Paleontological Research

The apparatuses of Triassic ellisonid conodonts: Cornudina breviramulis, Hadrodontina aequabilis, and Staeschegnathus perrii gen. et sp. nov. from the Taho Formation in Higashiuwa-gun, Ehime Prefecture, Southwest Japan and Furnishius triserratus from the Iwai Formation in Nishitama-gun, Tokyo were reconstructed on the basis of the multielement structure of natural assemblages previously reported. Ellisonia triassica was remarked on this occasion. These species well agree with the general septimembrate apparatus structure containing 15 elements: angulate or pastinate P1, angulate P2, breviform digyrate M, alate S0, extensiform digyrate S1 and S2, and bipennate S3 and S4 elements. Having compared the morphologic features of apparatus elements of the Ellisonidae, I propose herein the new subfamily Hadrodontinae within it. Among the Ellisonidae and other taxa of the Prioniodinina, a phylogenetic relationship is recognized only between E. triassica and Upper Devonian Hibbardella angulata. Other species of the Ellisonidae are more closely related to ozarkodinides with “ozarkodiniform” angulate P1 element than any previously reported prioniodininids with “oulodiniform” extensiform digyrate and carminate P1 elements.

Radiolarian assemblages in 69 surface sediment samples from the Japan Sea were moderately diversified, consisting of about 100 taxa in total, although only a few species accounted for a large proportion of most assemblages. First, the assemblages were often dominated by deep-dwelling species: Cycladophora davisiana, Actinomma leptodermum, A. boreale, A. langii, and adult forms of Larcopyle buetschlii. These species were restricted to great depths and were characteristic of the cold, oxygen-rich deep water that fills deep basins of this marginal sea, the so-called Japan Sea Proper Water. Second, although the observed Japan Sea assemblages included some subtropical elements, such as the Dictyocoryne and Euchitonia groups, Didymocyrtis tetrathalamus and the Tetrapyle octacantha group, many of the major temperate and subarctic elements of the North Pacific were essentially excluded. The semi-enclosed topography of the Japan Sea is most likely responsible for the dominance of certain subtropical surface dwellers as well as for the near-absence of transitional and cool water species from corresponding latitudes of the North Pacific. Q-mode cluster analyses of the relative abundance data of the radiolarian species distinguished three faunal provinces that reflect the modern surface water circulation and the distributions of the upper water masses, including the Tsushima warm current and the Liman cold current. These results suggest that the radiolarian assemblages are strongly related to the present hydrography of the Japan Sea and can therefore be used as environmental proxies in this region.

Inioidea, which consists of Iniidae and Pontoporiidae, includes only four extant species, all of which occur in South America. Inioids were, however, more diversified and widely distributed in the past, starting from the late middle Miocene, as recorded from the eastern Pacific, North and South Atlantic, North Sea and the riverine systems of South America. In this paper, the author describes a new extinct inioid, Awadelphis hirayamai gen. et sp. nov. (WU SILS G 408), from the uppermost Miocene Senhata Formation (6.3–5.7 Ma) of Chiba, central Japan. Morphological cladistic analysis supports placement of this new taxon within the Inioidea. The new species is characterized by a right premaxillary eminence with a pronounced overhang on the right maxilla and a short zygomatic process of the squamosal. This is the first fossil record of an inioid not only from Japan but also from the North Pacific. The discovery of this new taxon considerably extends the paleobiogeographic range of inioids, and indicates that inioids co-occurred with other small odontocetes of the western North Pacific, where delphinoids dominated. The new species is also the first valid occurrence of a small odontocete from tropical—subtropical climates of the late Miocene in the western North Pacific.

New fossil material from the latest early Miocene Manchar Formation in the Ranikot area (Sindh, Pakistan) provides information about the poorly known listriodontine suid Listriodon guptai Pilgrim, 1926 . Recognition of this taxon, initially described on the basis of a fragmentary upper molar, allows biochronologic correlations for the base of the Manchar Formation with penecontemporaneous formations of Pakistan where this taxon also occurs (Sulaiman Range and Potwar Plateau). Listriodon guptai may be closely related to the bunolophodont species Listriodon affinis of doubtful stratigraphic provenance and age in the Bugti Hills, as well as to the fully lophodont Listriodon pentapotamiae which occurs throughout the Chinji Formation in the Potwar Plateau. The early occurrence of these taxa pleads for an early diversification of the listriodonts in the Indo-Pakistan Subcontinent during the late early Miocene.

A new species of coelacanth, Whiteia oishii sp. nov. is described on the basis of complete specimens from West Timor in Indonesia. This species differs from other species in the genus Whiteia by the presence of five to ten long ridges on scales, nine rays on the first dorsal fin, pointed denticles on the anterior fin rays of the first dorsal fin, and an operculum ornamented with many tubercles. Whiteia oishii sp. nov. is the first coelacanth occurrence from Southeast Asia and its discovery sheds light on the worldwide distribution of the genus Whiteia in the marine Triassic.

A rhinocerotid fossil (NMNS-PV9600) from the lower part of the Isa Formation (Middle Pleistocene), Isa area, Mine City, Western Japan, is described. Previously, this specimen had been identified as Dicerorhinus nipponicus. However, the specimen shares the following features with the genus Stephanorhinus: molarised premolars; absence of P1; absence of metacone fold on P3 and P4; presence of crochet on the premolars and molars; presence of a crista on the molars; absence of medifossette on both premolars and molars; triangular-shaped M3. In this study, NMNS-PV9600 is redescribed as Stephanorhinus kirchbergensis based on morphological and metric comparisons with other Eurasian Pleistocene rhinocerotids. NMNS-PV9600 is close to S. kirchbergensis from Choukoutien, suggesting S. kirchbergensis could have reached the Japanese archipelago during the middle Pleistocene (before or around 0.43 Ma). NMNS-PV9600 was an adult individual around 25 years in age when it died, based on the tooth eruption and wear stage. The cheek teeth morphology suggests that S. kirchbergensis from Japan was probably a browser as the species has before been considered in occurrences from other countries.

Well preserved, toothed, pinnate leaves have been collected from the Upper Jurassic Tochikubo Formation, northeast Japan. This plant has been interpreted as a new species of Encephalartites, which was established by Vachrameev in 1962 for a plant from the Lower Cretaceous. Specimens recently collected from the Jurassic of Fukushima Prefecture, Japan, provide us with new morphological information about this genus. Leaflets are attached to the adaxial surface of the rachis. The base of the leaflet is symmetrical and its proximal portion narrows to the attachment area. Unfortunately the cuticle is not preserved and therefore the suprageneric taxonomic position is uncertain. The authors propose a new species of this genus, Encephalartites nipponensis sp. nov.

The stable carbon (δ¹³C) and oxygen isotopes (δ¹⁸O) of planktic foraminiferal tests have been widely used as proxies in paleoceanography and paleoclimatology. The ontogenetic isotopic profiles of foraminifers are also thought to record ecological information about species, such as changes in habitat depth and symbiotic relationships. However, isotopic profiles during “individual ontogeny” have rarely been examined. In this study, we report the ontogenetic isotopic information for three net-collected modern species, Globigerinoides sacculifer, Neogloboquadrina dutertrei, and Globorotalia inflata, together with several in situ oceanographic parameters of the water column in Sagami Bay, Japan (seawater temperature, salinity, nutrients, chlorophyll a content, δ¹³C of dissolved inorganic carbon [DIC], and δ¹⁸O of seawater). We examined the ontogenetic profiles of the foraminifers with chamber dissection and chamber-by-chamber analyses of δ¹³C and δ¹⁸O using a specially designed continuous-flow mass spectrometry system. The ontogenetic δ¹⁸O profiles showed overall ¹⁸O-enrichment in all three species, suggesting their ontogenetic migration toward deeper habitats. When these records were compared with the physicochemical profiles of the water column, all the ontogenetic records began within the uppermost thermocline or shallower, corresponding to the depth of relatively high chlorophyll content. Later in ontogeny, Gs. sacculifer and N. dutertrei migrated to the bottom of the level of maximum chlorophyll, whereas Gr. inflata descended to a depth of 200 m. The deviations of foraminiferal δ¹³C from the δ¹³C of DIC were largest in the juvenile stages, but were near zero at a test mass of ca. 10 μg for all three species. Contrary to the subsequent asymptotic profiles of this deviation in N. dutertrei and Gr. inflata, Gs. sacculifer alone showed a subsequent increase, of up to +1.0‰, reflecting its symbiotic relationship. We conclude that a certain ontogenetic test mass, in this case of around 10 μg, can be assigned to a preferable size class of foraminifers from which to reconstruct the paleo-δ¹³C of DIC in the water column, regardless of the species ecology.

A new periotic of Numataphocoena yamashitai from the upper part of the Horokaoshirarika Formation (lower Pliocene), Numata, Hokkaido, Japan shows younger ontogenetic features in comparison to the matured holotype (NFL 7). The referred specimen of N. yamashitai is smaller (approximately 90% in size) than the physically and sexually matured holotype periotic. The holotype (NFL 7) and referred specimen (NFL 2617) show several differences owing to ontogenetic changes, such as: the holotype (NFL 7) has a more strongly swollen medial part of the anterior process, blunt anterior keel, rounded anterodorsal angle, deeper and clearer anterior incisure, more rounded pars cochlearis, deeper and narrower hiatus epitympanicus rather than a flat area, longer posterior process with an extra edge posterolaterally. N. yamashitai differs from other fossil phocoenids in having a narrower and sharper anterior part of the internal acoustic meatus and a robust anterior process. Haborophocoena toyoshimai similarly has a relatively narrow internal acoustic meatus and a robust anterior process, compared with other fossil phocoenids. Between N. yamashitai and H. toyoshimai, N. yamashitai has a narrower internal acoustic meatus and a more robust anterior process. These similarities imply a close relationship between Numataphocoena and Haborophocoena. The new specimen provides diagnostic features and insight into ontogenetic variation of N. yamashitai.

This paper describes the first record of the heteromorph ammonoid Diplomoceras cylindraceum from the uppermost Maastrichtian Kawaruppu Formation of the Nemuro Group, southeastern Hokkaido, Japan. D. cylindraceum was found in a siltstone 15 m below the Cretaceous/Paleogene (K/Pg) boundary. The stratigraphical level of the fossil is estimated to be ∼66.8 Ma, based on the time scale for the geomagnetical polarity and the K/Pg boundary. D. cylindraceum, therefore, was living at least ∼0.8 m.y. before the K/Pg boundary event in the north Pacific region.