Vertebrate Zoology - V. M. Konstantinov 2011

Chordates
Terrestrial, or tetrapod, vertebrates — Amniotes
Class Mammals — Subclass Theria — Suborder Toothed whales — Order Sirenians

Sirens, much like whales, are fully aquatic mammals that never come ashore. Systematically, they are closely related to ancient ungulates, representing a branch of the latter that adapted to life in coastal marine environments.

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Fig. 203. Distribution of sirenians

Body length ranges from 2.5 to 5.8 m, with a body mass of up to 250 kg. The overall body shape resembles that of whales, yet the neck is distinct. Only the forelimbs are present, modified into flippers, though the digits still retain rudimentary hooves. The molars, much like those of true ungulates, feature flat chewing surfaces. Affinities with ungulates are also evident in The Structure of the multi-chambered Stomach. Sirenians inhabit coastal marine waters and frequently ascend into rivers. They feed on submerged vegetation and often gather in groups that "graze" on underwater "meadows".

Several species of manatees (Trichechidae) inhabit the western coast of Africa, the eastern coast of South America, and the Antilles. Dugongs (Dugongidae) are distributed throughout the coastal zone of the Indian Ocean, ranging from East Africa to Eastern Australia and the island of Taiwan (Fig. 203).

In 1741, the Russian academician Steller discovered a large sirenian species off the Commander Islands, which was named Steller's sea cow (Hydrodamalis gigas) in his honor. Herds of these remarkably trusting animals "grazed" in kelp beds. Shortly after Steller's discovery, they were ruthlessly hunted to extinction by fur traders and sealers. The last specimen was killed in 1768, just 27 years after science first learned of this species' existence.