| A | B |
|---|
| era | one of the three long units of geologic time bewteen the Precambrian and the present |
| invertebrate | an animal without a backbone |
| period | one of the units of geologic time into which geologists divide eras |
| epochs | subdivisions of the periods of the geologic time scale |
| amphibian | a vertebrate that lives part of its life on land and part of its life in water |
| mass extinction | when many types of living things become extinct at the same time |
| mammal | a warm-blooded vertebrate that feeds its young milk |
