Paleontological Research

We report the fourth record of a fossil whale-fall community in Japan. The new material consists of a single whale bone in association mainly with small bathymodiolin mussels, Adipicola sp., found in the Karumai Formation (late middle Miocene—early late Miocene) in the Hobetsu area of Hokkaido, Japan. This association of whale bone and Adipicola sp. and its mode of occurrence resembles the description of some other ancient whale-fall communities dominated by small mussels from the Olympic Peninsula in Washington State (early Oligocene), Shosanbetsu in Hokkaido (early middle Miocene) and Carpineti in northern Italy (middle Miocene) and constitutes an example of a chemosynthesis-based community sustained by whale-fall decay in the Miocene deep sea. The new example extends the Miocene distribution of bathymodiolin-dominated whale-fall communities to the northwestern Pacific Ocean.

Plant fossils are reported on from the Lower Cretaceous Upper Formation of the Sasayama Group, including the new species Otozamites toshioensoi sp. nov. The Sasayama Flora proposed here is characterized by an abundance of microphyllous conifers such as Brachyphyllum spp. and Pseudofrenelopsis sp., as well as by the rare occurrence of pteridophytes. These floristic components suggest that an arid climate prevailed in the land of the Sasayama Group and that the Ryoseki-type flora, which generally represents the Outer Zone of Japan during the Late Jurassic to Early Cretaceous, flourished in the Inner Zone of Japan during the late Early Cretaceous. Albian or Cenomanian ages have previously been proposed for the Upper Formation of the Sasayama Group by radiometric datings, but an Albian age is preferred here in regards to the extremely rare occurrence of possible angiosperms.

Late Devonian brachiopods from Choanji, South Kitakami Belt, northeastern Japan, are re-examined to determine their age. Two brachiopod assemblages are recognized in the Choanji area. The assemblage from locality CHJ2 in the middle part of the Choanji Formation consists of one species, Desquamatia (Seratrypa) sp., which indicates a Frasnian age. The assemblage from locality CHJ1 in the upper part of the Choanji Formation consists of 13 species in 10 genera, including a new species, Parallelora minatoi sp. nov., and is of late Famennian age. Thus, the Choanji Formation is correlated with the Upper Devonian (Frasnian–upper Famennian), although until quite recently it was considered to be the lowest Carboniferous (Tournaisian) in Japan.

A recently discovered, incomplete dentary with preserved incisor, premolar and first molar of a minute castorid from the Dota locality, Kani Basin, early Miocene (ca. 18.5 Ma) Nakamura Formation of the Mizunami Group in central Japan, is described as a new species of the trogontheriine beaver Euroxenomys. It represents the first record of Euroxenomys in Asia and one of the oldest records of this genus. Euroxenomys nanus sp. nov. is smaller than the type species, E. minutus from the Miocene of Europe and more hypsodont, but similar in size to E. inconnexus from the Barstovian of Montana. Dota is the first Asian locality with three cooccurring castorids, Youngofiber sinensis, Minocastor godai, and E. nanus.

A partial crocodylian skull was found among the paleontological collection at the Archaeological Museum, Waseda University in Honjo-shi, Saitama Prefecture. Following a bibliographical survey, this was verified as a specimen reported by Tokunaga as the first known crocodile from Taiwan but then regarded as lost due to World War II. Molten glass debris is attached to the specimen, suggesting that it had been subject to air raids during World War II in May 1945, most likely at the Waseda University campus. Based on its largest seventh maxillary alveoli and the prominent lateral expansion towards the posterior direction, this specimen was identified as Toyotamaphimeia sp.; up to now a single species under this genus has been reported. The discovery demonstrates that this crocodylian genus once had a wide geographical distribution during the Pleistocene.

A basal mosasauroid specimen, including a rib and a vertebra from the middle to posterior portion of the trunk, is reported from the lower Campanian Inoceramus (Platyceramus) japonicus Zone in Obira Town, northern Hokkaido, northern Japan. It is the second occurrence of basal mosasauroids sensu lato in Japan after the halisaurine Phosphorosaurus ponpetelegans, but represents a larger individual than the P. ponpetelegans holotype. The Obira specimen predates the early Maastrichtian P. ponpetelegans by about 10 million years, indicating colonization by basal mosasauroids of the northwestern Pacific by at latest the early Campanian age. While the overall morphology of the Obira specimen agrees well with that of a halisaurine vertebra, the presence of well developed zygantra (zygosphenes missing postmortem if present), the inclined condyle, and a distinct precondylar constriction uniquely align the specimen with Pannoniasaurus inexpectatus, a Santonian-aged basal mosasauroid from freshwater deposits in Hungary.

The Late Triassic conodont biostratigraphy of two pelagic chert sections (sections N and Q) in the Inuyama area, central Japan, was investigated to calibrate the Triassic radiolarian zonation proposed by Sugiyama in 1997 with the conodont zones and the standard Triassic timescale. Based on the stratigraphic distributions of marker species, six conodont zones were defined: the Paragondolella? tadpole interval Zone, the Quadralella tuvalica interval Zone, the Epigondolella quadrata interval Zone, the E. triangularis interval Zone, the Mockina postera interval Zone, and the M. bidentata Zone. These conodont zones are comparable to the standard Carnian and Norian conodont zones of North America and the Tethys. The Carnian-Norian boundary in the sections studied is tentatively placed between the last occurrence of a Carnian species (Q. tuvalica) and the first occurrences of Norian species (E. quadrata and E. spatulata). The intercalibrated conodont—radiolarian biostratigraphy from the sections we studied accurately calibrates the radiolarian zones in Japan with standard chronostratigraphic stages and substages.

The family Eschrichtiidae is presently only represented by Eschrichtius robustus, a relict species from the North Pacific. Because of the scarcity of fossil records of the Eschrichtiidae, their evolutionary history is not well understood. A finely preserved mysticete skeleton was recovered from the Lower Pleistocene (1.77–1.95 Ma) of Tokyo, Japan, in 1961. The fossil consists of a cranium, mandibles, cervical, thoracic, lumbar and caudal vertebrae, chevrons, ribs, and forelimb bones, including scapula, humerus, radius, ulna and digit bones. Here, we describe and diagnose this fossil as a new species of the Eschrichtiidae, Eschrichtius akishimaensis sp. nov. This is the first fossil species of the genus Eschrichtius and suggests that at least two lineages represented by the modern species of Eschrichtius and the new species described here survived as late as the Early Pleistocene. This expands our knowledge of the paleodiversity of the eschrichtiids.

Nine longiconic cephalopod species of Desmoinesian (Middle Pennsylvanian; upper Carboniferous) pseudorthoceratid orthocerids are described from the Buckhorn Asphalt Lagerstätte (Boggy Formation) in Southcentral Oklahoma, Midcontinent North America. The fauna consists of Pseudorthoceras knoxense (McChesney), Arbuckleoceras tricamerae (Smith), Bitaunioceras buckhornense (Smith), Cyrtothoracoceras? sp., Dolorthoceras boggyense sp. nov., Smithorthoceras unicamera (Smith), Sueroceras oklahomense (Smith), Sulphurnites taffi sp. nov. and Unklesbayoceras striatulum sp. nov. Bitaunioceras buckhornense represents the first Pennsylvanian and therefore the oldest record of this genus. Arbuckleoceras gen. nov. differs from a comparable genus, Shikhanoceras, in possessing a weak exogastric curvature with a circular cross section of the conch and in lacking a conspicuous inflation at the embryonic shell. Smithorthoceras gen. nov. resembles orthoceratids rather than pseudorthoceratids in characters of camerae and siphuncle; however it refers to the Pseudorthoceratidae by having endosiphuncular deposits. These similarities seem to be the result of convergent evolution. Endosiphuncular deposits in Sulphurnites gen. nov. initiate at apical and adoral junctions between septal neck and connection ring, whose characters are unique for pseudorthoceratids. Unklesbayoceras gen. nov. differs from Mitorthoceras in having the endogastric conch, longer camerae and a less eccentric siphuncle. Taxonomic status of these orthoceratids was uncertain in previous biogeochemical and morphological studies. Sediments in the Buckhorn Asphalt Lagerstätte were deposited in a tropical epeiric sea (the Midcontinent Sea). Small, restricted marine basins, like that in this Oklahoma occurrence, probably provided an orthocerid refuge habitat as indicated by the high diversity and provincialism in comparison with other Middle Pennsylvanian (= Moscovian) faunas in other regions of the world.

An upper Paleocene radiolarian-bearing succession (ZN1) located in the southern part of the north subzone of the Tethyan Himalaya is composed of siliceous siltstones and claystones. In this paper, 23 species belonging to 13 genera are reported. Three new species, Lychnocanium? pyramis Li and Matsuoka sp. nov., Lychnocanium? stypticum Li and Matsuoka sp. nov., and Pterocyrtidium sinense Li and Matsuoka sp. nov., are described. Radiolarian assemblages from this succession can be compared with the Bekoma campechensis Zone (RP6), indicating a time interval of 61.5–58.23 Ma, i.e., late Paleocene. The radiolarian assemblages from the Yamdrok mélange, the Zheba section, and the Jiazhu section indicate that they are of the same age as those from the Zhinadibu 1 section. The lack of calcareous and coarse-grained terrestrial materials in the ZN1 section proves that the strata of this section were deposited in a relatively deep marine environment below the calcium carbonate compensation depth (CCD) and more distal to the continents during the late Paleocene. Coeval deepwater sediments near Saga were accumulated near the CCD.