Digestive system of humpback whales stretching like an accordion. They allow whales to open their mandible to 90 degrees to gulp water and prey. They expel saltwater through the baleen plates, leaving behind krill and plankton. The whale's tongue plays a vital role in filter feeding, as it is used to push the water out of its mouth while keeping the prey trapped in its baleen bristles. As humpback whales do not have teeth, their food is swallowed whole and travels down the esophagus into the preliminary stomach (known as the forestomach) to be ground. Baleen whales have a multi-chambered stomach, divided into three distinct
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