Paleontological Research

This paper focuses on the molluscan faunal changes in the nonmarine deposits of the Lower Cretaceous (Barremian) Itsuki Formation of the Tetori Group, central Japan. In the lower part of the Itsuki Formation, the transition from brackish to freshwater environments is recognized by lithofacies, sedimentary structures, and fossil indices of molluscs and plant remains. The molluscan assemblages include eleven gastropod species and six bivalve species, and one new gastropod species, Probaicalia okurai sp. nov., is described. The brackish habitats in the delta-front were dominated by Myrene tetoriensis, which usually formed shell beds. The newly founded freshwater habitats on the fluvial environments were populated by Campeloma onogoense and Sphaerium coreanicum as pioneer immigrants. With the development of the fluvial floodplain, a variety of aquatic and semiaquatic microenvironments formed, thereby providing various habitats suitable for small gastropods.

In the history of baleen whales, the early Miocene is one of not a well-known time. Many baleen whales from the Miocene are known as “cetotheres”. The “cetothere” Isanacetus laticephalus was a baleen whale from the early to middle Miocene that is only known from Japan. The holotype and paratype of I. laticephalus are well-preserved, and the age of the species I. laticephalus makes it one of the earlier basal plicogulans; however, its phylogenetic position is unclear. Here, a new cranium including the periotic from an early Miocene deposit (about 18.5 to 17.0 Ma) of the Oi Formation, Ichishi Group in Mie, Japan is reported as cf. I. laticephalus. The specimen displays a combination of four periotic features: (i) a large lateral tuberosity; (ii) a rectangular pars cochlearis; (iii) a robust neck of the posterior process, and (iv) a cerebral aperture of the facial canal that is smaller than that of the dorsal vestibular area. This combination of features could be diagnostic for I. laticephalus, among basal plicogulans from the early and middle Miocene. Adding more referred specimens will expand the diagnoses of I. laticephalus, and firmly support phylogenetic trees with this species.

Moderately-preserved Late Triassic (late early to early middle Norian) and Late Triassic or Early Jurassic radiolarians have been identified from two sections of limestone and dolomitic limestone, respectively in the Tha Sao area, Kanchanaburi Province in western Thailand. Previously these limestones have been assigned as the Permian because of the similarity of lithology without any fossil evidence. Section 1 is a succession of about 18 m in thickness, consisting of calcareous mudstone, micritic limestone, and dolomitic limestone, in ascending order. Section 2 is about 30 m in thickness and consisting of dolomitic limestone. Middle micritic limestone of about 10 m thick of Section 1 yielded diversified radiolarians. Dolomitic limestone of Section 2 yielded rare radiolarians from two levels. We identified radiolarians from five levels of micritic limestone and two levels of dolomitic limestone and totally identified 53 species belonging to 39 genera including two species of which generic position is uncertain and one new species Colum tekini Sashida and Ito sp. nov. At the Late Triassic time, the Tethys Ocean between Sibumasu and Indochina blocks almost closed due to the collision of these two continental blocks. Upper Triassic radiolarian-bearing limestones have been deposited in the eastern to western Tethyan areas within 35° of both hemispheres. The present radiolarian fauna is representative of low latitude Tethyan fauna. The faunal similarity between the eastern and western Tethys indicates the uniformity of the establishment of the low latitude fauna by the Late Triassic.

The rare clam shrimp (spinicaudatan) genus Carapacestheria Shen, 1994 was originally described from the upper Lower–lower Middle Jurassic of Antarctica. Later, it was reported from the Middle Jurassic of Argentina and the lowest Cretaceous of the United Kingdom. Here, a new species, Carapacestheria cangshanensis sp. nov., is described from the Upper Jurassic Penglaizhen Formation in southwestern China. The subquadrate-shaped carapace of the new species has a distinct ornamentation pattern, which transitions from medium-sized reticulations to radial lirae. There are punctae within the polygonal cells and between the lirae. According to its geological range and geographical distribution, Carapacestheria first occurred in Antarctica, and later appeared in the northern hemisphere during the Late Jurassic.

We describe three well-preserved, articulated brittle stars from the Pliocene Hatsuzaki Formation, Hitachi Group of Ibaraki, central Japan, as a new species, Stegophiura takaisoensis. It differs from its congeners in having dorsal arm plates with a high ridge and large swollen disk scales. The only other previously described extinct species unambiguously assigned to Stegophiura is S. miyazakii from the Upper Cretaceous of Japan. The new species from the Pliocene significantly adds to the fossil record of the genus, representing the second extinct species known to date and filling part of the fossil record gap between the Upper Cretaceous and the Recent representatives. The specimens have the disk and arms in intact live position, without any signs of transportation before buried. We assume that they were buried alive in their original deep shelf to upper slope habitat by rapid channel fills.

Within the genera of fossil Echinodermata three junior homonyms are found and the following replacement names are proposed: Edrioblastocystis nom. nov. pro Blastocystis Jaekel, 1918 nec Aléxéieff, 1911 and consequently Edrioblastocystidae nom. nov. to replace Blastocystidae Jaekel, 1918 ; Euzonohymenosoma nom. nov. = Hymenosoma Lehmann, 1957 nec Desmarest, 1823 ; Pennsylvanicycloscapus nom. nov. = Cycloscapus Moore and Jeffords, 1968 nec Erdös and Novicky in Erdös, 1951 . Accordingly, also three new combinations (comb. nov.) are established.

A new gobiiform fish Odontobutis hayashitokuei sp. nov. is decribed from middle Miocene freshwater deposits of the Chojabaru Formation in Iki Island, Nagasaki, Japan based on a single specimen. This new species differs from other species of the genus Odontobutis in having 21 caudal vertebrae and a smaller head. Odontobutis hayashitokuei sp. nov. is most similar to O. obscura in having almost the same position of dorsal and anal fins. This is the first fossil species of the genus Odontobutis and suggests that the origin of the genus extends to the middle Miocene, about 15 Ma.

Ontogenetic development of ornamentation and whorl geometry of the Cretaceous ammonoids Parajaubertella kawakitana and P. zizoh are studied based on well-preserved specimens collected from the lower Cenomanian in the Horokanai area, Hokkaido, Japan. Our results indicate that their comparably sized immature stages share identical ornamentation and shell morphology, while the size of their adult shells is distinctly bimodal. They also share the same stratigraphic ranges in the lower Cenomanian and have overlapping geographic distributions in Northwest Pacific region, and lastly, they co-occur in the same concretions. This evidence strongly suggests that the two taxa should be considered as dimorphs, microconch and macroconch of a single species, which is herein described as P. kawakitana.

The Carboniferous Ichinotani Formation in the Hida Marginal Terrane has lithologic and paleobiogeographic features of a continental margin affinity significantly different from those of the coeval seamountoriginated limestones widely distributed in the Japanese Islands. The fusuline fauna of the former is also different from that of the latter as exemplified by the restricted occurrence of late Moscovian Neostaffella and Hidaella in the former. The occurrence of these two genera is confined to the Podolskian Beedeina lanceolata Zone and are completely absent in the underlying and overlying biozones of the middle part of the Ichinotani Formation. Fusuline species mainly described in this paper are Beedeina lanceolata, Neostaffella umbilicata, Hidaella kameii, and Ozawainella vozhgalica prolific in and almost restricted to the B. lanceolata Zone, and Fusiella hayashii also characteristic and dominant in this zone.