Their ancestors, such as mesonychus, were terrestrial ungulates, like cows and pigs. Some were carnivorous, others were herbivorous and many were adapted to coastal environments. Just offshore, marine habitats were relatively free of competitors due to the loss of a host of giant, marine reptiles. The diversity of marine environments posed a series of challenges to these mammals. Water draws heat away from a warm body more effectively than air.
Water creates more drag on a moving body, slowing locomotion.
There is an obvious difficulty drawing air into the lungs. Instead, massive heads supported racks of baleen. Made of the same material and the same structure as your fingernails, plates of bristly baleen filtered the food from a mouth full of food and water. Representatives of both suborders can be found on Stellwagen Bank and reflect this diversity.