Paleoparadoxia ("ancient paradox") is a genus of large, herbivorous aquatic mammals that inhabited the northern Pacific coastal region during the Miocene epoch. It ranged from the waters of Japan (Tsuyama and Yanagawa), to Alaska in the north, and down to Baja California, Mexico.

Description

Paleoparadoxia is thought to have fed primarily on seaweeds and sea grasses. The jaws and the angle of the teeth resemble a backhoe bucket. Its bulky body was well adapted for swimming and underwater foraging. Originally interpreted as amphibious, Paleoparadoxia is now thought to have been a fully marine mammal like their living relatives, the sirenians, spending most of their lives walking across the sea bottom like marine hippos. Studies on its habitat preference show that it favoured deep, offshore waters. Recent discoveries have extended the known geographical range of Paleoparadoxia, with the oldest record from the northwest Pacific suggesting a much earlier presence in this region. This finding implies that Paleoparadoxia had a wider distribution and potentially different migratory patterns than previously understood, hinting at a complex early evolution within the Desmostylia order. Size estimates of P. tabatai vary, with the Tsuyama specimen measuring 215 cm in length, 80 cm in height, and 582 kg in body mass, and the other specimens measuring 1048 kg and 3.2 MT in body mass. named the genus Cornwallius but synonymized it as a species of Paleoparadoxia.