The two subgroups have been distinct since the late Eocene. Both are fully aquatic, have fusiform bodies and tails flattened dorsoventrally as flukes.
Posterior limbs are absent externally, and the anterior limbs are enclosed in flippers.
The baleen in the Baleen whales filter out plankton in their buccal cavities, water being filtered through the comblike baleen plates, having been scooped up in huge mouthfuls. Toothed whales capture fish or small marine mammals. Most toothed whales are relatively small. All whales are sensitive to acoustic signals and the toothed whales are unusual in their use of echolation to identify the location and character of food, other objects, as well each other. Most whales are highly social.