Whales, dolphins and porpoises belong to the Order Cetacea. This title was derived from the Latin term cetus, meaning large sea animal, and the Greek term ketos, meaning sea monster. Along with bats, Cetaceans are some of the most derived animals in existence. Although the origin of modern cetaceans is not fully understood, it is believed that terrestrial animals, of the Suborder Mesonychia, may have given rise to modern cetaceans, as they colonized the sea in the Paleocene, roughly 60 million years ago. Fossils that are recognizable as Cetaceans have been found in rock strata dating back to the early Middle Eocene, roughly 50 million years ago. These fossils represent elongated quatic animals, of the Suborder Archaeoceti, which, with reduced hindlimbs and a small beak, resembled modern snakes or eels. During the Oligocene, roughly 38 million years ago, these ancient aquatic animals began to decline in abundance and were replace by the present day Suborders Odontoceti and Mysteceti. Members of the Suborder Mysteceti, which include the Rorqual and Right Whales, posses fine baleen plates which are used to filter food from the water column. In contrast, members of the Suborder Odontoceti, which include dolphins and porpoises, are referred to as the "toothed whales" because they posses teeth, rather than baleen.
Although both dolphins and porpoises are related to the squalodonts, or earliest true toothed whales, and the kentridontids, or ancestral dolphins, porpoises have been distinct from their dolphin relatives for approximately 11 million years. The geographic distribution of porpoise fossils suggests that they originated in the North Pacific and later spread to the Atlantic and southern waters.
Being one of the most derived groups of modern mammals, cetaceans have undergone a number of adaptations to prepare them for a completely aquatic lifestyle. One significant area of adaptation deals with the cetacean method of locomotion. Since cetaceans spend the entirety of their lives in the water, a number of modifications have taken place which help reduce the amount of drag present when the animal is on the move. For example, the cetacean body is cylindrical in shape, and the reproductive organs are internal. Cetacean limbs have also been modified to reduce drag and improve locomotion in the aquatic environment. The front limbs have adapted into broad, flat, paddle-like flippers. The hind limbs have disappeared completely from the external portion of cetaceans, though an internal remnant is still present. Likewise, the tail of cetaceans has adapted to the aquatic environment, as it has modified into a fluke that provides the propulsion power that enables these creatures to swim and dive so efficiently.
A similar adaptation that allows cetaceans to thrive in their environment is the modification of the skin. Unlike terrestrial mammals, cetaceans generally lack hair, which also aids in the reduction of drag in the water. Similarly, a well developed layer of blubber has developed to provide necessary insulation. Furthermore, nostrils in these mammals have become modified into blowholes to provide easy access to the surface of the water for breathing.
Porpoises as Mammals
One of the greatest misconceptions about porpoises, as well as all Cetaceans, is the belief that they are fishes, rather than mammals. Externally, cetaceans do resemble fishes, as they generally possess dorsal fins, flippers, and flukes. The Fin Whale, Whale Shark, and Vaquita, due to the dorsal fin arrangement, resemble sharks, but these animals, like all cetaceans, are more closely related internally to terrestrial mammals than fishes.
Like terrestrial mammals, cetaceans are warm blooded and must use part of their available energy to maintain a stable core body temperature. Blubber, the shunting of blood from peripheral areas, and Bradycardia, a reduced metabolic state, are tools used by cetaceans to maintain that core body temperature in their often cold aquatic environment.
Another similarity between cetaceans and terrestrial mammals is their method of respiration. Cetaceans possess lungs and must breath air, unlike fishes who take in oxygen across the gills. Reproductive similarities are also observed between cetaceans and terrestrial mammals, as cetaceans give birth to live young who suckle milk secreted by the mammary glands of the mother. Perhaps the most distinct characteristic that distinguishes a cetacean from a fish in the open sea is the movement of the fluke. A cetacean fluke is horizontal and exhibits an upward and downward movement. In contrast, the caudal fin in fishes is vertical and moves side to side.