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
Baleen whale toothed cetaceans (Archaeoceti) remained unknown until the extinct "Janjucetus hunderi" was discovered in the early 1990s in Victoria, Australia. Like a modern baleen whale, "Janjucetus" had baleen in its jaw and had very little biosonar capabilities. However, its jaw also contained teeth, with incisors and canines built for stabbing and molars and premolars built for tearing. These early mysticetes were exceedingly small compared to modern baleen whales, with species like "Mammalodon" measuring no greater than . It is thought that their size increased with their dependence on baleen. However, the discovery of a skull of the toothed "Llanocetus", the second-oldest