First extinction events

These Ordovician - Silurian extinctions took place about 444 to 445 million years ago.

The first mass extinction consisted of two extinction pulses.
The first extinction pulse resulted from a combination of cooling oceans, falling sea levels and changes in ocean chemical composition. The global average temperature cooled rapidly to 13°C.
The second pulse resulted from warming temperatures, rising sea levels and further changes in ocean chemical composition.

Plants shaped the terrestrial biosphere, burying carbon on continents and extracting nutrients from the land that washed into the oceans and stimulated life there as well. These changes likely caused the glacial melt and exterminated most small marine organisms as the planet rapidly cooled and froze before thawing again. Intense glacial and interglacial periods caused large sea level fluctuations and dramatically displaced coastlines. The tectonic uplift of what is now the mountains in eastern to northeastern North America caused much weathering, storing carbon dioxide (CO₂) in places such as underground geological formations. This changes in the chemical composition of the oceans resulting in the first extinction pulse. About a million years later, the glaciation abruptly retreated and warm conditions returned.
This second pulse was associated with intense global oxygen (O₂) depletion and toxic sulfide (S²⁻) production.
As each pulse reduced species richness, ecosystem complexity and resilience also declined, leading to a cascade of influences that added up to a catastrophic mass extinction.

Extinction rates:

• 86% species
• 57% genera
• 27% families

The earliest land plants evolved about 230 million years before the first extinction