Hydrodamalis gigas
Hydrodamalis gigas
Name Hydrodamalis gigas
Order Sirenia
Family Dugongidae
Class Mammalia
Period Late Holocene
Location North Pacific Coast, Japan and California
Diet Herbivorous
Length 8-9 metres (26-30 feet)

The Steller's Sea Cow (Hydrodamalis gigas), is an extinct genus of large herbivorous marine mammal. It reached 7 feet long and weighed 4 tons. Its small head ended smoothly into a huge trunk for sucking up sea plants into its mouth to eat, and its body ended with a forked tail, resembling a whale's tail. The Steller's Sea Cows hind pair of legs were a bit like that of a cetacean. It was discovered in 1741 by Mr. Steller. As a result of predatory fishing by 1768, Steller's Sea Cow had been completely destroyed.

In historical times, it was the largest member of the order Sirenia, which includes its closest living relative, the Dugong (Dugong dugon), and the manatees (Trichechus spp.). Formerly abundant throughout the North Pacific, its range was limited to a single, isolated population on the uninhabited Commander Islands. It was first described in 1741 by Georg Wilhelm Steller, chief naturalist on an expedition led by explorer Vitus Bering. Within 27 years of discovery by Europeans, the slow moving Steller's Sea Cow was hunted to extinction.

The sea cow grew to at least 8 meters (26 ft) to 9 meters (30 ft) in length as an adult, much larger than the manatee or dugong; however, concerning their weight, Steller's work contains two contradictory estimates: 4 and 24.3 metric tons. The true value is estimated to lie between these figures, at around 8 to 10 t. It looked somewhat like a large seal, but had two stout forelimbs and a whale-like tail and the fluke. According to Steller, "The animal never comes out on shore, but always lives in the water. Its skin is black and thick, like the bark of an old oak…, its head in proportion to the body is small…, it has no teeth, but only two flat white bones—one above, the other below". It was completely tame, according to Steller. They fed on a variety of kelp. Wherever sea cows had been feeding, heaps of stalks and roots of kelp were washed ashore. The sea cow was also a slow swimmer and apparently was unable to submerge.