the paired nostrils of a humpback;
a baleen whale
The order Cetacea, or whales, includes a diversity of mammals of shared ancestry, with flippers, fins and flukes. More uniquely, they have nostrils on the top of the head as opposed to the tip of the nose (manatees, also designed for a marine lifestyle, have flippers and flukes but their nostrils, like most mammals, are situated at the tip of the snout, or rostrum).
The first whale-like fossils appear in coastal sediments laid down about 60 million years ago. 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. In response, this lineage of mammals developed a series of traits in common: front legs modified to paddles for steering (the pectoral flippers); loss of hind legs and effective hip bones; flattened, horizontal flukes for propulsion; nostrils oriented on the top of the skull for efficient breathing at the surface; loss of extensive body hair to reduce drag with a gain in fat or blubber for warmth and a streamlined body shape; a highly modified respiratory system for diving and conserving body heat; and much more.
Around 30 million years ago, as different populations of Cetaceans moved into ever more environments, a major division appeared within the order: some species took advantage of plentiful shoals of small fish and plankton without the use of teeth. 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.
To this day the over seventy different species of whale are divided between two suborders: the Odontocetes (“toothed whales") and the Mysticetes (“mustached whales”) or baleen whales. Representatives of both suborders can be found on Stellwagen Bank and reflect this diversity.