Paleontological Research

Biostratigraphy of conodonts is widely used for age assignment of Permian strata. In this paper, we report conodonts from the limestone of the Nabeyama Formation deposited on a pelagic seamount in Panthalassa, which is an oceanic realm where Permian conodont data are scarce compared to other oceanic realms. Samples collected from the lower part of the Nabeyama Formation yielded Mesogondolella idahoensis (Youngquist, Hawley and Miller) and Sweetognathus hanzhongensis (Wang), which indicate a late Kungurian age. Previous studies and fusulinids obtained in this study indicate that the studied samples belong to the Parafusulina yabei biozone. Therefore, the Parafusulina yabei Zone includes the uppermost Kungurian. Mesogondolella idahoensis and Sweetognathus hanzhongensis are respectively regarded as cool and warm water species. Hence, the distribution of cool and warm water conodont species may have overlapped in pelagic Panthalassa during the late Kungurian.

A preliminary description is provided of a new assemblage of small, three-dimensional and charcoalified mesofossils from the Tamagawa Formation (late Turonian–middle Santonian; Upper Cretaceous) of the Kuji Group in northeastern Japan. The new mesofossils yield excellent structural details and include well-preserved circinate shoots of ferns together with conifer leafy-shoots, seeds and probable pollen cones, and variety of angiosperm fruits and seeds, including fruits of Cornales and seeds of Nymphaeales. The new mesofossil assemblage is complementary to the previously published macrofossil flora from the Kuji Group.

In this study, a new late Campanian (Late Cretaceous) heteromorph ammonoid, Didymoceras morozumii Misaki and Tsujino sp. nov., from the Hiketa Formation, Izumi Group, southwestern Japan, is described. It is characterized by a loose helical post-embryonic early whorl and relatively tight helical middle whorls, followed by a retroversal hook with a highly elevated apertural end, and two rows of irregular tubercles. Didymoceras morozumii sp. nov. occurs in the upper part of the Didymoceras sp. Zone, which is overlain by the Didymoceras awajiense Zone. Didymoceras awajiense-bearing beds of the Hiketa Formation in the study area is correlated to the lower part of that of the Toyajo Formation in the Kii Peninsula on the basis of the shell forms of D. awajiense. Didymoceras morozumii sp. nov. and D. awajiense have similar characteristics, and it is thought that D. awajiense evolved from D. morozumii sp. nov. As indicated in the descendants, some specimens of D. morozumii sp. nov. are also encrusted by anomiid bivalves.

Plotopterids, commonly known as “penguin-like birds”, are wing-propelled diving birds known from the latest Eocene to middle Miocene in the eastern and western Pacific Rim. Here, we describe two new specimens of the family Plotopteridae, a right femur from the Itanoura Formation (latest Eocene to earliest Oligocene), and a distal half of a right tibiotarsus from the Kakinoura Formation (early Oligocene), both at the lower part of the Nishisonogi Group, Saikai City, Nagasaki Prefecture, Kyushu, southwestern Japan. The femur is slightly younger than or potentially as old as the earliest plotopterid known from Japan and the U.S.A. CT scanning revealed that it has a dense cortical bone, justifying its taxonomic assignment to the family Plotopteridae. It resembles the femur of Olympidytes, which was previously considered endemic to North America, in its femoral neck, well developed trochanter femoris, and straight facies articularis antitrochanterica, but is not assignable to any known genus. The tibiotarsus resembles that of Olympidytes in the presence of a well-developed trochlea catilaginis tibialis, a large embossment lateral to the pons supratendineus, and a deep incisura intercondylaris, and therefore referable to Olympidytes. The possibility of hindlimb-propelled diving in the family Plotopteidae was also discussed. These new specimens suggest the early diversity of the family in Japan was higher than previously thought.

The Qom Formation deposits, located at 12 km southeast of Salafchegan (N: 34° 21′ 26″ and E: 50° 32′ 14″), have a thickness of 110 m. The formation includes thin, and medium-to-thick bedded limestone, as well as shale, overlying the Lower Red Formation (early Oligocene) above an erosional unconformity. Its upper boundary is covered by alluvium sediments. Biostratigraphic distributions of benthic Foraminifera were used to determine the age and paleoenvironmental conditions of the Qom Formation. Two assemblage zones were recognized. Assemblage zones 1 and 2 were indicative of a Rupelian–Chattian age. The inner shelf (restricted and semi-restricted lagoons) and middle shelf (open marine) settings were formed on an open shelf platform. Euphotic conditions were dominant during the early and late Rupelian in the studied area. During the middle Rupelian, photic conditions were variable between euphotic and mesophotic to oligophotic. During the early and middle Chattian, photic conditions varied between oligophotic, mesophotic to oligophotic, and euphotic. During the late Chattian, mesophotic-oligophotic conditions were dominant. Additionally, a high level of salinity (40–50 and > 50 psu) was present in the studied area during the early Rupelian. The level of salinity varied from normal (30–40 psu) to hypersaline (40–50 psu) during the late Rupelian. Salinity during early and late Chattian subages was normal (30–40 psu). However, hypersaline (40–50 and > 50 psu) and normal (30–40 psu) conditions were present in the middle Chattian. Eutrophic to mesotrophic-oligotrophic conditions were found in the early Rupelian age. However, mesotrophy-oligotrophy dominated during the middle and late Rupelian and Chattian ages. In addition, the paleo-water depth of the Qom Sea fluctuated from < 10 m to > 20 m. Foralgal and foramol associations are dominant in the studied area during the Rupelian-Chattian ages. The general depositional environment of the Qom Formation is associated with seagrass meadows.

We report carbon (δ¹³C) and oxygen (δ¹⁸O) isotope records of two modern giant clam (Tridacna squamosa) shells from two sites (Ishigaki-jima and Okinoerabu-jima) at different latitudes in the Ryukyu Islands, Japan. The δ¹³C profiles of samples from the inner shell layer on cross-sections along the maximum growth axis display no ontogenetic trends or seasonal variations. This finding suggests that the calcification site is likely to be unaffected by CO2 uptake and release resulting from the metabolic activity of the molluscan host and algal symbionts. The δ¹⁸O profiles show distinct seasonal cycles. After accounting for the influence of seawater δ¹⁸O, the time-series variations are consistent with variations in sea surface temperature, and the temperature dependency of oxygen isotope fractionation is nearly identical to previously published δ¹⁸O–temperature relationships for biogenic and synthetic aragonite. We conclude that δ¹⁸O records from pristine fossils of this species will enable accurate paleoenvironmental reconstructions at high temporal resolution.

Raususetarches sakurai gen. et sp. nov. is described from the late Miocene Koshikawa Formation in Kasuga, Rausu, Shiretoko Peninsula, Hokkaido, Japan based on several specimens found in the same rock, as a new genus and species. This new species belongs to the family Scorpaenidae of Scorpaenoidei in having short, triangular posterior spines of preopercle, two ridges and spines of opercle and the first pterygiophore of the anal fin inclined. This species belongs to the subfamily Setarchinae in having lateral line scales forming grooves and cycloid scales. This new species, however, differs from all other genera of the subfamily in having large triangular spines of preopercle; 14 spines and 13 soft rays of the dorsal fin; three spines and eight soft rays of the anal fin; 19 rays of the pectoral fin; hypurals 1 and 2 fused; and hypurals 3 and 4 are also fused. This is the first fossil species of the subfamily Setarchinae.

A nostoceratid ammonoid Yezoceras elegans sp. nov. is newly described from the Coniacian of the Haboro area in Hokkaido, northern Japan. Yezoceras elegans sp. nov. having loosely coiled whorls, a wide umbilicus, and two prominent tubercle rows concentrated in the lower part of the whorls, is distinguished from the other species by these characteristics. Yezoceras elegans sp. nov. might have originated from Y. nodosum, judging from the stratigraphic correlation. The restricted occurrences of three Yezoceras species (Y. elegans sp. nov., Y. nodosum and Y. miotuberculatum) in Hokkaido, northern Japan suggest that the speciation of Yezoceras occurred in the northwestern Pacific realm during the Coniacian age.

A new heteromorph ammonoid Sormaites teshioensis gen. et sp. nov. (Diplomoceratidae) is described from the upper Turonian (Upper Cretaceous) in the Nakagawa area, Hokkaido, northern Japan. Its near planispiral whorls consist of crioceratoid early whorls followed by straight shafts with U-shaped whorls connecting the successive shafts. The shell surface is ornamented with simple, straight, sharp tipped ribs throughout ontogeny, but infrequent flared ribs and constrictions occur on later whorls. Excluding its earliest whorls, its coiling and ornamentation are very similar to Scalarites mihoensis and Sc. densicostatus from the Turonian to Coniacian in Hokkaido and Sakhalin, suggesting that So. teshioensis was probably derived from one of these taxa in the Northwest Pacific during middle to late Turonian time.