Baleen whale modern mysticetes are obligate filter feeders, using their baleen to strain small prey items (including small fish, krill, copepods, and zooplankton) from seawater. Despite their carnivorous diet, a 2015 study revealed they house gut flora similar to that of terrestrial herbivores. Different kinds of prey are found in different abundances depending on location, and each type of whale is adapted to a specialized way of foraging. There are two types of feeding behaviors: skim-feeding and lunge-feeding, but some species do both depending on the type and amount of food. Lunge-feeders feed primarily on euphausiids (krill), though some smaller lunge feeders
Aetiocetus a family of toothless baleen whales, are contemporaneous with "Aetiocetus". Hence, whales whose feeding relied entirely on baleen made their stratigraphic appearance before "Aetiocetus", meaning that “true” baleen whales existed before "Aetiocetus". Baleen is made of keratin (the same material that comprises claws, hooves, nails, and hair) that grows throughout the whale's life. Development of mysticetes indicate that they had a toothed ancestor, as the fetal baleen whale forms tooth buds which are later reabsorbed and do not develop any further. However, "Aetiocetus" presents the evolutionary biologist with evidence for this transition in the fossil record. While baleen, as a