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Fossils 8th Grade Science What is a Fossil? The remains, or traces, of organisms that lived long ago. A fossil can be a bone, footprint, shell, etc. I. Fossils in Rocks Fossils are usually found in sedimentary rocks Most fossils were formed underwater and were buried very quickly. Take millions of years to form. Soft parts decay. Only hard parts like bone and shells are left. Sediments harden into rock. II. Molds and Casts Molds are a hollow impression of a once living thing found in a rock. Form when an organism is buried by sediments and the sediments change to rock. The organism decays and leaves a cavity, or opening, in the rock. The cavity is the mold. Casts occur when a mold is filled with sediments or minerals that harden or become a rock. III. Imprints or Trace Fossils Usually made from the soft parts of an organism. There are many fish and leaf imprints. Footprints, nests, burrows and tracks, are also imprints. These can tell us about the animal’s speed or stride, whether they walked on 2 or 4 legs, if they were lone stalkers or roamed in herds. IV. Petrification or Permineralization Occurs when plant or animal tissue is replaced by minerals. Carbonization or coalification is when the tissue is replaced by carbon only. Mineral replacement is when tissue is replaced with other types of minerals. V. True Form or Whole Organisms True Form fossils are fossils of the actual animal, plant, or body part. There are three substances that contribute to the formation of True Form Fossils. Fossils in Ice Freezing preserves things by preventing decay. Animals such as mammoths have been found totally preserved in ice. Fossils in Tar Tar pits were often covered with water, so when animals came to drink they became stuck in the tar. Example: Saber tooth tigers. Whole Organisms Continued… Fossils in Amber Hardened tree sap is called amber. Insects become trapped and are later found preserved.