A fossil formed when an animal, plant, or other organism dies and is covered by sediment, its flesh decays and bones deteriorate due to chemical reactions, and a cavity remains below the ground surface.

____________ is the science of determining the relative order of past events (i.e., the age of an object in comparison to another), without necessarily determining their absolute age, (i.e. estimated age).

Radiometric dating or ____________ is a technique used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed.

_____________. an element subject to spontaneous degeneration of its nucleus accompanied by the emission of alpha particles, beta particles, or gamma rays. All elements with atomic numbers greater than 83 are radioactive. Naturally occurring ______________ include radium, thorium, and uranium.

the time required for any specified property (e.g., the concentration of a substance in the body) to decrease by half 

(of a species, family, or other larger group) having no living members.

A policy of gradual reform rather than sudden change or revolution.

_____________ form when minerals replace all or part of an organism. Water is full of dissolved minerals. It seeps through the layers of sediment to reach the dead organism. When the water evaporates, only the hardened minerals are left behind.

_______________ is a term used by paleontologists  to refer to the total number of fossils that have been discovered, as well as to the information derived from them.

the hypothesis that evolutionary development is marked by isolated episodes of rapid speciation between long periods of little or no change.