The Ordovician-Silurian Extinction was a major extinction event that occurred around 443 million years ago, marking the end of the Ordovician period and the beginning of the Silurian period. It is estimated that about 85% of marine species went extinct, significantly impacting life in the oceans. This event is believed to have been caused by a combination of climate change, glaciation, and changes in sea levels. The extinction primarily affected marine organisms, including trilobites, brachiopods, and graptolites. The loss of biodiversity during this period paved the way for new species to evolve and thrive in the Silurian period, leading to the development of more complex ecosystems.