Yasuhisa Nakajima

Tokyo City University · Department of Natural Sciences

of the Early Triassic ichthyopterygian reptile Utatsusau-rus hataii: Implications for early ichthyosaur biology. Acta Palaeontologica Polonica 59 (2): 343–352. Ichthyosaurs were highly adapted to a marine lifestyle, as shown by their fish-like body shape and their assumed active swimming abilities and high metabolic rates. However, the processes of...

Among living turtles, highly terrestrial or highly aquatic modes of life are likely to have developed from a plesiomorphic semi-aquatic one. A taxonomically comprehensive data set of turtle humeri was examined to ascertain if adaptation to an aquatic or a terrestrial lifestyle affects the general internal bone structure. Three-dimensional and virtu...

Herein we report morphologically and histologically diagnostic trionychid specimens from the Barremian–Aptian (129.4–113.0 Ma) of Japan. One specimen (FPDM-V9487) is an associated skeleton that consists of a scapula, a humerus, an ischium, and hypoplastra. The limb and girdle bones are similar in morphology to those of modern trionychids. The hypop...

Ichthyopterygia is a major clade of reptiles that colonized the ocean after the end-Permian mass extinction, with the oldest fossil records found in early Spathian substage (late Olenekian, late Early Triassic) strata in the western USA. Here, we describe reptilian remains found in situ in the early Spathian Neocolumbites insignis ammonoid zone of...

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 incompl...

This paper presents a description of the Olenekian-Anisian sections in South Primorye, Far East Russia, containing reptile remains. The paper describes important ammonoid Flexoptychites cf. rifunus (Yabe et Shimizu), as well as Flexoptychites cf. compressus (Yabe et Shimizu), which are stratigraphically significant. New Anisian reptile remains were...

A paleoparadoxiid specimen from Kamado, Mizunami exhibits numerous bone fragment-like fossils ranging in diameter from a few millimeters to about 3 centimeters. These fossils are associated with various morphological features, such as rod-like arrangements depending on the preservation state, surface features rich in irregularities and nutrient for...

The histology of bone can be preserved virtually unaltered for hundreds of millions of years in fossils from all environments and all vertebrate taxa, giving rise to the flourishing field of paleohistology.1 The shafts of long bones are formed by the apposition of periosteal bone tissue, similar to the growth of wood, and preserve, an often cyclica...

Two additional trionychid costal fragments were recovered from the lowermost strata of Maastrichtian of the Isoai Formation, Nakaminato Group. These fragments belong together with the previously described specimens INM-4-16737 and INM-4-16738. INM-4-16737 and INM-4-16738 are thought to be a right costal (2nd?-5th?) and a left costal (3rd?-5th?), re...

Fifty-one fossil shark teeth including Hexanchiformes, Echinorhiniformes, Squaliformes, and Lamniformes are described from two localities in Nishichirashinai and Omagari formations of the Yezo Group in Nakagawa Town, Hokkaido, Japan. They include the first occurrence of Protosqualus from northwestern Pacific and suggest the onset of the adaptation...

Plotosaurus is a highly aquatically adapted mosasaur, which is supposed to inhabit the deep ocean basin. The geographic occurrence of this genus has been limited only to the west coast of North America. In this study, two Plotosaurus‐type mosasaur caudal vertebrae derived from the Upper Cretaceous Nakaminato Group in Ibaraki Prefecture, Japan, are...

INM-4-15300 was initially identified as a right scapula of the pterosaur in the previous study. The specimen is herein re-identified as the right humerus of a trionychid turtle based upon the internal structure observation using X-ray computed tomography images and comparisons of bone surface morphology with wider taxonomic groups beyond pterosaurs...

Elasmobranch fossils have been found from the upper part of the Late Cretaceous Tamagawa Formation（Turonian- Santonian/Coniasian）of the Kuji Group, Iwate Prefecture, Northeast Japan. Teeth belonging to Cretoxyrhinidae（Cretalamna）, Mitsukurinidae（Scapanorhynchus）, Odontaspididae, and Sclerorhynchidae were identified. All of these elasmobranch fossil...

The first tuna-shaped amniotes evolved among ichthyosaurs, but this group exhibits in fact a wide diversity of morphologies and swimming modes. The histology and microanatomical features of vertebral centra of a diversity of ichthyosaur taxa from most basal to highly derived illustrating this variability were analyzed. The occurrence of unusual par...

Preliminary report on Redox condition of Early Triassic shallow marine around the South Kitakami block

Secondary marine adaptation is a major pattern in amniote evolution, accompanied by specific bone histological adaptations. In the aftermath of the end-Permian extinction, diverse marine reptiles evolved early in the Triassic. Plesiosauria is the most diverse and one of the longest-lived clades of marine reptiles, but its bone histology is least kn...

United States of America Voles of the genera Phenacomys and Arborimus are represented in Pliocene and Pleistocene mammal faunas in North America, but patterns of dental variation remain poorly understood and continue to hamper taxonomic interpretations of fossils. Paleontologists historically worked primarily with the lower first molar, and establi...

The limb bones of terrestrial vertebrates exhibit tubular diaphysis with large internal medullary cavity, whereas those of aquatic vertebrates show extraordinary dense, or alternatively spongy internal structure without large open medullary cavity. As such relationship between bone tissue and ecology is found in multiple living clades of tetrapods,...

Although fossils of Trionychidae are found as far back as the Early Cretaceous, their evolutionary history, including the origin of the crown clade, is poorly understood. Here, we use a microCT scan of the skull of ‘Trionyx’ kyrgyzensis from the late Early Cretaceous (Albian) of Kyrgyzstan as the basis for a thorough redescription of the cranial mo...

Until now, the Hiraiso Formation was considered to be entirely of Smithian (early Olenekian, Early Triassic) age, but the discovery of the ammonoid Tirolites cf. ussuriensis in the lower part of the formation on the rocky coast between the Maekawara and Akaushi Ports, Motoyoshi Town, Kesennuma City, in the South Kitakami Belt, Northeast Japan, esta...

Eight specimens of Sphenodus are collected from the Coniacian, Upper Cretaceous of Nakagawa Town in Hokkaido, northern Japan. They are referred to as S. cf. lundgreni, S. spp. 1 and 2, and described in detail. Nearly complete specimens of Sphenodus were previously unknown from the Cretaceous in the Pacific region, and the Nakagawa specimens are the...

Lower Triassic marine deposits can potentially provide significant information on the diversity and trophic structure of marine ecosystems following the end-Permian mass extinction. Given the sparse body fossil record from the Lower Triassic, trace fossils may provide additional windows on paleo-ecosystems. Herein, we report a newly discovered copr...

The ichthyopterygian Utatsusaurus hataii Shikama et al. 1978 is the only valid reptilian taxon known from the Lower Triassic Osawa Formation in Minamisanriku Town, Miyagi Prefecture, which records the recovery of the marine ecosystem shortly after the end-Permian mass extinction. In this paper, we describe a fragmentary specimen of an indeterminate...

Recent studies have made remarkable progress in knowledge of bone histology of fossil and recent tetrapods. However, despite the three-dimensional nature of internal limb bone structure, two-dimensional comparison of single transverse thin sections as a convenient standard has been performed. Sections were most often sampled from middiaphyseal leve...

Background The paleoecology of desmostylians has been discussed controversially with a general consensus that desmostylians were aquatic or semi-aquatic to some extent. Bone microanatomy can be used as a powerful tool to infer habitat preference of extinct animals. However, bone microanatomical studies of desmostylians are extremely scarce. Method...

Consensual phylogenetic tree illustrating the relationships between the taxa sampled for the study of the femur. Modified from [4], [47]–[51]. (TIF)

Consensual phylogenetic tree illustrating the relationships between the taxa sampled for the study of the rib. Modified from [4], [47]–[51]. (TIF)

Consensual phylogenetic tree illustrating the relationships between the taxa sampled for the study of the humerus. Modified from [4], [47]–[51]. (TIF)

Institutional abbreviations appearing in the inventor numbers of specimens. (DOC)

In order to investigate the relationship between limb bone internal structure and the aquatic adaptation, we examined humeri of multiple carnivoran species of various lifestyles (terrestrial, semiaquatic, and aquatic) using micro-focus CT scanner. The transverse section image passing through the inferred center of ossification is analyzed in order...

Desmostylians are a group of extinct mammals known from the upper Oligocene through the middle Miocene marine strata of the northern Pacific Rim. The paleoecology of extinct desmostylians has been controversial since they were first discovered. The unique osteological and dental morphologies have hindered a consensus on their paleoecology. Bone his...

The internal limb bone structure of tetrapods varies depending on their habitat and manner of locomotion; limb bones of terrestrial taxa tend to show tubular structure with open medullary cavity, which enables both reduction in mass and resistance to stress; whereas, some aquatic taxa have anomalously dense limb bones. Here I conducted microanatomo...

Here the authors report on the fossil turtle collections in Hobetsu Museum. 43 of 159 fossil vertebrate specimens in this museum were identified as testudines, which takes the largest percentage. This report takes 42 specimens among them. All of 42 fossil turtles are from Hokkaido, JAPAN, and 38 of them are from Hobetsu area. 17 specimens of Mesode...

The South Kitakami Terrane (SKT) is a tectonic region located in Northeastern Japan. SKT contains a Paleozoic-Mesozoic continental shallow-marine sedimentary sequence whilst most of other tectonic regions in Japan consist of deep-sea sediments accreted onto the continental shelf. Because of this, understanding the structural development of the SKT...

Bone microstructure of derived ichthyosaurs is characterized by following two specializations: presence of rapid growing bone tissues (woven bone tissue and fibrolamellar complex) rarely seen in extant reptiles; and osteoporotic-like state indicating a light body mass advantageous for fast and maneuverable swimming. In order to look into the evolut...