Filter feeder gap between the two rows of baleen plates lets the water in together with the prey, while the baleens filter out the water. Rorquals such as the blue whale, in contrast, have smaller heads, are fast swimmers with short and broad baleen plates. To catch prey, they widely open their lower jaw — almost 90° — swim through a swarm gulping, while lowering their tongue so that the head's ventral grooves expand and vastly increase the amount of water taken in. Baleen whales typically eat krill in polar or subpolar waters during summers, but can also take schooling fish, especially
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