Question

When did most marine life go extinct?

Answer 1

During the industrial revolution, it was easier for people dispose of there garbage. And more cars meant more pollution oil, and garbage have been leading factors to as why marine life is going extinct.  <span />

Answer 2

While the other answers provides a viable answer, another possibility is during/right before any of the ice ages, as the freezing of water decreases the amount of livable space, and even changes the contours of the oceans (like the central american landbridge dividing the atlantic and pacific oceans), many species died out quickly (like the megalodon carcharodon).