| A | B |
|---|
| vertebrate | animal with a backbone |
| trilobite | animal with jointed legs whose extinction marks the end of the Paleozoic Era |
| radioactive element | element whose atoms have an unstable nucleus, which decays |
| Precambrian Era | earliest time period in geologic time |
| period | division of geologic time, shorter than an era, but longer than an epoch |
| Paleozoic Era | period of geologic time that began 570 million years ago and ended 225 million years ago |
| Mesozoic Era | period of geologic time that began 225 million years ago and ended 65 million years ago |
| invertebrate | animal without a backbone |
| interglacial | time between ice ages |
| inorganic | composed of material that was never alive |
| half-life | time it takes for one half the atoms in a radioactive sample to decay |
| geologic time | very long period of time in the Earth's existence |
| era | largest division of geologic time |
| epoch | division of geologic time shorter than a period |
| decay element | stable element into which a radioactive element breaks down |
| cycad | palm-like seed plant that lived during the Triassic and Jurassic periods |
| conifer | cone bearing plant, such as pine, cedar, spruce, or cypress |
| Cenozoic Era | current period of geologic time, that began 65 million years ago |
| brachiopod | clam-like marine animal that lived during the Paleozoic Era |
| amphibian | animal that lives part of its life in water, and part on land |
