Saturday, February 25, 2012

Reflections — Fossils 1

In 2010, I was nominated for the Presidential Award for Excellence in Math and Science Teaching—a prestigious honor for math and science teachers in the United States. The rigorous application process provided me with an excellent opportunity to reflect deeply on my classroom practice. Although I was not selected as a finalist, I value my experience in the process. Over the next three blog posts, I would like to share some of what I wrote for my application, which centered around a geologic unit on fossils.

The PAEMST application requires a written narrative on several dimensions of outstanding teaching, including the following areas:

  • Dimension 1: Mastery of Science Content
  • Dimension 2: Instructional Methods and Strategies
  • Dimension 3: Effective Use of Student Assessments

In this entry, I would like to share Dimension 1, Mastery of Science Content...

There are three essential questions to guide the study of fossils:
  1. What are fossils? 
  2. How do fossils form? 
  3. What can fossils tell us about past life? 
Planet Earth is billions of years old, and during its long history a diversity of life has evolved. Fossils are the remains of living organisms preserved in the geologic record through burial, and they provide valuable evidence for this long Earth history. Students need fundamental understanding of the geologic processes that led to fossil formation, the types of fossils that form as a result, and the types of clues that are found within the fossils that can be used to interpret past history.
Image courtesy of MorgueFile

Three main types of fossils—body, trace, and replacement—form over time periods of thousands to millions of years. Each type of fossil has unique physical characteristics and clues to help us infer past Earth history. Because fossils are found mainly within sedimentary rocks, it is essential that students understand the constructive and destructive processes involved in sedimentary rock formation, which ultimately lead to the preservation of once-living organisms.

Information about the environments in which fossilized organisms lived can be inferred from fossil clues when students rigorously apply the process of science. Making careful, detailed observations using different scientific tools (such as hand lenses and microscopes), asking a variety of probing “interview” questions about each fossil, referencing fossil identification guides and charts, and collaborating with other student scientists are all key to interpreting fossil clues in a non-biased, open-minded manner to unlock the information about their past environments.

Fossil evidence helps us infer geologic, environmental, and biological changes through time. By examining fossils and reconstructing the story they tell, students fine tune their “process of science” skills and develop a better understanding of the 3.5+ billion-year life history of planet Earth. Specifically, students engage in making detailed observations of fossils and asking questions in order to infer information about the geologic past.Students will apply what they learned during fossil identification to a future plate tectonics research project. The skills and knowledge developed in this lesson will be applied to understanding spatial and temporal changes of Earth’s tectonic plates—fossil evidence gives us clues that Earth’s crustal plates move and that continents were once connected.

This particular lesson on fossils contributes to the “Six Facets of Understanding”—a lesson planning tool I use to develop deep, engaging learning activities—as outlined below: 
  • Explanation: Students address the following conceptual questions: 1) What are fossils? 2) What the different types of fossils? 3) How do fossils form? 4) How can we infer information about past environments from examining fossils?
  • Interpretation: Students observe and “interview” fossils to determine the physical characteristics of organisms and infer the past environment in which they lived.
  • Perspective: Students make connections between life and conditions on planet Earth today and life and conditions during geologic eras and periods of the past.
  • Self-Knowledge: Students may inquire about or begin to assemble their own fossil collection, or seek opportunities to learn more about fossils (for example, museum visits).
  • Empathy: Students consider why geologists study fossils and develop a better understanding about the difficulties involved in fossil interpretation and reconstruction.
  • Application: Students engage as fossil scientists and create their own fossil identification cards that show physical characteristics and inferred information about past environments.

A variety of misconceptions and misunderstandings about fossils can arise in the science classroom. Here is a summary:

All rocks contain fossils.
Fossils are found mainly in sedimentary rocks. For an organism to become a fossil, a protective layer of sediment, such as sand or mud, must quickly cover up its remains. Otherwise, the remains are likely to be eaten, to decompose, or to be destroyed before fossilization can occur. Probing questions to ask students: 
  • Why don’t we find fossils inside rocks like granite, basalt, gneiss, or schist?
  • Why do we find fossils inside rocks like sandstone, shale, and slate?
Fossils are actual pieces of dead animals and plants, rather than preserved impressions of the original organisms. 
During the fossilization process, the living remains of organisms decay and are replaced by minerals, leaving behind an impression of the original organism. Exceptions to this include insects that are fully preserved in fossilized tree sap (amber). However, most organisms are preserved in an altered state during the fossilization process. Probing questions to ask students: 
  • Petrified wood, a fossil, is not wood anymore. What happened to the wood?
  • If you have a fossil shell or leaf imprint, what happened to the original organism?
All living organisms leave behind fossils, and the fossil record is complete. 
Hard structures such as bones, teeth, shells, and exoskeletons are more likely to be preserved during the fossilization process than soft body materials such as tissues, organs, and/or hair/feathers, which decay more readily. It is estimated that only 1% to 3% of organisms have fossilized and/or been described. While our fossil record keeps growing as we explore more and more of Earth’s crust, we will never have a 100% fossil record because not all species leave behind fossils and not all fossils can ever be found or recovered. Probing questions to ask students: 
  • Why do scientists keep looking for fossils, even today?
  • Why don’t we find fully preserved, fully intact organisms (like dinosaurs) buried in the ground? Why do we usually just find skeletons, bones, teeth, etc.?
  • When an organism dies, what parts decay and what parts are preserved, and why? 
Geologic time is short, and fossils are relatively young. 
Geologic time is vast. The fossilization process, which occurs during the sedimentary rock formation process, can take thousands and millions of years to occur. Various dating techniques allow scientists to accurately determine the ages of rocks and fossils. Fossils are thousands, millions, and in some cases, billions of years old. Probing questions to ask students: 
  • How do you think fossilized seashells got to the top of the Rocky Mountains?
  • What does finding fossilized seashells in the Rocky Mountains tell us about the past?
Few species have gone extinct during Earth’s history. 
More than 99% of all species (which number in the millions) that ever lived on planet Earth have gone extinct. The fossil record leaves us evidence of species that are no longer alive and what the environments in which they lived were like. Probing questions to ask students: 
  • Have you ever seen an organism like this (show pictures of extinct organisms)?
  • What happened to these organisms?


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