A fossil humerus of Desmostylus highlights the spatio-temporal distributional changes of its genus on both sides of the Pacific coastlines

Desmostylus is an extinct genus of large, quadrupedal, herbivorous marine mammals from the Oligocene to Miocene strata of the North Pacific. Despite abundant fossil discoveries, the taxonomic, ontogenetic, and ecological aspects of this genus remain insufficiently understood. In this study, we reexamined a previously misidentified specimen (NMNS-PV 44111) from the Miocene Chikubetsu Formation in Hokkaido, Japan. Archival records rediscovered in 2024 enabled precise documentation of the locality and its stratigraphic context. Comparative anatomical analysis of the humerus revealed that the specimen represented a juvenile Desmostylus, with a body size larger than that of early desmostylians but smaller than that of fully mature Desmostylus individuals. The growth stage suggests a rapid increase in body size during the early ontogenetic phases. Stratigraphic and paleontological evidence places the specimen within the Middle Miocene Climatic Optimum (MMCO), a period linked to marine transgression and warm conditions that may have facilitated a northward range expansion of this genus. Our findings underscore the importance of reassessing legacy specimens using updated stratigraphic and comparative frameworks to better understand the evolutionary history of marine mammals in the North Pacific.