Good question. This (PhD from Smithsonian interview) might be part of the answer (details specific to PT extinction). To quote just a bit: "[90% of ocean speci

Good question. This (PhD from Smithsonian interview) might be part of the answer (details specific to PT extinction). To quote just a bit: "[90% of ocean species dying] basically hit a reset button on ecosystems in the ocean ... These animals were living in coastal environments, eating seafood that washed up on the beach, and over time became more and more dedicated to living in the ocean." [https://www.splendidtable.org/story/2015/04/10/how-land-animals-became-marine-animals-they-followed-their-stomachs https://www.splendidtable.org/story/2015/04/10/how-land-anim...]