Whale than males. Baleen whales have no teeth; instead they have plates of baleen, a fringe-like structure used to expel water while retaining the krill and plankton which they feed on. They use their throat pleats to expand the mouth to take in huge gulps of water. Balaenids have heads that can make up 40% of their body mass to take in water. Toothed whales, on the other hand, have conical teeth adapted to catching fish or squid. Baleen whales have a well developed sense of "smell", whereas toothed whales have well-developed hearing − their hearing, that is adapted for both
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