Specific environmental conditions are necessary in order for fossils to form.

Only a tiny percentage of living things became fossils. Fossils are preserved remains or traces of animals, plants, and other organisms from the past. Only a tiny percentage of living things became fossils. Fossils can form in several ways.

Permineralization occurs when minerals carried by water are deposited around a hard structure. Skeleton of a veloci-raptor

An organism dies in a location, such as a river bed, where sediments can rapidly cover its body. Over time, pressure from additional sediment compresses the body, and minerals slowly replace all hard structures, such as bone. Earthquakes or erosion may expose the fossils millions of years after formation.

Crinoid - marine animal A natural cast forms when flowing water removes all of the original tissue, leaving an impression. Crinoid - marine animal

Trace fossils record the activity of an organism. Footprints from a Dimetrodon dinosaur

Amber-preserved fossils are organisms that become trapped in tree resin that hardens after the tree is buried. Amber-preserved wasp

Preserved remains form when an entire organism becomes encased in material such as ice or volcanic ash. Ice preserved remains of a 5000 year old man found in the Alps

Relative dating estimates the time during which an organism lived. It compares the placement of fossils in layers of rock.

Radiometric dating uses decay of unstable isotopes to calculate the age of a material. Isotopes are atoms of an element that differ in their number of neutrons. neutrons protons

A half-life is the amount of time it takes for half of an isotope to decay.

Half Life Problems If an element has a half life of 20 years, how many years would it take for the element to decay to 25% of its original amount? 40 years If an element has a half life of 50 years and 150 years have gone by than what percent of the original amount is left? 12.5 %