Life on Earth nearly died out at the end of the Paleozoic Era 250 million years ago. At that time, all the continents had just come together to form a supercontinent called Pangaea, and a single superocean called Panthalassa. The global climate had warmed by several degrees, especially at the poles.
If the air was warmer at the poles, then the water would be too. And without cold water and ice formation at high latitudes, there would be little downwelling and not much of a global conveyer belt in the deep sea. So all in all ocean circulation would probably have been much weaker than it is today. Oxygen in the deep sea would have been used up, and without strong downwelling, there was no way to replenish it.