Joumocetus

Joumocetus is a genus of extinct baleen whale in the family Cetotheriidae containing the single species Joumocetus shimizui. The species is known only from a partial skeleton found in Miocene age sediments of Japan.

Joumocetus shimizui was described from a mostly complete cranium and associated portions of both mandibles, cervical and thoracic vertebrae. The skeleton was recovered from an outcrop of the earliest late Miocene, Serravallian age Haraichi Formation exposed along the Kabura River near Yoshii, Gunma Prefecture, Japan (36°18′N 139°00′E / 36.3°N 139.0°E / 36.3; 139.0, paleocoordinates 37°48′N 137°12′E / 37.8°N 137.2°E / 37.8; 137.2). The Haraichi Formation includes several tuff layers which have been used for dating. The type skeleton was found between the older Baba tuff and the younger Kamikoizawa tuff. Biotite specimens taken from the Baba tuff have yielded an ⁴⁰argon–³⁹argon date of 11.26 million years ago, while sanidine specimens gave a ⁴⁰argon–³⁹argon date of 11.29 million years ago. The overlying Kamikoizawa tuff has not been dated, sedimentation rates have been used to estimate an age of the earliest late Miocene.