Baleen this issue. A whale's baleen plates play the most important role in its filter-feeding process. To feed, a baleen whale opens its mouth widely and scoops in dense shoals of prey (such as krill, copepods, small fish, and sometimes birds that happen to be near the shoals), together with large volumes of water. It then partly shuts its mouth and presses its tongue against its upper jaw, forcing the water to pass out sideways through the baleen, thus sieving out the prey, which it then swallows. People formerly used baleen (usually referred to as "whalebone") for making numerous items where
Baleen whale mysticete, yielded a total length of , indicating filter feeding was not a driving feature in mysticete evolution. The discovery of "Janjucetus" and others like it suggests that baleen evolution went through several transitional phases. Species like "Mammalodon colliveri" had little to no baleen, while later species like "Aetiocetus weltoni" had both baleen and teeth, suggesting they had limited filter feeding capabilities; later genera like "Cetotherium" had no teeth in their mouth, meaning they were fully dependent on baleen and could only filter feed. However, the 2018 discovery of the toothless "Maiabalaena" indicates some lineages evolved toothlessness before baleen. "Mystacodon