| A | B |
|---|
| geologic column | ordered arrangement of rock layers that is based on the relative ages of the rocks-oldest rocks are at the bottom |
| era | unit of geologic time that includes two or more periods |
| period | a unit of geologic time that is longer than an epoch but shorter than an era |
| epoch | a subdivision of geologic time that is longer than an age but shorter than a period |
| evolution | heritable change in characteristics within a population from one generation to the next; new types of organisms develop from preexisting ones |
| Precambrian time | the interval of time in geologic time scale from Earth's formation to beginning of Paleozoic era-4.6 billion to 542 million years ago |
| Paleozoic Era | the geologic era that followed Precambrian time--lasted from 542 million to 251 million years ago |
| mass extinction | an episode during which large numbers of species become extinct |
| Mesozoic Era | the geologic era that lasted from 251 million to 65.5 million years ago; also called Age of Reptiles |
| Cenozoic Era | the current geologic era, which began 65.5 million years ago; also called Age of Mammals |
