Before the ordovician-silurian extinction, the diversity of life on earth was growing enormously due to
shallow, warm, continental seas
cold, glaciated oceans
unknown reasons
comets hitting the earth

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.