Habitat preferences of the enigmatic Miocene tethythere Desmostylus and Paleoparadoxia (Desmostylia; Mammalia) inferred from the depositional depth of fossil occurrences in the Northwestern Pacific realm

Abstract Desmostylus and Paleoparadoxia are extinct marine mammals belonging to the order Desmostylia that existed in the period between the late Oligocene and middle Miocene. All occurrences of their fossils are limited to marine strata along the coasts of the North Pacific Ocean. Although these two genera have similar body form, their paleoecologies including habitat preferences are thought to be different because their cranial structures are distinctive as well as they have been known separately in different localities. We estimated the depositional depths of their fossil occurrences on the basis of the associated mollusks and benthic foraminiferal assemblages from 45 desmostylian localities. Only data on complete or partial skeletal specimens were considered in order to exclude cases of reworking and pre-burial drift of carcasses that would confound our inference. Our results indicate that the depositional environment of Desmostylus specimens was restricted to the inner sublittoral zone shallower than 30 m in depth whereas that of Paleoparadoxia specimens ranged from the inner sublittoral (0–50 m) to upper bathyal zone (between 150 and 400 and 500 m). This finding indicates that Desmostylus lived in nearshore water while Paleoparadoxia foraged in a relatively deep, offshore water. The depositional segregation of these two genera most likely reflects their different habitat preferences.