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
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