Steller's sea cow
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.