Fossils are the remains of ancient animals and plants, the impressions of living things from past geologic ages or the traces of their activities. Fossils have been found on every continent. The word “fossil” comes from the Latin word fossils, which means "dug up." Most fossils are excavated from sedimentary rock layers. Sedimentary rock was formed from sediment such as sand, mud and small pieces of rocks. Over long periods of time, these small pieces of debris are squeezed as they are buried under more layers of sediment. Eventually, they are compressed into sedimentary rock. The layers deeper in the earth are older than the top layers.

Types of Fossils

• Impression or mold fossils – These are fossils made by organisms left in sediments. If a small, soft organism is covered with sediments, the organism will leave its body prints on the surrounding mud. It will eventually decay with only the prints left on the sediments. Impression fossils are broken into two categories: (1) imprint fossils and (2) mold and cast fossils. A third category is trace fossils.
  • Imprint – These are impressions of thin organisms such as feathers, leaves or fish that had fallen into sediment before it hardened. Later the organisms decomposed, leaving only the carbon remains on the sedimentary rock.
  • Mold and cast
    • Mold fossils are impressions made from larger organisms. When an organism dies and is covered by sediments, it decomposes slowly. The cavity left in the rock will retain the exact shape and size and is called the mold.
    • The cavity in the sedimentary rock may later fill with sediments and may take the shape of the mold. This is the cast. It looks just like the original organism on the outside.

  • Trace fossils are marks or tracks left by ancient organisms that have been preserved in sedimentary rock. These fossils show organisms once passed through an area. They include:

    • Tracks or footprints made in soft mud
    • Trails or paths left by a moving body in soft mud
    • Burrows made in soft mud
    • Coprolites or dinosaur dung
    • Eggshells
    • Gastroliths or digestive stones
    • Body imprints

• Preserved fossils – These are fossils that are unaltered, and the original organism stays intact. The soft body parts as well as the hard parts are preserved.

  • Freezing – Sometimes whole organisms will be encased in ice or snow that never melts. Freezing the organism prevents decay.
  • Amber – Sometimes an insect gets trapped in tree sap. Over time the sap fossilizes to amber.
• Mineral replacement – The fossils we are most familiar with are mineral replacement fossils (also known as mineralized, petrified and fossilized). When an animal is buried in sediments, the soft parts of the organism decay quickly. The hard parts such as bones, teeth and claws will not decay right away. Since they stay unchanged for many years, they might be mineralized. For example, when the original bone is buried, water must seep through the sediment and pass through the bone. At this point, mineral replacement can take place. The seeping water dissolves the bone, but as the water is dissolving it, the minerals in the water replace the bone one cell at a time, changing it to stone. Wood changes to stone in the same way except that wood is often covered with volcanic ash instead of sediments. When a volcano explodes and sends ash into the air, it lands in thick layers. If a forest is nearby, it will cover the trees and prevent the wood from rotting. Rainwater falls on the ashes for many years and seeps through the ash to the wood. The water dissolves the wood and replaces each cell with minerals, changing it to stone.
• Importance – Plants and animals have come and gone as a natural process. We can study causes of these changes, such as natural disasters and environmental changes.  Since man has been in certain areas, he has often impacted this process by over-hunting, polluting, etc.  Conservation agencies and concerned citizens work to monitor the balance within nature and try to correct imbalances to prevent unnatural extinctions of species.

Fossil Excavations
After being found, a fossil must be carefully freed from the rocky matrix that encased it for millions of years without damaging it. First, the fossils should be labeled and photographed (while still encased in the rock). Its position should be carefully noted. Most of the overlying rock (the overburden) is removed using large tools such as picks and shovels, but the two to three inches of rock closest to the fossil are removed with smaller hand tools such as trowels, hammers, whisks and dental tools. The exposed fossil is photographed and labeled again. Frequently, only some of the overlying rock is removed at the dig site. The rest of the overburden can be removed later in the lab. Small fossils are easily excavated with small hand tools. Large fossils require more effort and bigger tools in order to expose the specimen. These tools include shovels, picks, jackhammers or even explosives. Small and large fossils are excavated differently, but both have to be treated very carefully to avoid breaking them. Before removing a crumbling or fragile fossil, quick-setting glue can be applied to it (with a brush or sprayer). Then the fossil can be removed from the surrounding rock. The fossil must be packed very carefully to be moved to the lab. Small fossils can be packed in boxes or bags. Large fossils can be first wrapped in paper or burlap with a layer of plaster applied, similar to setting a broken bone.