Masaichi Kimura

THE OLDEST RECORD OF ESCHRICHTIIDAE (CETACEA: MYSTICETI) FROM THE LATE PLIOCENE, HOKKAIDO, JAPAN

Abstract A new eschrichtiid mysticete was recovered from the Late Pliocene Yuchi Formation, Teshio Town, Hokkaido, Japan. The specimen is relatively well preserved, comprising a posterior part of the skull, periotics and tympanic bullae, several incomplete cervical and thoracic vertebrae, ribs, and forelimb elements including scapula, humerus, ulna, and several phalanges. Its small size, unfused epiphyses of vertebrae and long bones (humerus, radius, and ulna) indicate that the individual is physically immature. It is identified as eschrichtiid based on diagnostic characters, including paired occipital tuberosities, large occipital condyles, the posteriorly concave exoccipital lateral to the occipital condyle, the cochlear portion of the periotic extending anteromedially, the relatively small and somewhat triangular anterior process of the periotic, and the proportionally large vertebral foramen relative to the vertebral body. Because all verified eschrichtiids are of Pleistocene age, the Teshio eschrichtiid significantly extends the temporal range of the group.

Haborophocoena toyoshimai, a new early Pliocene porpoise (Cetacea; Phocoenidae) from Hokkaido, Japan

Abstract Haborophocoena toyoshimai (family Phocoenidae), from the early Pliocene Mochikubetsu Formation in northwestern Hokkaido, Japan, is a new genus and species of porpoise. The holotype is an incomplete skull, tympanoperiotic, mandible, and several teeth. Its features include: a robust and asymmetrical skull; a premaxilla with a less atrophied posterior extremity; a low but wide premaxillary eminence; a conspicuous knob-like frontal boss; narrow palatines posteriorly separating the pterygoids; a thin lateral lamina of the palatine that extends posteriorly; well-excavated air sinus fossae on the basicranium; a relatively small temporal fossa; an abruptly sloping supraoccipital shield; a periotic with massive anterior process; and tiny teeth with blunt tips. Some features of living phocoenids may be paedomorphic; examples include the relatively short rostrum, the rounded cranium, and the small body size. “Precocious” reproduction and a short interval of the reproductive cycle, compared to other delphinoid dolphins, suggest that paedomorphic characters of living phocoenids are progenetic. Progenetic evolution possibly occurred in harsh environments caused by glacial oscillations of the Pleistocene; molecular divergence times for porpoises are compatible with this idea.

A NEW FOSSIL PORPOISE (CETACEA; DELPHINOIDEA; PHOCOENIDAE) FROM THE EARLY PLIOCENE HOROKAOSHIRARIKA FORMATION, HOKKAIDO, JAPAN

Abstract A new fossil odontocete genus and species, Numataphocoena yamashitai is from the Early Pliocene Horokaoshirarika Formation, Numata, central Hokkaido, Japan. The porpoise was probably buried in relatively shallow marine sediments. This specimen consists of an incomplete skull, a right periotic and tympanic bulla, a posterior part of the right mandible, 23 isolated teeth, a nearly complete vertebral column, a chevron, incomplete right and well-preserved left scapulae, a right forelimb including humerus, radius, ulna, and a few indetermined carpals, and several ribs, all the same individual. The ankylosis of the epiphyses to vertebral bodies and to long bones, such as humerus, radius and ulna, shows that this individual reached both sexual and physical maturity; the animal in life was probably about 220 cm long. The cranial vertex has an elevated frontal knob, and the tooth crown is spatulated; both are characteristic of phocoenids. Numataphocoena yamashitai shows many primitive features such as premaxillae with less atrophied posterior extremities extending far posteriorly almost to hinder margin of the narial fossa, robust zygomatic processes, a periotic with massive anterior process and rugose posterior process, humerus long relative to radius or ulna, and free cervical vertebrae. The axial skeleton is significantly different from that of extant relatives in its a small number of vertebrae with anteroposteriorly broad transverse processes, and a long humerus relative to the radius and ulna.

Reinterpretation of some cranial structures of Desmostylus hesperus (Mammalia: Desmostylia): a new specimen from the Middle Miocene Tachikaraushinai Formation, Hokkaido, Japan

Abstract A new skull of Desmostylus hesperus Marsh, 1888 is described from the late Middle Miocene Tachikaraushinai Formation, Utanobori, Hokkaido, northern Japan. The specimen, GSJ-F7745, is a nearly complete skull and mandible, but the posterior halves of their left sides are missing. It was a juvenile individual, having part of the worn fourth deciduous molar (dp4) in the right upper jaw. Another specimen (GSJ-F7743) was uncovered in the same formation near the same area where GSJ-F7745 was collected. Both fossils represent nearly the same ontogenetic stage and were of the same gender. There are two different opinions regarding the position of the external acoustic meatus of Desmostylus. The cranial cavity of specimen GSJ-7745 is observable and reveals that the meatus should be identified as being the foramen on the lateral surface of the posterior portion of the zygomatic arch.

New material of Haborophocoena Toyoshimai (Odontoceti: Phocoenidae) from the Lower Pliocene Embetsu Formation of Hokkaido, Japan

Abstract. A new specimen of the Early Pliocene porpoise Haborophocoena toyoshimai, comprising an incomplete cranium, was discovered from the Embetsu Formation near the locality of H. minutus, Haboro Town, Hokkaido, Japan. The specimen is a juvenile skull of H. toyoshimai, comparable in size to the congeneric H. minutus. It confirms and/or adds some characters that define H. toyoshimai. It is now clear that the structure of the posterior part of the palate is unique to H. toyoshimai among phocoenids. It is markedly deep dorsoventrally at the level of the midlength of the orbit. In ventral view, the palatal face of the pterygo-palatine complex is distinct in having a sharp and sinuous lateral edge. The palatal surface of the pterygo-palatine complex is gently concave. A prenarial triangle anteromedial to the premaxillary eminence is depressed. The new specimen also reveals possible ontogenetic variation.