The Silurian time period (named after Silures - an ancient Celtic tribe that live on the territory of modern Wales) was the third time period of the Paleozoic Era. It lasted from 435 to 400 MYA, +/- 10 million years.
Geography and climate
100 million years since the Cambrian have passed. And the fight for survival had filled the Silurian seas with variety. Some creatures in the oceans would be recognizable today. But land at this time was like an alien planet. It was a barren expense of roasting rock hotter than the Sahara. The air would be toxic to humans. Back then, it had much less oxygen and 300 times more carbon dioxide than today.
The Silurian was marked by a noticeable geographic change: at the beginning of this time period almost all of Earth was covered by sea, at its end the sea largely withdrew, creating new mountain ranges in the process. The Silurian fauna introduced many new species, including sea cucumbers, molluscs, brachiopods, graptolites. First true scorpions (i.e. Brontoscorpio) appear.
On the other hand, the end of the Silurian is marked by the extinction of the many groups of ancient corals, sea lilies, etc.
The Silurian vertebrates are represented by fish; jawless fish still dominate, but first jawed fishes (acanthodians) begin to appear as well. The Silurian flora is represented mostly by algae and similar plants, but first land plants (i.e. Cooksonia) begin to appear as well. First fungi began to appear at this time as well.
400 MYA the Silurian end and the Devonian time period began.