture into the mold. Allow the plaster to dry. Drying time will depend on the size of the shell and the plaster mixture. Smaller shells, about an inch in diameter, are easier to work with.

Number each cast to correspond with the number on the shell used to make the cast so the shell and cast pairs do not get mixed up during the lessons. Extra casts may be needed in case some get broken during student use.

“Now it’s your turn to make your molds. Follow the procedure that I modeled for you. The procedure is also listed on the worksheet. Answer the questions on the worksheet. Be prepared to contribute to a class discussion of your findings.” Distribute the mold-modeling worksheets (S-7-2-1_Modeling Molds of Shells Worksheet and KEY.doc), the clay, and the paper plates. Monitor students as they create their shell molds, providing feedback to students.

Facilitate a short class discussion, summarizing group answers on the board. For example:

1. The mold has the same shape as the shell; the mold has the same patterns as the outside of the shell; the mold is the same size as the shell.

2. The mold is filled with empty space (bows down, is a low spot in the clay); the plaster form is a filling of the empty space with copies of the shell patterns on the outside.

3. Answers will vary, but the shell has dissolved so it is no longer there.

4. The impression is a mold, an empty space representing the dead creature that was once there. The plaster is a filling of that space, conforming to the mold of the dead creature.

“We call the filling of a mold a cast.” Facilitate a class discussion of casts students are familiar with, like crayons, gummy bears, and bouncy balls.

Molds and Casts. “Now let’s compare your shells to casts I made of them using plaster of Paris. Compare and discuss the casts and the shells that were made to use them. Answer the questions on the worksheet. Be prepared to contribute to a class discussion of your findings.” Distribute the casts and the cast worksheet (S-7-2-1_Cast Worksheet and KEY.doc). Monitor students and provide feedback.

Facilitate a class discussion of student findings. For example:

1. The cast has the same designs as the shell; the cast is the same size as the shell; the cast is the same shape as the shell.

2. The cast is like a filling of the space inside the shell; the cast is made of different material than the shell.

3. First the organism dies. Next it is buried by sediment, most likely in oceans. A mold of the buried organism may form when the organism rots and the hard parts dissolve, leaving an empty space. If sediment fills the empty space, then a cast may form.

4. The dead organism must have been present to begin with for a mold to form. The mold records what the organism looked like.

5. Indirect evidence, since there are no remains of the organism.

Animated models can be viewed at https://www.youtube.com/watch?v=SEDfRy6DQns&feature=related and http://www.classzone.com/books/earth_science/terc/content/visualizations/es2901/es2901page01.cfm?chapter_no=visualization.

“We modeled how molds and casts of marine organisms form because these are the most common types of fossils found in the geologic record. When you made an impression of the shell, you modeled how a mold of an organism forms. When you filled the mold with plaster, you modeled how a cast of an organism forms. Molds and casts are trace fossils since the remains of the organism are no longer there. Bacteria decomposed the soft parts of the organism, and the shell dissolved, leaving indirect evidence of the organism’s existence.”

Trace Fossils. “These molds and casts we examined today are trace fossils. Trace fossils are evidence of ancient plants and animals preserved in rocks of the Earth’s crust, not the fossilized remains themselves. Trace fossils, then, are indirect evidence of the original organism. What other types of trace fossils do you think we might find in rocks?” Tracks, trails, burrows, excrement, stomach stones (rounded pebbles believed to be from the stomachs of animals that ground the food in the stomach into smaller pieces).

Connecting Modeling with Earth Processes. “Now let’s review the Earth processes that produce molds and casts in the fossil record. Are all fossils marine (ocean) fossils?” [No. Some are of land animals (like mammoths and dinosaurs) and fresh water organisms (like fresh water fish).] “Most fossils we find are the remains or traces of organisms that lived in oceans. Burial is most likely to happen in the ocean because sediments are more likely to cover and preserve organism remains there. We used seashells as an example of the fossilization process in class today. We also modeled the most common type of fossilization—molds and casts of animals that lived in the ocean. Perhaps only 1 in a million organisms has a chance of becoming a fossil.”

Facilitate a class discussion reviewing the fossil modeling to Earth processes. Note the steps on the board:

1. First the organism dies.

2. Next it is buried by sediment, most likely in oceans.

3. A mold of the buried organism may form when the organism rots and the hard parts dissolve, leaving an empty space as an impression (mold) of the dead organism.

4. If sediment fills the empty space, then a cast may form.

“We call these fossils trace fossils because they do not contain remains of the dead organism, but they do give us evidence that the organism once existed.”

Students continue working in the same groups. Call students’ attention to the class definition of the term “fossil” that was posted on the board at the beginning of the lesson. Give students time to read and mentally critique the definition in light of information gained from the lesson. “Now discuss and record a revised group explanation of what a fossil is.” In general, fossils are the remains or traces (evidence) of ancient plants and animals preserved in rocks of the Earth’s crust.

Hand out the image of Phacops rana (S-7-2-1_Phacops rana Image.doc) “Your final task as a group is to write a short, illustrated fossil history of the now extinct trilobite, Phacops rana, the state fossil of Pennsylvania. Phacops rana once lived in the ancient ocean that covered the area we call Pennsylvania today. Your history must include:

1. the name of the organism.

2. a sketch of the fossil.

3. the fossilization history of the trilobite in paragraph form—in other words, how did the trilobite become the fossil that is pictured in the photograph?

4. an explanation of whether the fossil is a mold or a cast, explaining the difference between the two types of fossilization process.”

Extension:

• Students performing above and beyond the standards can investigate other types of fossilization processes, such as freezing (frozen mammoths in Siberia and Alaska), natural mummification, and preservation in amber.

• For students requiring further practice with the standards, limit the scope of the fossil histories in order to meet student needs.