Extinct
a species that has died out.
Paleo
prehistoric, early or primitive.
Paleontologist
researchers who study fossils to learn about ancient life .
Relative dating
places rock layers and their fossils in a sequence based on which came first and which came last .
Index fossil
distinct fossils used to establish and compare the relative ages of rock layers and the fossils they contain.
Radiometric dating
uses the proportion of radioactive to stable isotopes to calculate the actual age of a sample.
Half-life
the time it takes for half of the radioactive atoms in a sample to decay.
Geologic time scale
a timeline of the earth's history that is based on both relative and absolute dating .
Eon
largest division of the geologic time scale.
Era
the smaller units that eons are divided into.
Period
smaller units that eras are divided into.
Plate tectonics
movements of the earth's solid, crustal plates moving over the mantle.
Evolution
change over time.
Mass extinction
many species become extinct over a short period of time .
Gradualism
slow and steady evolution over time .
Evolutionary equilibrium
species structures do not change much , but they continue to change genetically .
Punctuated equilibrium
evolutionary equilibrium is interrupted by short periods of rapid change.
Adaptive radiation
species or small group of species that evolve over short time into several different forms that live in different ways .
Convergent evolution
when evolution produces similar structures and characteristics in distantly related organisms over time .
Coevolution
two species evolve in response to changes in each other over time .