Marine Mammals I: The Cetaceans

Whales and other cetaceans have long held a fascination for humans. Maybe that is where the worldwide mythology of the mermaid comes from, the longing to be able to swim like a porpoise. Porpoises can be seen in the ancient art of many sea-side cultures, and the skeletons of whales are displayed in many Natural History Museums. The American Museum of Natural History is well-known for its model of a blue whale, and the newly re-opened North Carolina Museum of Natural Sciences boasts the skeletons of five whales, plus an "aquarium" that takes the vistor through the development of whales. (See my article "A Visit to the North Carolina Museum of Natural Sciences" for more information.)

Whales and porpoises (which includes the dolphins) are called cetaceans. The name comes from the Greek word "cetus" which means "whale". The cetaceans are divided into three groups. The first group is the Achaeoceti, the early whales, all members of this group are now extinct. The second group is the Odontoceti, or toothed whales, porpoises and killer whales are in this group. The last group is the Mysticeti, or baleen whales.

The Archaeoceti first appear in the fossil record in the early Eocene Period (55 to 36 million years ago). It is believed that they developed from land-living ungulates (hooved mammals that chew cud, such as deer) during the early Paleocene Period (65 to 55 million years ago).

Until recently, the earliest known whale is Pakicetus, a small cetacean from the early Eocene Period of Pakistan, known from its skull. Pakicetus had teeth that are very similar to the teeth of Paleocene Period ungulates, indicating their relationship. The ears of Pakicetus were not well adapted for functioning underwater, and the discovery of other land-living animals in the same deposits as Pakicetus indicate that it was probably amphibious, and spent much of its time on land. Pakicetus probably resembled modern seals, with four paddle-shaped limbs that allowed it move on land awkwardly, but allowing it to be more at home in rivers and estuaries.

Recently, an older cetacean was discovered. Named Himalayeceteus the teeth and jawbone of a 53.5 million year old cetacean were found in northern India at the foothills of the Himalayan mountains.

The next cetacean discovery is Protocetus, which lived only 8 million years after Pakicetus, but had become much more whale-like in appearance. Its body was more streamlined, with flat, paddle-like forelegs and greatly reduced hindlegs. The nostrils of Protocetus had begun to move back along its skull, away from the tip of the snout (seen in Pakicetus), and its ears were developed for hearing underwater, although Protocetus had probably not developed the sophisticated system of echolocation and songs seen in modern whales.

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