A new specimen of Paleoparadoxia found from a marine lower Miocene deposit in the Chikubetsu area, Hokkaido, Japan, is described. The material consists of a distal part of the scapula, proximal end of the humerus from the right side, as well as a fragmentary rib, preserved in a float of calcareous fine sandstone. The specimen is referred to the order Desmostylia and subsequently to the genus Paleoparadoxia sp. The well preserved shoulder girdle of this specimen provides the first detailed morphology of this anatomical region in Paleoparadoxia. We compared the specimen with a wide range of desmostylid samples to reveal new diagnostic characters for the genus, such as the greater tubercle extending toward the proximal side above the head and the distinct lesser tubercle located on the medial side, projected medially. The lower Miocene Sankebetsu Formation outcrops in the area where the float was found, and the lithology and associated fossil fauna of the float indicate that it was derived from there. The fossil pollen assemblages and molluscs found in the formation indicate that it was deposited under cool to temperate conditions. The new specimen thus suggests that Paleoparadoxia had inhabited a cool environment in addition to warm areas, as suggested in previous studies. The published age estimate of the formation places the present specimen between 23.8±1.5 and 20.6±1.0 Ma. This represents the oldest record of the genus Paleoparadoxia, exceeding the previous record of ca. 19 Ma for a specimen found from the Chichibu Basin, and nearly matches the oldest record of Paleoparadoxiinae in the Northwestern Pacific. This indicates that the ages of the oldest occurrences of basal and derived paleoparadoxiines overlapped (i.e., Archaeoparadoxia from the Northeastern Pacific Region and Paleoparadoxia from the Northwestern Pacific, respectively). It is also likely that the geographic range of Paleoparadoxiinae had already expanded from the Northeast Pacific to the Northwest Pacific in the early stage of their evolution.

Background Desmostylia is a clade of extinct aquatic mammals with no living members. Today, this clade is considered belonging to either Afrotheria or Perissodactyla. In the currently-accepted taxonomic scheme, Desmostylia includes two families, 10 to 12 genera, and 13–14 species. There have been relatively few phylogenetic analyses published on desmostylian interrelationship compared to other vertebrate taxa, and two main, alternative phylogenetic hypotheses have been proposed in previous studies. One major problem with those previous studies is that the numbers of characters and OTUs were small. Methods In this study, we analyzed the phylogenetic interrelationship of Desmostylia based on a new data matrix that includes larger numbers of characters and taxa than in any previous studies. The new data matrix was compiled mainly based on data matrices of previous studies and included three outgroups and 13 desmostylian ingroup taxa. Analyses were carried out using five kinds of parsimonious methods. Results Strict consensus trees of the most parsimonious topologies obtained in all analyses supported the monophyly of Desmostylidae and paraphyly of traditional Paleoparadoxiidae. Based on these results, we propose phylogenetic definitions of the clades Desmostylidae and Paleoparadoxiidae based on common ancestry.

Here, we report a new ‘discovery’ of a desmostylian fossil in the geological collection at a national university in Japan. This fossil was unearthed over 60 years ago and donated to the university. Owing to the original hand-written note kept with the fossil in combination with interview investigation, we were able to reach two equally possible fossil sites in the town of Tsuchiyu Onsen, Fukushima. Through the interviews, we learned that the fossil was discovered during construction of a debris flow barrier and that it was recognized as a ‘dinosaur’ bone among the locals and displayed in the Village Hall before/until the town experienced a fire disaster in 1954. As scientific findings, the fossil was identified to be a right femur of Paleoparadoxia (Desmostylia), which shows well-preserved muscle scars on the surface. The age was estimated to be 15.9 Ma or younger in zircon-dating. This study shows an excellent case that historical and scientific significances could be extracted from long-forgotten uncatalogued specimens as long as the original information is retained with the specimens.