Fossilization is a long shot ! Death is a sure bet . . . Fossilization is a long shot ! Fossils are usually formed when an organism is covered by sediments that then harden into sandstone, slate, mudstone or flint. Most of the time, organisms die in locations that are not conducive to fossilization. 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.

Fossils: Evidence of Ancient Life 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

Body Fossils Unaltered remains 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. Unaltered skeletal material, tar impregnation, amber entombment, refrigeration, mummification

Body Fossils 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 . Altered Remains Permineralization, dissolution/replacement, carbonization

Trace Fossils Mold Cast Borings and Burrows Copralites Gastroliths 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. Mold Cast Borings and Burrows Copralites Gastroliths Gnawings

Dating Fossils: Drawbacks: Relative Dating Techniques Law of superposition (Nicolaus Steno) Drawbacks: You cannot use this technique to get the actual date of a specimen

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

What About Carbon 14 ? 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). Also…You can only date organic remains using carbon 14 as well 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.

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

Project: Becoming a Fossil 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.

Next Time: Cladistics and Phylogeny The study of speciation patterns in the form of an evolutionary tree diagram, showing relative relationships. Looking at evolutionary relationships among species, starting with an ancestral form and including branches leading to descendants