Fossils are the remains, imprints, or traces of prehistoric organisms. Fossils have helped determine approximately when life first appeared, when plants and animals arrived on land, and when organisms became extinct. CONDITIONS NEEDED FOR FOSSIL FORMATION Whether or not a dead organism becomes a fossil depends upon how well it is protected from scavengers and agents of physical destruction, such as waves and currents.

An organism must be buried quickly by sediment in order to be fossilized. Hard parts such as teeth, bones, and shells have a better chance of becoming fossilized because scavengers are less likely to eat them. TYPES OF PRESERVATION When an organism dies, the soft materials in the hard parts decay, tiny spaces in these parts become empty. If these parts are buried, groundwater can seep in and deposit minerals in the spaces This is called Permineralized Remains. Some original material may become encased in the minerals and DNA can be recovered.

The tissues of organisms are made of compounds that contain carbon. When sediments bury a dead organism, the remains are subjected to pressure and heat. These conditions force gases and liquid from the body and a thin film of carbon residue is left forming a silhouette of the original organism called Carbon Film. COAL When large volumes of plant matter accumulate in swampy regions, these deposits become carbonized becoming coal.

Impressions from when seashells or other hard parts of organisms fall into soft sediments such as mud. The object and sediments are then buried by more sediment. Compactation, together with cementation, turns the sediment into rock. The hard part might decay or dissolve, leaving behind a cavity in the rock called a mold. Mineral rich water and sediment fills the cavity, forms new rock and produces a copy or cast of the original object.

Sometimes conditions allow original soft parts of organisms to be preserved. Examples: insects trapped in amber, mammoths frozen in Siberia, remains found in natural tar pits such as the La Brea tar pits in California. TRACE FOSSILS Fossilized tracks and other evidence of the activity of organisms. These tracks can tell you more about how an organism lived than any other type of fossil. Trails and burrows of worms and other animals are also called trace fossils.

The remains of species that existed on Earth for relatively short periods of time, were abundant, and were widespread geographically. Index fossils lived only during specific intervals of geologic time, so geologists can estimate the ages of rock layers based on the particular index fossils they contain. Not all rocks contain index fossils, in that case the age of the rock layers is to compare the span of time, or ranges, over which more than one fossil appears. The estimated age is the time interval where the fossil ranges overlap.

Scientist use fossils to determine what the environment and climate of an area was like long ago. Fossils can provide information about the past life on Earth and also about the history of the rock layers that contain them.

Relative dating – is the process of placing events in sequence in which they occurred, does not identify actual dates Principle of superposition – states that in an undisturbed sequence of sedimentary strata, the oldest rock layer will at the bottom and the youngest at the top Principle of cross-cutting relationships – states that an igneous intrusion is always younger than the surrounding layers

Igneous intrusion – magma that has intruded into the rock layer and hardened Fossil – the remains, impression, or any other evidence of life from another geologic age preserved in rock Original remains – remains of an organism that have not been totally replaced by minerals Replaced remains – remains that are slowly replaced by rock forming minerals Mold – a hollow depression in rock formed when a fossil dissolves out of the rock; shows the original shape and the surface of the fossil

Unconformity – indicates where layers of rock are missing from the strata sequence Casts –an object created when minerals seep into a mold, forming a copy of the original Relative time – an estimated time based on events that happened before or after Relative dating – the process of placing events in the sequence in which they occurred, does not identify actual dates

Strata – the layers of rock that form after particles settle out of a fluid and are compressed over time Unconformity – indicates where layers of rock are missing in the rock sequence Principle of superposition – states that an in undisturbed sequence of strata, the oldest rock layer will be at bottom and the youngest at the top Index fossil – the fossilized remains of organisms that lived and died within a particular time segment of Earth’s history and that can be used t correlate rock layers

Trace fossil – any indirect evidence of life preserved as an impression in rock; trails, footprints, tracks, burrows, and bite marks Absolute time – actual dates of event Radioactivity – A phenomenon resulting from an instability of the atomic nucleus Radioactive decay – the process by which radioactive active isotopes emit or capture tiny particles Parent isotope – the original element that will after radioactive decay, become an isotope of a different element with a different atomic number

Daughter isotope – an element that is the product of radioactive decay Half- life – the time it takes half the atoms in a radioactive element to decay to stable end product Uniformitarianism – the idea that geologic processes that operate today also operated in the past

What are the necessary conditions that must be met for a dead organism to become a fossil? Whether or not a dead organism becomes a fossil depends upon how well it is protected from scavengers and agents of physical destruction, such as waves and currents. An organism must be buried quickly by sediment in order to be fossilized. Hard parts such as teeth, bones, and shells have a better chance of becoming fossilized because scavengers are less likely to eat them

What are some of the ways unconformities in rock layers can occur? An intrusion of igneous rock may occur. Also layers may become tilted and then the top layers can erode away or be worn down. Strata may also be missing because they were never deposited

How is radiometric dating used to determine the absolute age of rocks? Scientist use radioactivity and half-lives of elements to measure absolute time. The scientist measure the amounts of a parent and daughter isotope within a mineral or rock and use the ratio between the two to find the age of the rock. Radiometric dating can also be used on organic material, such as preserved remains, to determine the time of the organism’s death