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

What are some things we can learn about fossils? In an evolutionary sense, relating fossils, to the history of life on Earth, what else do they offer us?

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. Why are fossils commonly found near ancient or modern lakebeds and floodplains? What role does groundwater play in the middle figure?

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.

Compare and contrast chart on absolute and relative dating. Activity Compare and contrast chart on absolute and relative dating. Relative dating Absolute dating What they have in common

Activity Solve: “On a research expedition, scientists uncover 2 different fossils in 2 different places. The first appeared to be a human-like skull in a stratum associated with a period of approximately 4 million years ago; the second is a mummified human found in a crypt. Write proposals for determining the age of each set and the reasons why they are the appropriate ones to use.