| A | B |
|---|
| Precambrian Era | earliest and longest division of geologic time, lasting about 4 billion years |
| Paleozoic Era | division of geologic time lasting about 345 million years, after the Precambrian Era and before the Mesozoic Era |
| Mesozoic Era | division of geologic time, lasting about 160 million years, after the Paleozoic Era and before the Cenozoic Era |
| Cenozoic Era | division of geologic time that has lasted about 65 million years after the end of the Mesozoic Era |
| trilobite | animal that is an important index fossil for the Paleozoic Era |
| Triassic Period | subdivision of geologic time at the beginning of the Mesozoic Era |
| Jurassic Period | middle subdivision of geologic time in the Mesozoic Era |
| Cretaceous Period | subdivision of geologic time at the end of the Mesozoic Era |
| evolution | change in species over time |
| adaptation | change that increases an organism's chancees of survival |
