Morphological Evolution in Whales

views updated

Morphological Evolution in Whales

Whales are the only group of mammals to have adopted an exclusively aquatic lifestyle. Their entire life cycle, from birth until death, is carried out in an aquatic habitat . The terms "whale" and "cetacean" are usually used interchangeably, cetaceans being the scientific term for whales, dolphins, and porpoises. However, "whale" is also sometimes used to distinguish some of the larger species of the order Cetacea from the other two major groups, the dolphins and porpoises.

Cetaceans are found in almost all oceans and their connecting seas, as well as in estuaries (wide tidal river mouths) and rivers. Whales range in size from the 1-meter-long (3 feet) dolphins and porpoises to the 30-meter-long (100 feet) blue whale. The seventy-seven extant species of whales each belong to one of two suborders, the Odontoceti (toothed whales) and the Mysticeti (baleen whales). The toothed whales—porpoises, dolphins, and most of the smaller whales—have homodont (uniform in size and shape) teeth or are toothless. The odontocetes use their teeth to feed on fish and crustaceans . The mysticetes are toothless, but strain plankton and small crustaceans using a dense fringe of bristled plates called baleen (whalebone) that hangs from the roof of their cavernous mouth.

The extinct Archaeoceti appeared in the early part of the Eocene epoch (about 50 million years ago). Although they do not show all the specialized body characteristics of the odontocetes and mysticetes, their spindle-shaped body and horizontal tail show that they were well adapted to an aquatic lifestyle. Unlike the extant whales, archaeocetes have heterodont (different in size and shape) teeth and their nostrils are located midway along the snout. Based on the fossil evidence, archaeocetes went extinct slowly, disappearing sometime during the Pliocene epoch (2.5 to 7 million years ago).

Morphological Specializations of Extant Whales

Whale form and structure departed dramatically from that of terrestrial mammals, adopting a fishlike appearance. The whale body is streamlined, with a large head and nondifferentiated neck, and has a dorsal fin made up of connective tissue and skin on the back. The tail is a T-shaped, horizontally flattened, boneless fluke, which serves as the major propulsive force during locomotion. Whales swim using an up-and-down wavelike motion rather than the side-to-side bending motion used by most fish.

Like other mammals, whales have lungs and breathe air. They inhale and exhale through a single nostril or a pair of nostrils on top of their head, toward the back. The waterspout that emerges from this "blowhole" is simply water vapor expelled from the lungs during exhalation along with a small amount of water collected near the edges of the blowhole.

The whale skull has undergone other changes in addition to the relocation of the nostril(s) and the development of homodont teeth and baleen. The upper jaw is thicker at the back end, while the lower jaw is a horizontal bar rather than an L-shape, such as in humans. The cranium and brain have become wider so that their widths are greater than their lengths, a very unusual situation in mammals. This elongation of the cranium and the brain is referred to as a "telescoping of the skull." The relative sizes and shape of the various skull bones have changed as a result of this telescoping.

Whale skin is hairless except for a few tactile hairs. In addition, the skin lacks sweat or oil glands and feels like smooth rubber. A thick layer of blubber lies underneath the skin, aiding in buoyancy, insulating the body, and storing energy in the form of fat.

When compared to the limbs of terrestrial mammals, whale forelimbs have been transformed into flippers, while the hind limbs have been lost. The bones of their upper forelimbs have been reduced in size and the number of phalanges in each digit has increased, resulting in elongated "hands." Although whales do not have hind limbs, all species have a pelvic bone, and a few species have thighbones.

Fossil Evidence for the Origin of Whales

Certain details concerning the evolution of whales remain unknown. Scientists agree that whales evolved from some kind of primitive ungulate (hoofed mammal), but because we do not have a fossil of the animal that was intermediate between the terrestrial and aquatic whale ancestors, it is difficult to classify whales relative to their closest mammalian relatives, the artiodactyls (camels, pigs, ruminants , and hippopotamuses). Fossil data suggests that whales and artiodactyls share a recent ancestor, but whales differ from other ungulates in a number of significant skeletal characteristics.

The important morphological differences between the two orders of extant whales, the odontocetes and the mysticetes, also lead scientists to believe that the branch that eventually gave rise to modern whales split off from the rest of the mammalian tree long ago. The first fossil representatives of both orders appeared during the Oligocene epoch (about 38 to 25 million years ago). Some paleontologists look for clues about whale origins in the Cenozoic era, particularly in the Paleocene and early Eocene epochs (50 to 65 million years ago). Other paleontologists who believe that a Paleocene or Eocene origin would not have allowed sufficient time for the extreme morphological changes look for answers within the Cretaceous period of the Mesozoic era (65 to 130 million years ago).

Molecular Evidence for the Origin of Whales

Molecular DNA studies performed during the 1990s provide a more precise placement of whales on the mammalian phylogenetic tree (diagram representing the evolutionary relationships between mammal groups). Several different scientific teams working on different parts of the cetacean genome have all concluded that whales should be classified as artiodactyls, not as a sister group as suggested by the fossil evidence. The molecular evidence indicates that whales are most closely related to hippopotamuses. The affinity to water exhibited by hippopotamuses has led some scientists to wonder about the aquatic tendencies of the most common ancestor of whales and hippos.

Despite the agreement of the results drawn of molecular studies performed on cetacean DNA, certain aspects of the origin of whales remain under dispute. Paleontologists maintain that whales should not be classified as artiodactyls and warn against drawing conclusions about the nature of whale ancestors based on a hypothesized relationship between whales and hippos.

see also Fossil Record; Geological Time Scale; Morphology.

Judy P. Sheen