Fossils Ch. 13 Section1

Fossils  Remains, imprints, or traces of prehistoric organisms  Scientists who study fossils are paleontologists.  Fossils tell us when, where, and how organisms lived.  Fossils are found in sedimentary rocks.

Conditions needed for fossil formation  For a dead organism to become a fossil it must be protected from scavengers and physically destructive things like waves and currents.  One way a dead organism can be protected is that is can be buried quickly by sediments.  Organisms with hard parts, like bones, shells, or teeth have a better chance of becoming fossils for 2 main reasons. 1) Scavengers are less likely to eat hard parts. 2) Hard parts take longer to decay.

Types of Preservation  Hard parts of organisms have tiny spaces within them.  Permineralized remains are fossils in which the spaces inside are filled with minerals from ground water.

Carbon Films  Tissues of organisms are made up of compounds.  As sediment piles up on dead organisms, the remains are subjected to pressure and heat.  A thin film of carbon is left that forms a silhouette of the original organism, called a carbon film.  Carbonized plants become coal.

Molds and Casts  Seashells and hard parts or organisms fall into the sediment.  They get buried by more sediment, and compaction and cementation change the sediment into rock.  The hard part might decay and leave behind a cavity called a mold.  Sediments or minerals might enter the mold and produce a cast, or copy of the original object.

Original Remains  Sometimes conditions allow the soft parts of organisms to be preserved.  Insects trapped in amber, a form of sticky tree sap.  Frozen ground and tar pits can also preserve organisms.

Trace Fossils  Fossilized tracks are evidence of an organism’s activities.  These can be tracks left in mud of stone.  Trails and burrows left by worms and other animals can tell you how they lived.

Index Fossils  Index fossils are the remains of species on Earth that: 1) Live short periods of time 2) were abundant 3) widespread geographically.  Scientists can estimate the ages of rock layers based on the index fossils they contain because they only lived during a specific time.

Fossils and Ancient Environments  Fossils can tell us: 1) The past climate of a region 2) Type of environment that existed when the rock layer formed.