| A | B |
|---|
| geologic time scale | a record of the geologic events and life forms in Earth's history |
| era | one of the 3 long units of geologic time between the Precambrian and the present |
| period | one of the units of geologic time into which geologists divide eras |
| epoch | subdivisions of the periods of the geologic time scale |
| invertebrate | an animal without a backbone |
| because the time span of Earth's history is so great | Why do geologists use the geologic time scale to measure geologic time instead of years or centuries? |
| 4.6 billion years | How old is Earth? |
| less than a second before midnight | If Earth's 4.6 billion year history was squeezed into a 24-hour day, when would humans first appear? |
| Precambrian Time | the first 88% of geologic time that occured before the 3 eras |
| Paleozoic, Mesozoic, & Cenozoic | What are the names of the 3 eras that occurred after Precambrian time? |
| Paleozoic | Many animals that lived during this time were invertebrates. |
| Paleozoic | This word means ancient life. |
| Mesozoic | This word means middle life. |
| Mesozoic | During which era did dinosaurs live? |
| Cenozoic | This word means recent life. |
| Cenozoic | Which era is sometimes called the Age of Mammals? |
| periods | What are Triassic, Jurassic, and Cretaceous examples of? |
| Cenozoic | During which era is the fossil record much more complete, so further subdivisions called epochs are used? |
| a mass of dust and gas | What did Earth form from? |
| studying rock layers & index fossils to determine relative age | What methods did geologists use when they first developed the geologic time scale? |
| It depends on events in the history of life on Earth | How did geologists decide where one division of the geologic time scale ends and the next begins? |
