A large otter that existed from late Miocene to early Pleistocene. It could live in freshwater and marine coastal habitats, and its fossils were found in several places in Florida and California.
>Otters, or members of the mustelid subfamily Lutrinae, fall into into two different groups based on the anatomy of their carnassial teeth, the upper fourth premolar (P4) and the lower first molar (m1). One group is called the “fish-eating” otters (Repenning, 1976; Lambert, 1997) and includes the extinct otter genus Satherium and the extant otter genera Lutra, Lontra, Pteronura, Amblonyx, and Aonyx. These otters are characterized by more blade-like carnassials, unlike the other group of otters, the bunodont otters. Bunodont otters possess non-blade-like carnassials with thick enamel and rounded cusps. The bunodont otters include the extant genus Enhydra (sea otters) as well as the extinct genera Enhydritherium and Enhydriodon (Lambert, 1997).
>The genus Enhydritherium is thought to be closely related to the Enhydra and Enhydriodon, two genera found in late Miocene to early Pleistocene fossil sites of Europe, Asia, and Africa (Berta and Morgan, 1985; see Figure 2). This clade may be differentiated from all other members of the subfamily Lutrinae by the lack of the first premolar, a shorter rostrum, and bunodont cusps on the lower first molar (node 2, Figure 2). Enhydriodon is overall larger in size and is characterized by an “isolated” hypocone on the upper fourth premolar, which is different from Enhydritherium and Enhydra, which share several features including similar body size and the shapes of the upper fourth premolars and lower first molars (node 3, Figure 2). Enhydritherium may be distinguished from both Enhydriodon and Enhydra by the anterior and medial position of the protocone in the upper fourth premolar (Figure 3) and by the absence of the protostylid and presence of the metastylid on the lower first molar (Figure 4; Berta and Morgan, 1985; Lambert, 1997). Currently, Enhydritherium terraenovae is the only species in the genus.
Tl;dr: There are two overall categories of otters, based on teeth structure. Enhydritherium is closely related to sea otters.