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

Fossils can form in several ways. Permineralization occurs when minerals carried by water are deposited around a hard structure.

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

Trace fossils record the activity of an organism.

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

Preserved remains form when an entire organism becomes encased in material such as ice.

Specific conditions are needed for fossilization. Only a tiny percentage of living things became fossils.

Radiometric dating provides an accurate way to estimate the age of fossils. Relative dating estimates the time during which an organism lived. It compares the placement of fossils in layers of rock. Scientists infer the order in which species existed.

Radiometric dating uses decay of unstable isotopes. Isotopes are atoms of an element that differ in their number of neutrons. neutrons protrons

Radiometric dating uses decay of unstable isotopes. Isotopes are atoms of an element that differ in their number of neutrons. A half-life is the amount of time it takes for half of the isotope to decay.

KEY CONCEPT The geologic time scale divides Earth’s history based on major past events.

Index fossils are another tool to determine the age of rock layers. Index fossils can provide the relative age of a rock layer. existed only during specific spans of time occurred in large geographic areas Index fossils include fusulinids and trilobites.

The geologic time scale organizes Earth’s history. The history of Earth is represented in the geologic time scale. 100 250 550 1000 2000 PRECAMBRIAN TIME Cyanobacteria This time span makes up the vast majority of Earth’s history. It includes the oldest known rocks and fossils, the origin of eukaryotes, and the oldest animal fossils.

Eras last tens to hundreds of millions of years. consist of two or more periods three eras: Cenozoic, Mesozoic, Paleozoic

Periods last tens of millions of years. most commonly used units of time on time scale associated with rock systems. Epochs last several million years.