| A | B |
|---|
| Precambrian | Comprises about 88% of geologic time (4500 million years ago)First multicelled organisms First one-celled organisms Origin of Earth |
| Cambrian | 540 million years ago. "Age of Invertebrates". Trilobites dominant First organisms with shells. |
| Ordovician | 490 million years ago. First fishes |
| Silurian | 443 million years ago. "Age of Fishes" Fishes dominant First land plants. |
| Devonian | 417 million years ago. First insect fossils |
| Carboniferous: | 354 million years ago. "Age of Amphibians". First reptiles Large coal swamps Large Amphibians abundant. |
| Permian | 290 million years ago. Extinction of trilobites and many other marine animals |
| Triassic | 248 million years ago. "Age of Reptiles". First birds Dinosaurs dominant. |
| Jurasic | 206 million years ago. |
| Cretaceous | 144 million years ago. First flowering plants |
| Tertiary | 5 million years ago. "Age of mammals" Extinction of dinosaurs and many other species. |
| Quaternary | present: Humans develop |
| Ordovician-Silurian mass extinction | 443 million years ago. End of the Ordovician period. Start of the Silurian period. During the Ordovician, most life was in the sea, so it was sea creatures such as trilobites, brachiopods and graptolites that were drastically reduced in number. In all, some 85% of sea life was wiped out. An ice age has been blamed for the extinctions - a huge ice sheet in the southern hemisphere caused climate change and a fall in sea level, and messed with the chemistry of the oceans. |
| Late Devonian mass extinction | 359 million years ago. End of the Devonian period. Start of the Carboniferous period. Three quarters of all species on Earth died out in the Late Devonian mass extinction. Life in the shallow seas were the worst affected, and reefs took a hammering, not returning to their former glory until new types of coral evolved over 100 million years later. In fact, much of the sea bed became devoid of oxygen, rendering it effectively out of bounds for anything except bacteria. Changes in sea level, asteroid impacts, climate change and new kinds of plants messing with the soil have all been blamed for these extinctions. |
| Permian mass extinction | 248 million years ago. End of the Permian period. Start of the Triassic period. The Permian mass extinction has been nicknamed The Great Dying, since a staggering 96% of species died out. All life on Earth today is descended from the 4% of species that survived. Many causes have been proposed for the event: asteroid impact, flood basalt eruptions, catastrophic methane release, a drop in oxygen levels, sea level fluctuations or some combination of these. |
| Triassic-Jurassic mass extinction | 200 million years ago. End of the Triassic period. Start of the Jurassic period. Climate change, flood basalt eruptions and an asteroid impact have all been blamed for this loss of life. Many types of animal died out, including lots of marine reptiles, some large amphibians, many reef-building creatures and large numbers of cephalopod molluscs. Roughly half of all the species alive at the time became extinct. Strangely, plants were not so badly affected. |
| Cretaceous-Tertiary mass extinction | 65 million years ago. End of the Cretaceous period. Start of the Palaeocene epoch. The Cretaceous-Tertiary mass extinction - also known as the K/T extinction - is famed for the death of the dinosaurs. However, many other organisms perished at the end of the Cretaceous including the ammonites, many flowering plants and the last of the pterosaurs. 's suggested that the decline was due to flood basalt eruptions affecting the world's climate, combined with drastic falls in sea level. Then a huge asteroid or comet struck the seabed near the Yucatan Peninsula in Mexico and was the straw that broke the camel's back. |
