Perhaps something to do with most fossils being land species, a large dropoff in marine species may not be as noticeable overall

FYI the link you provide says nothing about diatoms, benthic or planktonic.

However, in "The origin of Cretaceous black shales: a change in the surface ocean ecosystem and its triggers (2015)" the authors state

>During OAE 2, up to 25% of marine invertebrate species became extinct