Steller's sea cow was a member of the genus Hydrodamalis, a group of large sirenians, whose sister taxon was Dusisiren.
Like those of Steller's sea cow, the ancestors of Dusisiren lived in tropical mangroves before adapting to the cold climates of the North Pacific.
Hydrodamalis and Dusisiren are classified together in the subfamily Hydrodamalinae, which diverged from other sirenians around 4 to 8 mya.
Steller's sea cow is a member of the family Dugongidae, the sole surviving member of which, and thus Steller's sea cow's closest living relative, is the dugong (Dugong dugon).

Steller's sea cow was a direct descendant of the Cuesta sea cow (H. cuestae), an extinct tropical sea cow that lived off the coast of western North America, particularly California.
The Cuesta sea cow is thought to have become extinct due to the onset of the Quaternary glaciation and the subsequent cooling of the oceans.
Many populations died out, but the lineage of Steller's sea cow was able to adapt to the colder temperatures.
The Takikawa sea cow (H. spissa) of Japan is thought of by some researchers to be a taxonomic synonym of the Cuesta sea cow, but based on a comparison of endocasts, the Takikawa and Steller's sea cows are more derived than the Cuesta sea cow.
This has led some to believe that the Takikawa sea cow is its own species.
The evolution of the genus Hydrodamalis was characterized by increased size, and a loss of teeth and phalanges, as a response to the onset of the Quaternary glaciation.
Steller's sea cow was a member of the genus Hydrodamalis, a group of large sirenians, whose sister taxon was Dusisiren. Like those of Steller's sea cow, the ancestors of Dusisiren lived in tropical mangroves before adapting to the cold climates of the North Pacific. Hydrodamalis and Dusisiren are classified together in the subfamily Hydrodamalinae, which diverged from other sirenians around 4 to 8 mya. Steller's sea cow is a member of the family Dugongidae, the sole surviving member of which, and thus Steller's sea cow's closest living relative, is the dugong (Dugong dugon).
Steller's sea cow was a direct descendant of the Cuesta sea cow (H. cuestae), an extinct tropical sea cow that lived off the coast of western North America, particularly California. The Cuesta sea cow is thought to have become extinct due to the onset of the Quaternary glaciation and the subsequent cooling of the oceans. Many populations died out, but the lineage of Steller's sea cow was able to adapt to the colder temperatures. The Takikawa sea cow (H. spissa) of Japan is thought of by some researchers to be a taxonomic synonym of the Cuesta sea cow, but based on a comparison of endocasts, the Takikawa and Steller's sea cows are more derived than the Cuesta sea cow. This has led some to believe that the Takikawa sea cow is its own species. The evolution of the genus Hydrodamalis was characterized by increased size, and a loss of teeth and phalanges, as a response to the onset of the Quaternary glaciation.
Steller's sea cow