Subject: Earth Science

Fossils

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· Lesson Objectives

· The student will investigate and understand the rock cycle as it relates to the origin and transformation of rock types and how to identify common rock types based on mineral composition and textures.

· Key concepts include sedimentary (clastic and chemical) rocks, scales, diagrams, maps, charts, graphs, tables, and profiles are constructed and interpreted.

· The student will investigate and understand that many aspects of the history and evolution of the Earth and life can be inferred by studying rocks and fossils.

· Key concepts include traces and remains of ancient, often extinct, life are preserved by various means in many sedimentary rocks, superposition, cross-cutting relationships, index fossils, and radioactive decay are methods of dating bodies of rock, absolute and relative dating have different applications but can be used together to determine the age of rocks and structures; and rocks and fossils from many different geologic periods and epochs are found in Virginia.

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Geological Time

When we look at human history, we often talk about things in terms of hundreds or thousands of years. During the majority of this time we have some sort of written records to analyze. When we talk about looking at the history of the Earth, and the evolution of organisms on it, we are looking at time periods spanning millions and billions of years. Understanding this portion of history requires detective work: gathering evidence and making comparisons.

 

 

 

People who study this kind of history have to use a greater time scale than that used for human history; this time scale is referred to as the geologic time scale. Think of it like a book, with the rocks as its pages. Some of the pages are torn or missing, and the pages are not numbered, but geology gives us the tools to help us read this book.

 

Long before geologists had the means to recognize and express time in numbers of years before the present, they developed the geologic time scale. This time scale was developed gradually, mostly in Europe, over the eighteenth and nineteenth centuries. Earth's history is subdivided into eons, which are subdivided into eras, which are subdivided into periods, which are subdivided into epochs. The names of these subdivisions, like Paleozoic or Cenozoic, may look daunting, but to the geologist there are clues in some of the words. For example, zoic refers to animal life, and paleo means ancient, meso means middle, and ceno means recent. So the relative order of the three youngest eras, first Paleoozoic, then Mesozoic, then Cenoozoic, is straightforward.

 

 

 

 

Fossils are the recognizable remains, such as bones, shells, or leaves, or other evidence, such as tracks, burrows, or impressions, of past life on Earth. Scientists who study fossils are called paleontologists. Remember that paleo means ancient; so a paleontologist studies ancient forms of life. Fossils are fundamental to the geologic time scale. The names of most of the eons and eras end in zoic, because these time intervals are often recognized on the basis of animal life. Rocks formed during the Proterozoic Eon may have fossils of relative simple organisms, such as bacteria, algae, and wormlike animals. Rocks formed during the Phanerozoic Eon may have fossils of complex animals and plants such as dinosaurs, mammals, and trees.

 

 

 

 

 

 

 

This video shows what the earth was like during the Pre-Cambrian.  Notice how simple the life forms were and where they all were found:

 

 Now let's compare what life looks like throughout the Phanerozoic Eon. Notice how life changes.

 

Sedimentary Rocks

Most of the rocks exposed at the surface of Earth are sedimentary--formed from particles of older rocks that have been broken apart by water or wind. The gravel, sand, and mud settle to the bottom in rivers, lakes, and oceans. These sedimentary particles may bury living and dead animals and plants on the lake or sea bottom. With the passage of time and the accumulation of more particles, and often with chemical changes, the sediments at the bottom of the pile become rock. Gravel becomes a rock called conglomerate, sand becomes sandstone, mud becomes mudstone or shale, and the animal skeletons and plant pieces can become fossils.

 

As early as the mid-1600's, the Danish scientist Nicholas Steno studied the relative positions of sedimentary rocks. He found that solid particles settle from a fluid according to their relative weight or size. The largest, or heaviest, settle first, and the smallest, or lightest, settle last. Slight changes in particle size or composition result in the formation of layers, also called beds, in the rock. Layering, or bedding, is the most obvious feature of sedimentary rocks.

 

Sedimentary rocks are formed particle by particle and bed by bed, and the layers are piled one on top of the other. Thus, in any sequence of layered rocks, a given bed must be older than any bed on top of it. This Law of Superposition is fundamental to the interpretation of Earth history, because at any one location it indicates the relative ages of rock layers and the fossils in them.

 

Layered rocks form when particles settle from water or air. Steno's Law of Original Horizontality states that most sediments, when originally formed, were laid down horizontally. However, many layered rocks are no longer horizontal. Because of the Law of Original Horizontality, we know that sedimentary rocks that are not horizontal either were formed in special ways or, more often, were moved from their horizontal position by later events, such as tilting through plate tectonics or earthquakes.

 

Rock layers are also called strata (the plural form of the Latin word stratum), and stratigraphy is the science of strata. Stratigraphy deals with all the characteristics of layered rocks; it includes the study of how these rocks relate to time.

 

This video shows how we use relative age to date rock layers:

 

 

More on the Time Scale

How can we add numbers to a time scale based on rock sediment? How have geologists determined that:

· Earth is about 4.6 billion years old?

· The oldest known fossils are from rocks that were deposited about 3.5 billion years ago?

· The first abundant shelly fossils occur in rocks that are about 570 million years old?

· The last ice age ended about 10,000 ago?

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Geologic time scale showing both relative and numeric ages. Ages in millions of years are approximate

 

Nineteenth-century geologists and paleontologists believed that Earth was quite old, but they had only crude ways of estimating just how old. The assignment of ages of rocks in thousands, millions, and billions of years was made possible by the discovery of radioactivity.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Now we can use minerals that contain naturally occurring radioactive elements to calculate the numeric age of a rock in years.

 

As you know, the basic unit of each chemical element is the atom. An atom consists of a central nucleus, which contains protons and neutrons, surrounded by a cloud of electrons.  Isotopes  of an element are atoms that differ from one another only in the number of neutrons in the nucleus. For example, radioactive atoms of the element potassium have 19 protons and 21 neutrons in the nucleus (potassium 40); other atoms of potassium have 19 protons and 20 or 22 neutrons (potassium 39 and potassium 41). A radioactive isotope (the parent) of one chemical element naturally converts to a stable isotope (the daughter) of another chemical element by undergoing changes in the nucleus.

 

The change from parent to daughter happens at a constant rate, called the  half-life . The half-life of a radioactive isotope is the length of time required for exactly one-half of the parent atoms to decay to daughter atoms. Each radioactive isotope has its own unique half-life. Precise laboratory measurements of the number of remaining atoms of the parent and the number of atoms of the new daughter produced are used to compute the age of the rock. For dating geologic materials, four parent/daughter decay series are especially useful: carbon to nitrogen, potassium to argon, rubidium to strontium, and uranium to lead. Age determinations using radioactive isotopes are subject to relatively small errors in measurement--but errors that look small can mean many years or millions of years. If the measurements have an error of 1 percent, for example, an age determination of 100 million years could actually be wrong by a million years too low or too high.

 

Parents and daughters for some isotopes commonly used to establish numeric ages of rocks.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isotopic techniques are used to measure the time at which a particular mineral within a rock was formed. To allow us to assign numeric ages to the geologic time scale, a rock that can be dated isotopically is found together with rocks that can be assigned relative ages because of their fossils. Many samples, usually from several different places, must be studied before assigning a numeric age to a boundary on the geologic time scale.

 

This video explains how we find the absolute age of a rock or fossil using half-life:

 

The geologic time scale is the product of many years of detective work, as well as a variety of dating techniques not discussed here. The details will change as more and better information and tools become available. Many scientists have contributed and continue to contribute to the refinement of the geologic time scale as they study the fossils and the rocks, and the chemical and physical properties of the materials of which Earth is made.

 

Fossil Succession

 

When most people think of fossils, they tend to think of dinosaurs, but dinosaurs only form a small fraction of the millions of species that live and have lived on Earth. The great bulk of the fossil record is dominated by fossils of animals with shells and microscopic remains of plants and animals, and these remains are widespread in sedimentary rocks. It is these fossils that are studied by most paleontologists.

 

Three concepts are important in the study and use of fossils:

 

(1) Fossils represent the remains of once-living organisms.

(2) Most fossils are the remains of extinct organisms; that is, they belong to species that are no longer living anywhere on Earth.

(3) The kinds of fossils found in rocks of different ages differ because life on Earth has changed through time.

 

 

If we begin at the present and examine older and older layers of rock, we will come to a level where no fossils of humans are present. If we continue backwards in time, we will successively come to levels where no fossils of flowering plants are present, no birds, no mammals, no reptiles, no four-footed vertebrates, no land plants, no fishes, no shells, and no animals. The three concepts are summarized in the general principle called the Law of Fossil Succession: The kinds of animals and plants found as fossils change through time. When we find the same kinds of fossils in rocks from different places, we know that the rocks are the same age.

 

The Law of Fossil Succession is very important to geologists who need to know the ages of the rocks they are studying. The fossils present in a rock exposure or in a core hole can be used to determine the ages of rocks very precisely. Detailed studies of many rocks from many places reveal that some fossils have a short, well-known time of existence. These useful fossils are called index fossils.

 

 

Today the animals and plants that live in the ocean are very different from those that live on land, and the animals and plants that live in one part of the ocean or on one part of the land are very different from those in other parts. Similarly, fossil animals and plants from different environments are different. It becomes a challenge to recognize rocks of the same age when one rock was deposited on land and another was deposited in the deep ocean. Scientists must study the fossils from a variety of environments to build a complete picture of the animals and plants that were living at a particular time in the past.

 

If we look at the Law of Fossil Succession together with the concept of evolution, we see that first there were simple single celled organisms, then more complex organisms, then movement from the oceans onto land by both plants and animals, then formation of larger animals on land. We can study the transitions of evolution through studying the fossil record--though the record will not always be complete.

 

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Bonus Content (This section is not required)

If you are interested in more information about some of the topics from the lesson OR if you need help with some of the questions, you can watch these  OPTIONAL  (you can watch if you want but they are not required) v ideos for help!

 

Watch the following videos to find out more information on what life was like during the Paleozoic, Mesozoic, and Cenozoic time periods.

 

Paleozoic Life

 

Mesozoic Life

 

Cenozoic Life (Time period we are in right now)

· Grading Rubric

Rubric Earth Science Lesson 8

 

Note: For this class it is necessary to post each question, then the work/explanation, then the answer. Failure to do so will result in asking for a revision. No grade will be given for incomplete work.

Mastery of this lesson will be determined when your total points are 8 or higher. Revisions will be requested when your total points are below 8. Points are earned according to the chart below.

10:  A total score of 10 on your first submission, or within the first revision.

9.5: A total score of 9.5, or all questions are correct after the first revision.

9:    A total score of 9.

8.5: A total score of 8.5.

8:    A total score of 8.

 

Short Answer  5 points total	Answers are clearly written, accurate, and student uses their own words. 1 points awarded	Answer is clearly written. May have 1 factual omission or error.  0.5 point awarded	Answer is not clearly written. There are several factual omissions or errors.     0 points awarded
Apply Your Knowedge 3 points total  (1 point per question)	Questions are thoroughly answered in complete sentences and in student’s own words. URLs are included for outside research.    1 point awarded	Questions are answered but may have incomplete responses or contain minor errors. Responses are in the student’s own words. URLs are included for outside research.      0.5 point awarded	Questions contain inaccurate information and/or are not written in the student’s own words. URLs for outside research are not provided.       0 points awarded
Essay 2 points total	Question is answered thoroughly describing how the meteor killed the dinosaurs and at least 2 pieces of evidence are provided. 2 points awarded	Question is answered partially and/or one piece of evidence is missing.   1 point awarded	Question is not answered thoroughly and evidence is not provided.   0 points awarded

 

· Assignment

 

Do not submit text that you have copied from sources, including websites. All of your work should be in your own words. Using copied text would be considered plagiarism. For more information, review our page on Plagiarism and Citation. Cite the complete web page source under each answer. Always put the question on top of the answer, and answer in complete grammatically correct sentences.

Many of these answers will require Internet research to answer. Make sure you rewrite all answers into your own words and be mindful of our page on  Plagiarism and Citation .

 

Short Answer

 

1. How are sedimentary rocks formed?

 

2. Why are fossils found in sedimentary rocks instead of other rock types? Why can't they be found in igneous or metamorphic?

 

3. How are radioactive isotopes used to determine the age of rock? Use the word half life in your answer. Explain how it works.

 

4. How can the Law of Superposition tell us how old one fossil is compared to another found in the same general area?

 

5. What are fossils?

 

Apply Your Knowledge

 

6. What are index fossils? Give at least 2 characteristics of index fossils.

 

7. What Cenozoic fossils have been found in the region where you live? Name at least two and the location where they were found. Remember Cenozoic is the time period we are in right now. Be sure you cite your source.

 

8. What Mesozoic fossils have been found in the region where you live? Name at least two and the location where they were found. Be sure you cite your source.

 

Essay 

9. Write a paragraph about the asteroid theory of dinosaur extinction. Be sure to include at least 2 pieces of evidence to support this idea--Make sure to use iridium as one piece of evidence & explain why it is so important.