Paleontological Research

A new sinobelemnitid belemnite genus, Tohokubelus, is described based on T. takaizumii sp. nov. from the late Olenekian (late Early Triassic) mudstone of the Osawa Formation belonging to the South Kitakami Belt in the Utatsu area, Miyagi Prefecture, Northeast Japan. The present discovery is important because this species is the oldest belemnite and extends downwardly the range of the Belemnitida from the Carnian (early Late Triassic). We concluded that the order appeared first in the earliest Triassic in the low-latitude area of the westernmost part of the Panthalassa, then belemnites spread to habitats in the Paleo- and Neo-Tethys in the Early Jurassic by the equatorial currents from the Panthalassa to the Tethys.

We designate the lectotype of Ptilophyllum pachyrachis Oishi ( 1940 ), collected from the Upper Jurassic (Tithonian) to Lower Cretaceous (Berriasian) Ashidani Formation of the Tetori Group exposed in Mochiana, Ohno City, Fukui Prefecture, Central Japan. We further propose a reclassification of this species to the genus Pterophyllum. We found that Oishi's ( 1940 ) syntypes included one Ptilophyllum specimen that reflected Eurosinian-type vegetation, contrary to the widely accepted idea that typical Siberian-type vegetation flourished in the Tetori Group region during the Tithonian to Berriasian.

Disarticulated fossil lateral arm plates of brittle stars from the Middle Pleistocene Miyata Formation, Miura, Kanagawa Prefecture, eastern Japan, are described. They are assigned to Stegophiura sladeni on the basis of their microstructural morphology. This is the first description based on disarticulated fossil lateral arm plates of brittle stars from Japan and should encourage further exploration of the Japanese ophiuroid microfossil record.

The Osawa Formation of the South Kitakami Belt is correlated with the upper Olenekian (Lower Triassic) based on its rich ammonoid fauna. Although two ammonoid zones, the Subcolumbites Zone below and Arnautoceltites Zone above, were established in the formation, its uppermost part has been almost barren of ammonoids. An “Olenekian–Anisian mixed” ammonoid fauna found from the uppermost Osawa Formation at Asadanuki, comprising eight genera, is the only one representing the uppermost part. It is unique in the generic composition and new ammonoid zone, the Eodanubites Zone, is proposed in the uppermost part of the Osawa Formation. The fauna consists of typical Olenekian genera such as Pseudosageceras, Ceccaisculitoides, Procarnites and Eodanubites, the Olenekian–Anisian genus Leiophyllites and Keyserlingitidae, and “Anisian” genera Japonites and Procladiscites. The last two genera, however, actually range down into the uppermost Olenekian, and therefore, based on the ammonoids, the Eodanubites Zone is considered to be latest Olenekian in age. This fauna is similar to the uppermost Olenekian Ziyun fauna in Guizhou, South China, and the Eodanubites Zone is correlated with the uppermost Olenekian Deslicairites Beds of Romania and the Neopopanoceras haugi Zone of Nevada.

A new whole-plant bennettite—Kimuriella densifolia—is described and restored based on excellently preserved fossils from the Oxfordian (Upper Jurassic) Tochikubo Formation of Northeast Japan. Recent excavations revealed plant fossils with axes, sterile foliage and ovuliferous Williamsonia type flowers in organic connection. The sterile foliage of the plant is identical to what is described as Zamites nipponicus when found isolated. Microsporangiate flowers of the Weltrichia type were preserved together with the articulated macrofossils and conspecificity is strongly warranted. The absence of an appropriate plant genus to accommodate the restored plant based on the articulated fossils necessitated the erection of the new genus Kimuriella for whole-plant bennettites with a divaricate growth habit and Zamites type foliage. Kimuriella densifolia is compared with a number of coeval bennettites, amongst which the very similar Williamsonia gigas and Williamsonia pecten from the Bajocian of Yorkshire, UK. Nomenclatural issues of the latter two bennettite species are discussed in detail when research on the different organs assigned to them raised some questions connected with the classical circumscription of the taxa complying with the concept of form-classification of fossil plant organs. Finally, some interpretations of the coastal/river delta environment and habitat, in which Kimuriella densifolia thrived, are discussed.

Detailed biostratigraphic study of a lower Neogene section with abundant, moderate to well preserved radiolarians from central Indian Ocean Drilling Program (ODP) Site 714 has resulted in the precise correlation of many radiolarian bioevents with calcareous nannofossil and planktonic foraminiferal biostratigraphy. The interval studied at Site 714 is divided into five radiolarian zones, RN6 to RN2. In this study, Zones RN5 (Dorcadospyris alata Interval Zone), RN4 (Calocycletta costata Interval Zone) and RN3 (Stichocorys wolffii Interval Zone) are divided into subzones based on the last occurrences of Calocycletta costata and Carpocanopsis cingulata, and the first occurrence of Liriospyris stauropora, respectively. Designation of these zones/subzones improved the biostratigraphic resolution (average of one subzone/zone per 1.2 million years). A new species, Pterocanium maldives, is described.

Comparison of results from Site 714 with those of previous studies from the tropical Pacific Ocean enables identification of synchroneity and diachroneity of radiolarian datum events between the tropical Indian and Pacific Oceans. Of 64 radiolarian bioevents in the tropical Indian Ocean from the early to late Miocene, 22 were apparently synchronous or nearly synchronous. These radiolarian events provide useful biohorizons for stratigraphic correlation and age determination of lower Neogene sequences from the tropical Indian to Pacific Oceans. Patterns of synchroneity and diachroneity of first occurrences were different near the middle/late Miocene boundary. Below this transition, the first occurrences were synchronous from the tropical Indian and Pacific Oceans. Above this transition, they were earlier in the tropical Indian Ocean than in the tropical Pacific Ocean. Changes in these patterns might to have been influenced by the effective closure of the Indo–Pacific seaway. In contrast, the large majority of last occurrences showed synchroneity from the tropical Indian and Pacific Oceans during the early to late Miocene. This suggests that last occurrences during the early to late Miocene were affected by global climatic or oceanographic events.

The specimen collected from the Cretaceous Yezo Group of Mikasa area, central Hokkaido, Japan by T. Matsumoto and T. Omori in 1955, deposited in the University Museum, the University of Tokyo is determined as a second specimen of Apsopelix miyazakii, which is the crossognathid fish described from Teshionakagawa area, northern part of Hokkaido in 2012. Some characters unconfirmed in the holotype, including the dorsoventrally long opercle and subopercle; the well-preserved preopercle having many radial openings of the sensory canal; the sensory canal of the second infraorbital bone having many branches; the presence of the forth infraorbital bone; the large supracleithrum; and the scale-like postcleithrum are preserved in the second specimen. An estimated standard length of the second specimen is more than about 10 cm larger than that of the holotype. The emended diagnosis is proposed for A. miyazakii based on the holotype and the second specimen. The discovery of the second specimen extends the range of A. miyazakii throughout the Upper Turonian. Also, Apsopelix miyazakii possibly lived in a shelf environment near the open ocean.

The anterolateral scar is a raised and generally rounded ossification on the anterolateral face of the femoral head of certain archosaurs, located anterior to the ‘greater trochanter’ and posterior to the femoral head. The presence and shape of this structure in early dinosaurs is still poorly understood. In the present study, the shape and variation of the anterolateral scar is preserved in two early sauropodomorph dinosaurs Pampadromaeus barberenai and Buriolestes schultzi from the Upper Triassic of Brazil. These sauropodomorphs are particularly significant because of their phylogenetic position as basal members of the clade and because they were coeval with the oldest of dinosaurs. Upon analysis, the specimens revealed distinct surface morphologies related to the anterolateral scar throughout their femoral series. Small specimens (putative juvenile individuals) of each species lack any sign of an anterolateral scar. On the other hand, the structure is well-developed in larger specimens (putatively adult individuals). Thus, an absence/presence of the anterolateral scar on the femora of early dinosaurs is affected by ontogeny, as observed in silesaurids. The presence of the anterolateral scar seems more widely distributed in ornithodirans than previously thought. Further, the absence of a raised anterolateral scar in massopodan sauropodomorphs appears to be a neotenic trait, since putatively immature early sauropodomorph individuals share this absence, while full adults develop a remarkable scar.