Osteology and osteometry of the postcranial elements of the first Utanobori specimen (GSJ F07743) of Desmostylus hesperus from Utanobori, Esashi-cho, Hokkaido are described and compared with the adult Keton specimen, another nearly complete skeleton of Desmostylus. I concluded that the main developmental modifications of D. hesperus to be as follows: the transverse process of the lumbar vertebra decreases in a caudal inclination and the lumbar vertebra with a cranially inclined spine shifts backward; the humeral shaft becomes S-shaped and the coronoid fossa shallows; the olecranon bends more backward; the foremost point of the iliac crest shifts medially and the acetabulum faces more backward; the lateral inclination of patella weakens.
The cervical vertebrae and fibula, missing in the Keton specimen, are compared with Paleoparadoxia for the first time.The open epiphyseal lines in vertebrae and isolated epiphyses in some limb bones of the juvenile Utanobori specimen to indicate the degree and stage of development are recorded. The reconstructed skeletal model of the specimen with movable metal joints proved that Desmostylus, with the lateral-type limb, could walk by swinging each limb sagittally without lateral undulation of the body axis. The well-preserved bone surface of the specimen presents remarkable areas of tuberosity and tuberculum for muscle attachment. The main support muscles and retractors are estimated using elastic bands to show muscle action on the skeletal model during support and recovery phases.