Fossils are usually formed when an organism is covered by sediments that then harden into sandstone, slate, mudstone or flint.

Most organisms do not fossilize and those that do are usually destroyed by geological processes or they never surface for examination.We have only discovered about 250,000 fossilized species. This tells us that there are many gaps in the fossil record.

Most of the time, organisms die in locations that are not conducive to fossilization.

By the 1700s it was accepted that fossils were the buried remains and impressions of organisms that had lived in the past.

Biologists generally recognize two types of fossils:

• Body Fossils

• Trace Fossils

• Unaltered remainsUnaltered skeletal material, tar impregnation, amber entombment, refrigeration, mummification

This category includes those fossils that have undergone little or no change in structure and composition. As a general rule an organism which lived fairly recently has a greater probability of being unaltered than a more ancient one.

• Altered Remains

As the sediments that entomb fossils become compressed by the weight of overlying sediments, they slowly become rocks. The same processes that turn sediments into rocks can alter organic remains .

Permineralization, dissolution/replacement, carbonization

• Mold

A fossil is any trace of past life. A fossil does not have to be an actual piece of an ancient organism. Many fossils are bones, shells, or other body parts. However footprints and other traces are fossils too.

• Cast• Borings and Burrows• Copralites

• Gastroliths• Gnawings

• Relative Dating Techniques

Law of superposition (Nicolaus Steno)

You cannot use this technique to get the actual date of a specimen

To find the specific age of rocks, scientists use radiometric dating.By using the radioactive isotopes naturally present in rocks, and understanding the rates at which these isotopes decay, scientists can determine approximate age of the rocks.

Radioactive isotopes are atoms with unstable nuclei that break down, or decay, over time, giving off radiation. The isotope is eventually changed into, or replaced by another element over time. By measuring the relative amounts of parent and daughter isotopes, you can determine the age of a specimen.

Atoms of the same element with differing atomic weights, due to differing numbers of neutrons, can be naturally found in the environment, and are called isotopes.

• The element potassium-40 decays to argon-40 and has a half-life of approximately 1.3 billion years• The element carbon-14 decays more rapidly into Nitrogen-14, and has a half-life of only 5,730 years.

RADIOMETRIC DATING TECHNIQUES

Because Carbon 14 has such a short half-life, it can only be used to date things that died recently in geologic history (under 60,000 years ago).

If you need to date rocky material from farther back in our geologic past, you must use another method, such as Potassium/Argon, which has a much longer half-life.

• Also…You can only date organic remains using carbon 14 as well

Some isotopes, however, decay slowly, and several of these are used as geologic clocks. The parent isotopes and corresponding daughter products most commonly used to determine the ages of ancient rocks are listed below:

Most radioactive isotopes have rapid rates of decay (that is, short half-lives) and lose their radioactivity within a few days or years.

Carbon samples are converted to acetylene gas by combustion in a vacuum line. The acetylene gas is then analyzed in a mass spectrometer to determine its carbon isotope composition.

Parent Isotope Stable Daughter Product

Currently Accepted Half-Life Values

Uranium-238 Lead-206 4.5 billion years

Uranium-235 Lead-207 704 million years

Thorium-232 Lead-208 14.0 billion years

Rubidium-87 Strontium-87 48.8 billion years

Potassium-40 Argon-40 1.25 billion years

Samarium-147 Neodymium-143 106 billion years

A half-life is the amount of time it takes of half of the radioactive isotope to change into the daughter material.

The graph below could represent any of the radioactive isotopes we’ve learned about. As you can see, after about 7 half-lives, there is little, if any of the original parent isotope left.

How old would a specimen have to be before 14C would be beyond its useful potential?

Go to the following link, and watch the short clip on how Lucy became a fossil:http://www.pbs.org/wgbh/evolution/library/04/3/l_043_01.html

Your task is to:

• pick any organism (plant or animal) from Earth’s geologic past that intrigues you.

• Write a short story about how your organism lived and died (be specific). Tell how your organism became a fossil (be specific about which type of fossil

your organism became, and how it was that it happened that way), and was once again unearthed in the future by some paleo-scientist.

• You must include a colorful drawing of your organism. (NOT a photo)

 Be creative. You may prepare a video presentation,, much like the short video clip you watched in class, or a Prezi. Use any and all technologies available to you! Upload it to netschool when complete.

Looking at evolutionary relationships among species, starting with an ancestral form and including branches leading to descendants

The study of speciation patterns in the form of an evolutionary tree diagram, showing relative relationships.