Paleontological Research

A palynological study was conducted for the first time on the Lower Devonian Nakazato Formation in the South Kitakami Belt, northeast Japan. The palynomorphs were primarily examined using a scanning electron microscope as they exhibited opaqueness due to very high thermal maturity and coalification. The palynological assemblage comprises cryptospores, spores, tubular remains and conducting structures. However, species-level identification was proven challenging for many obtained palynomorphs. Nevertheless, the assemblage exhibited notable similarities to the Pragian to early Emsian palynoassemblages from South China in terms of composition and representation of genera. The age estimate derived from the palynological data slightly predates those obtained from marine invertebrates (Emsian to Givetian). To reconcile this discrepancy, a comprehensive comparison between terrestrial and marine biostratigraphies in Asia is necessary.

Two specimens of fossil lipotid have been recovered from the Upper Miocene of Gunma and Tochigi prefectures, Japan. The specimens consist of crania including periotic and tympanic bullae. We describe and diagnose the specimens as a new genus and species of the family Lipotidae, Eolipotes japonicus, gen. et sp. nov. The paratype of E. japonicus, GMNH-PV-1011, was found stratigraphically between two tuff layers and a well constrained age interval (11.29–11.25 Ma) can be proposed, indicating that E. japonicus is the oldest precisely dated lipotid species yet described. Phylogenetic analysis revealed that E. japonicus is more closely related to Parapontoporia spp. than to the recently extinct Yangtze river dolphin, Lipotes vexillifer. Given the phylogenetic relationships and parsimonious distribution of the event to fluvial invasion in Lipotidae, it is suggested that at least two independent invasions to the freshwater habitat occurred in Lipotidae.

A very small-sized ammonoid, i.e., Tetragonites pusillus sp. nov., was newly found from the upper lower to middle Maastrichtian (Pachydiscus flexuosus Zone) in the Makarov area, southern Sakhalin, Russian Far East. The new taxon is characterized by a sub-rounded whorl section and a fairly narrow umbilicus with an oblique wall, and an aperture whose ventral margin either runs straight across the venter or exhibits a slightly concave sinus. Adult shells, 13–23 mm in diameter, exhibit no signs of dimorphism. Tetragonites pusillus sp. nov. morphologically most resembles T. superstes, which is known from the Santonian to the Campanian of South Africa and Madagascar and from the lower Maastrichtian of northeastern Mexico. It is suggested that the former species gave rise from the latter during the migration from the Gulf of Mexico to the Northwest Pacific region via the tropical eastern Pacific in the early Maastrichtian time. In the Northeastern Pacific region, T. pusillus sp. nov. could have survived at least until early late Maastrichtian time.

A fish fossil found in the Miocene Kuri Formation of Kamikimachi, Shinji Town, Matsue City in Shimane Prefecture, Japan is described as a new species of the genus Vinciguerria of the family Phosichthyidae of the order Stomiiformes. This new species differs from other congeners in having the more posterior position of the anal fin; 25 abdominal vertebrae; 15 photophores of the ventral series between the bases of the pectoral and pelvic fins; eight photophores of the ventral series between pelvic fin base and anal fin origin; 13 photophores of the lateral series between the bases of the pectoral and pelvic fins; and nine photophores of the lateral photophore series between the pelvic fin and anal fin.

The bias in fossil sampling impacts how we understand the macroevolution of life. Past research has explored the bias triggered by socioeconomic elements as well as geological and paleoclimatic factors, but there is a gap in our knowledge on how the modern climate and vegetation may impact fossil sampling. Here, an assessment has been conducted to study the relationship between the modern climate and fossil sampling biases. The Paleobiology Database (PBDB), WorldClim, and Maximum Green Vegetation Fraction (MGVF) datasets have been utilized to analyze the potential impacts of contemporary climatic variables (temperature, precipitation, and wind speed) and vegetation coverages on fossil sampling. Results of Kolmogorov-Smirnov and chi-squared tests indicate that the distributions of fossil sampling localities across modern climatic variables and vegetation coverage are different from the expected ones assuming these variables do not impact the fossil sampling. These results suggest that a portion of the spatial heterogeneity in fossil sampling rates could be explained by modern climate and the vegetation cover, with temperature being the most dominant potential factor. This study also found that highly productive localities are more likely to be distributed in regions with higher temperatures and more precipitation, and that the temperature range in which a higher concentration of fossil sampling has slightly broadened after 2000.

Nipponitrigonia yebisu sp. nov. is a newly described species from the mid-Cretaceous Goshoura Group on Shishi-jima and Goshoura-jima islands, Kyushu, Japan. This species is characterized by its smooth shell surface ornamentation, lacking concentric costae near the umbo and anterior part of the shell surface, in contrast to other known species. The shell inflation shows considerable intraspecific variation. The species occurs from the Mortoniceras sp. Zone of the upper Albian to the Graysonites adkinsi–Desmoceras kossmati Zone of the lowermost Cenomanian and would have survived to the Turonian. Nipponitrigonia yebisu sp. nov. occur from gravelly sandstone and conglomerate in upper shoreface environment, in which the valves are usually disarticulated. We discovered five still-articulated individuals. Among them, three individuals were interpreted in situ, recording the burial orientations (direction and plunge angles of the commissure planes and marginal carina). According to the burial position, their posterior edge area was oriented upward, and the plunge angles after tilt correction were approximately 10–20 degrees to the bedding. This burial position closely resembles that of extant Neotrigonia species, and it is suggested that these individuals were buried in situ in a high-energy depositional environment. Nipponitrigonia tends to lose shell ornamentation stratigraphically, and Nipponitrigonia yebisu sp. nov. is the last representative of the genus as far as known.

A lowermost Campanian nostoceratid ammonoid Eubostrychoceras valdelaxum was discovered, accompanying with Platyceramus japonicus, Sphenoceramus naumanni, and Damesites cf. damesi from the Kunitan Formation of the Kuji Group in the Iwate Prefecture, northeastern Japan. This is the first report of the occurrence of this species outside of the Yezo Group, Hokkaido, northern Japan. Eubostrychoceras valdelaxum occurs in a wide range of sedimentary facies in the multiple areas of the same horizon and might be beneficial for stratigraphic correlation of the lowermost Campanian (Platyceramus japonicus Zone) in the northwestern Pacific region. Besides E. valdelaxum, the ammonoids of the Kunitan Formation seem to have much in common with the species described from the Yezo Group, but more detailed understanding of the ammonoid fauna requires further accumulation of fossil records from the Kunitan Formation.

Of the four main Japanese islands, Late Cretaceous dinosaur fossils have been discovered in Hokkaido, Honshu, and Kyushu. Here, we report the first Late Cretaceous dinosaur from the remaining main island Shikoku, an isolated dorsal vertebra from the upper Campanian Hiketa Formation (Izumi Group) in Sanuki, Kagawa Prefecture. The vertebra is incomplete, comprising a robust centrum with a subrectangular profile that is generally characteristic of dinosaurs. Histological examination of the centrum reveals moderately packed trabecular bone and the complete absence of pneumatic structures, which precludes referral of the specimen to Saurischia or Pterosauria. Instead, we interpret it as a hadrosauriform ornithischian based on the slightly expanded anterior articular surface of the centrum; heart-shaped outline of the intervertebral surface, resulting from its dorsally wide and ventrally narrow shape; ventral keel; and neural canal partially embedded into the centrum. As the ventral positioning of the neural canal is common to Bactrosaurus and higher taxa within Hadrosauriformes, we tentatively refer the specimen to Hadrosauroidea. This discovery provides additional evidence that hadrosauriforms had dispersed into present-day Japan by the Campanian and highlights the potential of the Hiketa Formation to provide additional evidence to improve our understanding of the dinosaur diversity at the extreme eastern continental margin of Eurasia.

A total of 35 ostracode species belonging to 16 genera are reported for the first time from the Middle Miocene Kaigarabashi Formation in Imagane Town, Hokkaido, northern Japan. Temperate and cryophilic ostracode species are found in the Kaigarabashi Formation. Temperate species comprise Cornucoquimba moniwensis, Cythere omotenipponica, Hermanites posterocostatus, Paracytheridea neolongicaudata, and Schizocythere kishinouyei, whereas cryophilic species and genera are composed of Schizocythere okhotskensis and Hemicythere. The co-occurrence of temperate and cryophilic species and genera suggests that the marine environment was strongly influenced by both warm and cold-water masses. The species diversity and equitability of the fossil ostracode assemblages indicate that the depositional topography was a bay mouth area facing the open sea. Two new species are described: Callistocythere imaganensis Mukai and Tanaka sp. nov. and, Aurila hokkaidoensis Mukai and Tanaka sp. nov.