The periods of the last 539 million years
Twelve periods of the Phanerozoic eon, oldest first. Start dates are from the International Commission on Stratigraphy chart, version 2024/12, in millions of years ago (Ma). Older charts and textbooks give slightly different numbers, because the dates are refined as rocks are re-dated. For example, the Cretaceous used to start about 145 Ma and the Ordovician 485.4 Ma; the matching accepts both. In the United States the Carboniferous is often split into the Mississippian and Pennsylvanian. "Tertiary" is an older name for the Paleogene and Neogene together.
These twelve periods are the story of complex life, from the first animals with shells to today. They are the names you see in museums, on fossil labels and in dinosaur books.
Use a first-letter sentence for the order: "Cows Often Sit Down Calmly; Perhaps To Just Chew Peacefully, Not Quickly." Then chunk by era: six Paleozoic periods (old life), three Mesozoic (dinosaurs), three Cenozoic (mammals). Give each period one picture from the 'what lived' column and place them along a long hallway, oldest by the front door. Learn the order first; add the start dates once the order is solid.
Encode make it vivid → Space bring it back on day 1, 3, 7, 21 → Retrieve pull it from your head. From the infographic Remember It on Purpose.
The periods of the last 539 million years
| Order | Period | Era | Began (million years ago) | What lived and happened |
|---|---|---|---|---|
| 1 | Cambrian | Paleozoic | 538.8 | Many animal groups appear in the fossil record within a few tens of millions of years (the "Cambrian explosion"). |
| 2 | Ordovician | Paleozoic | 486.85 | Sea life spreads; the first simple plants reach land; it ends in a mass extinction. |
| 3 | Silurian | Paleozoic | 443.1 | Small plants with veins grow on land; fish with jaws spread. |
| 4 | Devonian | Paleozoic | 419.62 | The "Age of Fishes"; the first forests; the first four-legged animals near the end. |
| 5 | Carboniferous | Paleozoic | 358.86 | Vast swamp forests later became coal; giant insects; the first reptiles. |
| 6 | Permian | Paleozoic | 298.9 | Land animals spread across one giant continent, Pangaea; it ends in the largest mass extinction known. |
| 7 | Triassic | Mesozoic | 251.902 | Life recovers; the first dinosaurs and the first mammal relatives appear. |
| 8 | Jurassic | Mesozoic | 201.4 | Giant long-necked dinosaurs; the first birds. |
| 9 | Cretaceous | Mesozoic | 143.1 | Flowering plants spread; it ends 66 million years ago with an asteroid impact and the end of the non-bird dinosaurs. |
| 10 | Paleogene | Cenozoic | 66.0 | Mammals spread and grow larger in the empty world the dinosaurs left. |
| 11 | Neogene | Cenozoic | 23.04 | Grasslands spread; the earliest human relatives appear in Africa. |
| 12 | Quaternary | Cenozoic | 2.58 | Ice ages come and go; mammoths roam; modern humans appear and spread across the world. |
Use a first-letter sentence for the order: "Cows Often Sit Down Calmly; Perhaps To Just Chew Peacefully, Not Quickly." Then chunk by era: six Paleozoic periods (old life), three Mesozoic (dinosaurs), three Cenozoic (mammals). Give each period one picture from the 'what lived' column and place them along a long hallway, oldest by the front door. Learn the order first; add the start dates once the order is solid. · teddyrookbook.com/memory/challenges/geologic-periods/
Sources
- International Commission on Stratigraphy: International Chronostratigraphic Chart v2024/12
- U.S. Geological Survey: Divisions of Geologic Time (Fact Sheet 2018-3054)
Checked October 30, 2026. Progress is kept on this device only (per learner). More lists: all memory challenges · memory games.