an ordered arrangement of rock layers that is based on the relative ages of the rocks and in which the oldest rocks are at the bottom.
a unit of geologic time that includes two or more periods.
a unit of geologic time that is longer than an epoch but shorter than an era.
a subdivision of geologic time that is longer than an age but shorter than a period.
the process of change by which new species develop from preexisting species over time.
the geologic era that followed Precambrian time and that lasted from 542 million to 251 million years ago
the interval of time in the geologic time scale from Earth's formation to the beginning of the Paleozoic era, from 4.6 billion to 542 million years ago.
an episode during which large numbers of species become extinct
the geologic era that lasted from 251 million to 65.5 million years ago
the current geologic era, which began 65.5 million years ago