Abstract
So far, 41 sirenian fossil records have been known in Japan. Most of them belong to the subfamily Hydrodamalinae and the fossil localities are mainly distributed from eastern Honshu to Hokkaido. The Hydrodamalinae is the only cold-adapted taxon among sirenians. A new fossil was found from the Late Pliocene Zukawa Formation in Takaoka, Toyama Prefecture. It is the first record of sirenian fossil in Toyama Prefecture and its locality is close to the southwesternmost of fossil distribution of Hydrodamalis in the Sea of Japan side. Collected fossils are partial bones of skull, scapula, humerus, vertebrae and ribs, and they are presumed to be a same individual because of no overlapping parts in the skeleton. The fossil is identified as the genus Hydrodamalis (Hydrodamalinae) based on several distinct features of the skull and considered to be a young individual with loose bone sutures. Furthermore, some unique morphological features in temporal, mandible and humerus are different from recognized species of Hydrodamalis. The fossil age is estimated to be around 2.7 Ma, because assumed fossil bearing bed is about 6 meters upper from the Datum A. The Datum A is a boundary of changing to cold assemblage in calcareous nannofossils and its age estimated to be 2.75 Ma. The fossil is important on the paleobiogeography of Hydrodamalis, because the fossil shows clearly that sirenianʼs southward movement and climate cooling are closely related in the Sea of Japan side. So the latest Pliocene climate of Toyama area was suitable for Hydrodamalis. In this way, cold-adapted Hydrodamalis may have migrated southward intermittently from the northern area to Honshu during the Pliocene to Pleistocene in response to cooling climate change.
