The answer is shallow, warm, continental seas.
The Ordovician-Silurian extinction happened about 450-430 million years ago. Before the extinction, most multicellular organisms lived in the seas. It was a rich life in shallow, warm, and continental seas. The climate was favorable. Reef-forming corals, as well as the first true vertebrates, some fish, appeared. Trilobites and mollusks were diverse and rich. Green algae were common in the seas.
