Where Fossils Occur Almost Exclusively in Sedimentary Rocks Heat of Melting or Metamorphism Would Destroy Almost Every Type of Fossil Rare Exceptions: Some Fossils in Low-grade Metamorphic Rocks Trees Buried by Lava Flow To Be Preserved, Organisms Have to Be: Buried Rapidly After Death Preserved From Decay

Types of Fossils There are three different types of fossils that all form in their own special way: preserved fossils petrified fossils trace fossils

Preserved Fossils Mummification Occurs in hot, arid climates, which can dehydrate organisms before their soft tissue has decayed fully. Complete preservation in a relatively undamaged state of hard and soft body part A few mummified remains of animals have been found in some caves where the conditions are dry and sterile

Preserved fossils Amber - Organisms preserved in hardened tree sap. Sticky tree sap trapped insects, spiders and small animals such as frogs, preserving their external structures.

Preserved fossils cont… Tar Seeps – tar seeps formed by thick petroleum oozing to the earth’s surface – usually covered by water which attracts and traps animals

Preserved Fossils - Freezing Cold temperatures prevent bacteria from consuming dead organisms Organisms preserved in frozen soil or ice do not decay

2. Petrification A process where organic substances such as wood and shells are replaced by silica. Can occur when organisms are buried in volcanic ash The silica rich water penetrated into these tree trunks. As the silica solution filled in the cells, the cell walls were replaced and gradually the wood was turned to stone, or petrified

Trace Fossils Trace fossils are the preserved remains or signs of animals left behind as they went about their lives. They include footprints, like these dinosaur footprints, or tracks, burrows, nests, eggs and feces.

Trace Fossils - Coprolites Coprolites are the fossilized excrement of ancient animals. Can indicate diet of animals.

Trace Fossils - Gastroliths Gastroliths are used to pound the food into smaller pieces to aid in digestion

Trace Fossils - Molds Internal molds are fossilized replicas of the internal structure of an organism For example, when a clam dies, over time the soft parts of the animal decay. The shell is left empty and this becomes filled with sediment. Eventually the shell dissolves away, but the sediments have hardened and remain as a fossil.

Importance of Fossils Index fossils: organisms whose range zones have been used to represent named divisions of the geologic time scale To be considered an index fossil, must be found in different rocks around the world, have distinguishing fossil features and lived in a relatively short period of time. Fossils used: Brachiopods, Trilobites (extinct), Plants, Reptiles including Dinosaurs (extinct), Mammals, Amphibians and Sharks

Fossils of Interest Mollusks: Shelled invertebrates Bivalvia – two valves connected by a hinge Gastropoda – Single spiral shell Cephalopoda – “Squids with shells” Ammonoids – similar to some nautiloid cephalopods but include ribs and bumps on shell

Fossils of interest Echinoderms – calcite skeletons, basic 5-part symmetry (e.g. starfish) - Blastoidea – calcite rosebud shaped Crinoidea – columnar stem, head and arms

Fossils of Interest Brachiopods - two-shelled, marine animals that somewhat resemble clams externally

Fossil Lab In groups of three, you are required to sketch, classify and date the replica fossils that are provided. When sketching, do not shade - use dots instead. Use a pencil. Classify as specific as possible When dating – provide the period(s) in which they can be found