Raven pins a tall chart to the Sky Deck board. "The geologic time scale," she says. "Oldest at the bottom, youngest at the top."
"Like the layers," Rocket says. "But these are names, not rocks."
"What do you notice about the names?" Raven asks. "Paleozoic, Mesozoic, Cenozoic. They all end the same way."
Nova hovers beside the chart. "Zoic means animal life," she says. "Paleo means ancient. Meso means middle. Ceno means recent."
"Ancient life, middle life, recent life," Rocket says. "So the order is in the words."
Raven taps the bottom of the chart. "And look how much comes before any of them." Nova hums. "Most of the book."
The United States Geological Survey says Earth's history is divided into eons, which are divided into eras, then periods, then epochs.
Eras are broad spans based on the general character of life that existed during them. Periods are shorter spans based partly on major disturbances of Earth's crust.
The three youngest eras are easy to order from their names. Paleo means ancient, meso means middle and ceno means recent. Zoic refers to animal life.
Rocks from before these eras may hold fossils of simple organisms such as bacteria, algae and wormlike animals. Rocks from the eras may hold complex animals and plants.
The open textbook Physical Geology gives the dates. Ma means millions of years ago.
The Paleozoic era ran from 540 to 251 Ma. The Mesozoic ran from 251 to 65 Ma. The Cenozoic runs from 65 Ma to the present.
Those three eras together make up the Phanerozoic eon, which means visible life. It covers only the last 540 million years.
The three eons before it make up almost 90 percent of Earth's history. Most of the story has no complex life in it.
Most boundaries between divisions were fixed where the fossil record shows major changes in the kinds of living things.
| Division | Dates from the textbook | Meaning of the name | Sheets on a 100-million-year roll |
|---|---|---|---|
| Before the eras (three older eons) | 4,600 to 540 Ma | simple life, or none | about 40 and a half sheets |
| Paleozoic era | 540 to 251 Ma | ancient life | about 3 sheets |
| Mesozoic era | 251 to 65 Ma | middle life | about 2 sheets |
| Cenozoic era | 65 Ma to present | recent life | less than 1 sheet |
| Statement | True or false? |
|---|---|
| The Mesozoic era came before the Paleozoic. | ? |
| The three eras cover only the last 540 million years. | ? |
| Most of Earth's history had complex animals and plants. | ? |
| Boundaries in the time scale were set where fossils change greatly. | ? |
The chapters are in order. Tomorrow is Sky Deck Lab: you unroll the whole book down the hall.