pachycormids

Science, however, shows that millions of years before these massive mammals evolved, huge fish fruitfully employed the same approach-and that it wasn't necessarily these cetaceans 'skill, but rather changing climate and new food source diversity, that allowed them to eventually dominate their domain. prehistoric big fish known as the pachycormids was known to have used filter feeding (in which water is sucked into the mouth and small organisms such as plankton are trapped inside when the water is filtered back out through baleen).

Scientific American 2010

But by reexamining fossils housed-and largely forgotten-in several museum collections, a team of researchers found that the pachycormids actually roamed the prehistoric seas for more than 100 million years.

Scientific American 2010

And the new findings imply that rather than being an unsuccessful, empty niche, the filter-feeding function was dominated by these large fish from about 170 million to 65 million years ago, "suggesting that familiar modern groups of planktivores diversified into the ecospace vacated by giant pachycormids."

Scientific American 2010