https://en.wikipedia.org/wiki/Cenomanian-Turonian_boundary_event

> Although the cause is still uncertain, the result starved the Earth's oceans of oxygen for nearly half a million years, causing the extinction of approximately 27 percent of marine invertebrates, including certain planktic and benthic foraminifera, mollusks, bivalves, dinoflagellates and calcareous nannofossils.[16] The global environmental disturbance that resulted in these conditions increased atmospheric and oceanic temperatures.