| A | B |
|---|
| amphibian | vertebrate animals that breathe air and live on land but must return to water to reproduce |
| Cenozoic era | the most recent era of Earth's geologic history; began 66 million years ago when dinosars became extinct |
| cyanobacteria | blue-green bacteria though to be one of the earliest life-forms on earth |
| endangered | describes a species that has only a relatively small number of individuals living and thus is in danger of dying out |
| epoch | subdivisions of periods on the geologic time scale |
| era | four largest subdivisions of the geologic time scale based on differences in life-forms |
| geologic time scale | a record of Earth's history, beginning 4.6 billion years ago |
| habitat | any place where organisms live, grow, and interact with one another and with the environment |
| Mesozoic Era | middle era of Earth's geologic history; began about 245 million years ago; reptiles and gymnosperms were the dominant land life-forms |
| natural selection | process by which organisms with traits best suited to an environment survive and reproduce |
| organic evolution | the gradual change in life-forms over time |
| Paleozoic Era | second-oldest division of geologic time; began about 544 million years ago |
| period | subdivisions of ears on the geologic time scale, based on life-forms and geologic events |
| Precambrian time | the oldest and longest division of geologic time, including about 90 percent of Eath's history; spans 4.6 billion to about 544 million years ago |
| reptile | scaly skinned, vertebrate animals that evolved from the same ancestors as amphibians but do not return to water to reproduce |
| species | a group of individuals that normally breed only among themselves |
