Shinji Isaji

Sauropod dinosaurs from the Lower Cretaceous of eastern Asia: taxonomic and biogeographical implications

Sauropod dinosaurs are poorly represented in the Lower Cretaceous of eastern Asia. Here, we describe a number of isolated sauropod teeth from the Kuwajima Formation (?Berriasian–?Hauterivian) of Shiramine, Japan. The mosaic of shared derived characters and symplesiomorphies displayed by the teeth indicate that they are referable to a basal member of the titanosauriform radiation. A taxonomic review of previously described sauropod specimens from eastern and south–eastern Asia reveals that a diversity of sauropods (including a titanosaurian, a basal titanosauriform and a ?euhelopodid, as well as several forms of indeterminate systematic position) was present in this region in the Early Cretaceous. This diversity conflicts with previous suggestions that eastern Asia was biogeographically isolated from the rest of Laurasia until the late Early Cretaceous and that the sauropod fauna was limited to the endemic East Asian clade Euhelopodidae. The presence of titanosauriform sauropods in the basal Cretaceous of Japan and Thailand indicate that the proposed faunal isolation of eastern Asia ended approximately 20 myr earlier than usually suggested.

An Early Cretaceous mammal from the Kuwajima Formation (Tetori Group), Japan, and a reassessment of triconodont phylogeny

ABSTRACT We describe here one of the oldest known Japanese mammals, a triconodont lower jaw from the Early Cretaceous Kuwajima Formation, Tetori Group, Japan. This mammal is recognized as the type of Hakusanodon archaeus, new genus, new species, and as closely related to Eurasian and North American Jurassic “amphilestid” triconodonts (Simpson 1925b–c; Jenkins and Crompton 1979; Sigogneau-Russell 2003a), a group of poorly understood early mammals. To better understand the relationships of Hakusanodon, we present an inclusive cladistic analysis of most triconodont mammals and relatives. Our analysis highlights the central role of triconodonts for early mammalian history and corroborates recent findings indicating that traditional groups of Mesozoic mammals such as Triconodonta (Jenkins and Crompton 1979), Amphilestidae (sensu Jenkins and Crompton 1979; McKenna and Bell 1997) and Symmetrodonta (sensu Lillegraven et al. 1979; McKenna and Bell 1997) are artificial. A dental pattern similar to that of “amphilestids” is generalized, playing a central role in mammalian dental evolution and being primitive for triconodontids and spalacotheriids, and therefore for the archaic members of the therian lineage. Australosphenidans (Luo et al. 2001a, 2003; Martin and Rauhut 2005), with fully reversed triangles in their lower dentitions (uppers are not known in any of them with the exception of monotremes), also are likely to have evolved from ancestors with an “amphilestid”/sym-metrodont-like dental morphology. The enigmatic Amphidon aequicrurius Simpson, 1925a, from the Jurassic of North America (Simpson 1925a) has been treated as related to acute angle symmetrodonts or to triconodonts. Re-study of the type and only specimen suggests that Amphidon Simpson, 1925a, is likely an old, worn-down “amphilestid” (Rougier et al. 2001), a conclusion also supported by the results of the cladistic analysis. Jeholodens jenkinsi Ji et al., 1999, postulated as a basal member of the triconodontid lineage, or eutriconodont (Ji et al. 1999), is more likely a generalized “amphilestid,” which makes this taxon more relevant for the understanding of the stem members of the therian lineage by providing a relatively complete out-group.

Palaeontology: A refugium for relicts?

Luo suggests that the vertebrate fauna from the Yixian Formation (Liaoning Province, China) shows that this region of eastern Asia was a refugium, in which several typically Late Jurassic lineages (compsognathid theropod dinosaurs, ‘rhamphorhynchoid’ pterosaurs, primitive mammals) survived into the Early Cretaceous (Fig. 1). Data from slightly older sediments in the Japanese Early Cretaceous, however, suggest that the faunal composition of this region can only be partly explained by the concept of a refugium.

Kappachelys okurai gen. et sp. nov., a New Stem Soft-Shelled Turtle from the Early Cretaceous of Japan

Kappachelys okurai gen. et sp. nov. is named and described based on two isolated carapacial elements (right seventh costal and left seventh peripheral) from the Lower Cretaceous (?upper Neocomian) Akaiwa Formation of west-central Honshu, Japan. Kappachelys is a small turtle (shell length ~10 cm) that exhibits a unique combination of three features: coarse and deep vermiculate sculpture on carpace; no scute sulci; and well-developed peripherals. The form of the sculpture and lack of scute sulci both suggest affinities with the Trionychidae (soft-shelled turtles), whereas the plesiomorphic retention of well-developed peripherals indicates Kappachelys lies outside the Trionychidae. Given this combination of primitive and derived features, we interpret Kappechelys as a stem trionychid. In the same region of Japan, the overlying Lower Cretaceous (Aptian) Kitadani Formation contains some of the oldest known, unequivocal trionychid fossils. Based on its slightly older age, similar geographical distribution, and more primitive shell morphology, Kappachelys could be ancestral to the trionychids of the Kitadani Formation.

Chelonian eggshells from the Lower Cretaceous Kuwajima Formation of the Tetori Group, Central Japan

ABSTRACT Chelonian eggshell fossils from the Lower Cretaceous Kuwajima Formation of the Tetori Group are described. The eggs were originally spherical or ellipsoidal in shape. The eggshells consist of a single layer of spherulitic shell units composed of needle-like crystallites originating from a nucleation center. This corresponds to modern rigid-shelled chelonian eggs. The eggshell fragments are commonly found buried together and are known from deposits of subaerial environments, such as vegetated swamps. In contrast, the vast majority of turtle remains are known primarily from nearby shallow lake deposits, indicating that they are aquatic; no terrestrial turtles are known from this formation. These taphonomic settings suggest that the eggs were laid on land by lacustrine turtles in a process still apparent today.

Terrestrial and Freshwater Pulmonate Gastropods from the Early Cretaceous Kuwajima Formation, Tetori Group, Japan

Abstract. Five species of pulmonate gastropods are described from the fluvial deposits of the Early Cretaceous Kuwajima Formation (Tetori Group) of Shiramine district, Hakusan City, Ishikawa Prefecture, central Japan. Gyraulus sp., Aplexa kasekikabe sp. nov. and Zaptychius kuwajimaensis sp. nov. appear to be inhabitants of the marginal zone in shallow lakes. Pseudarinia wangyingensis and Tetoripupa costata gen. et sp. nov. are land snails that probably inhabited vegetated swamps located near shallow lakes. T. costata is the earliest record of the stylommatophoran pulmonate clade Pupilloidea. This fauna contains a common and closely related species to the Jehol molluscan fauna from the Early Cretaceous of northern China, and also appears to be utilizable for paleoenvironmental analysis of terrestrial to freshwater microhabitats.

Molluscan larvae from the Carboniferous Ichinotani Formation, Fukuji, Gifu Prefecture, central Japan

Abstract. Molluscan larval shells are described from the upper Carboniferous (Moscovian) Ichinotani Formation at Fukuji, Okuhida-onsen-gou, Takayama City, Gifu Prefecture, Japan. The vast majority of these larval shells are isolated and not metamorphosed. The larvae appear to represent species with planktotrophic larval development representing pseudozygopleurid, streptacidid and indeterminate gastropods and pteriomorph and anomalodesmacean bivalves. This is the first report of isolated molluscan larval shells from Palaeozoic deposits in Japan.

Palaeontology. A refugium for relics

Luo suggests that the vertebrate fauna from the Yixian Formation (Liaoning Province, China) shows that this region of eastern Asia was a refugium, in which several typically Late Jurassic lineages (compsognathid theropod dinosaurs, ‘rhamphorhynchoid’ pterosaurs, primitive mammals) survived into the Early Cretaceous (Fig. 1). Data from slightly older sediments in the Japanese Early Cretaceous, however, suggest that the faunal composition of this region can only be partly explained by the concept of a refugium.

Adulomya from the Miocene Aokiyama Formation, Hota Group, in the Boso Peninsula, Japan

Vesicomyid bivalves are representative of the chemosynthetic deep-sea fauna, and their successive fossil records have been reported from the Cenozoic deposits of Japan (Amano and Kanno, 2005; Majima et al., 2005). Amano and Kiel (2007) have proposed that vesicomyid bivalves in the North Pacific region are assigned to four genera: Calyptogena, Archivesica, Adulomya and Hubertschenckia. Among these taxa, the fossil record for Adulomya in Japan has been investigated in detail, resulting in five well established species and several uncertain records (Amano and Kiel, 2011). The specimen discussed in this report is one of these uncertain records. The specimen was originally reported as Calyptogena (Ectenagena) sp. from the Aokiyama Formation, Hota Group, in the Boso Peninsula (Kuramochi et al., 1999). However, these authors determined only the external features of the left valve of the articulated specimen, and its internal features have remained unknown. Therefore, as Amano and Kiel (2011) indicated, the assignment of the specimen to Adulomya? sp. is based only on the shell size and outline. I reexamined the specimen described by Kuramochi et al. (1999), which is in the collections of the Natural History Museum and Institute, Chiba (CBM-PS 2670-1). The specimen was revealed to preserve the right valve in good condition and to be attached with another conjoined individual (CBM-PS 2670-2). I also observed its internal features by nondestructive analysis using an X-ray microfocus CT system (TESCO TXS320-ACTIS) at the National Museum of Nature and Science, Tokyo, under experimental conditions of 315 kV and 200 μA. The resolution of the square slice CT was 81 mm per 512 pixels, and the spacing between each CT slice was 0.1 mm. Analysis and surface rendering were performed using the software OsiriX version 3.9.2 32-bit (an open-source DICOM viewer for Macintosh).

A refugium for relicts?: Palaeontology

Luo suggests that the vertebrate fauna from the Yixian Formation (Liaoning Province, China) shows that this region of eastern Asia was a refugium, in which several typically Late Jurassic lineages (compsognathid theropod dinosaurs, ‘rhamphorhynchoid’ pterosaurs, primitive mammals) survived into the Early Cretaceous (Fig. 1). Data from slightly older sediments in the Japanese Early Cretaceous, however, suggest that the faunal composition of this region can only be partly explained by the concept of a refugium.