Paleontological Research

A star-shaped trace fossil here assigned to Asteriacites quinquefolius (Quenstedt) was found in the Miocene Shirahama Formation, Wakayama Prefecture. This is the first report in Japan and stratigraphically youngest record for the ichnospecies. The fossil has five distinct arms and wide striations on both lateral sides of each arm, and the shape is bilaterally symmetrical. To clarify the fossil producing process, we conducted burial experiments of extant asteroids in aquaria and in situ for the first time. Asteroids buried themselves in the substratum using the tube-feet, and when asteroids were covered with thin sand, they escaped slantingly upward onto the sand tilting their body in a bilaterally symmetrical posture. As a result, the remaining trace was very similar to the fossil of A. quinquefolius from the Shirahama Formation. Accordingly, the results suggest the present fossil was formed by the behavior of escaping from thin sand cover by asteroids.

In order to recognize and verify the lower Miocene deposits and the Aquitanian-Burdigalian boundary, benthic foraminifera from different localities in the north of the Central Iran Zone are considered. These facies-controlled foraminifera are useful for biostratigraphical studies, especially in the absence of planktic foraminifera. We investigated eleven stratigraphic sections of the north and northwest Central Iran Zone, and found datum levels of benthic foraminifera such as Borelis melo curdica, Peneroplis farsensis, Elphidium sp. 14, Meandropsina anahensis, Meandropsina iranica, and Austrotrillina howchini. The most significant event is the first occurrence of Borelis melo curdica which appears at the beginning of the Burdigalian. Hence, this datum can be helpful to recognize the Aquitanian-Burdigalian boundary.

A new deep-sea shark of the genus Scymnodalatias (Squaliformes, Somniosidae), S. kazenobon sp. nov., is described from the middle Miocene Yatsuo Group in Central Japan. This is the first fossil record of the genus Scymnodalatias from the Miocene strata and its first occurrence in the Pacific region. This discovery seems to indicate that major distributional changes occurred in the Pacific region sometime during the late Cenozoic era.

Carboniferous and Permian limestones, as well as pre-Cretaceous cherts and basaltic volcaniclastic and volcanic rocks, are tectonically intermingled with lower Jurassic to upper Cretaceous siliciclastic rocks in the Yura area, Southern Chichibu Terrane of the western part of Kii Peninsula, Japan. They are seamount originated and highly fossiliferous. Fifty species assignable to 26 genera of Carboniferous foraminifers were distinguished from the limestone blocks in the Yura area (Kaimori, Tatego, and Shirasaki). Among them, 21 species assignable to 11 genera are systematically described, including two new species (Akiyoshiella kaimoriensis and Semistaffella vachardi). Foraminiferal biostratigraphic constraints on the limestone blocks result that the Kaimori block ranges from the middle Bashkirian to Kashirian and the Tatego blocks from the Visean/Serpukhovian to upper Serpukhovian or partly to Bashkirian, while a part of the Shirasaki block, from which the Carboniferous was first distinguished, is lower and middle Kasimovian. Characteristic species are Akiyoshiella kaimoriensis, Profusulinella prisca, and Bradyina nautiliformis in Kaimori; Eostaffella mosquensis and Bradyina sp. A in Tatego; and Protriticites variabilis, Obsoletes burkemensis, and Montiparus matsumotoi in Shirasaki.

This study describes the right upper postcanine teeth of a single individual of a Pleistocene rhinocerotid (Mammalia, Perissodactyla) from the lower to lower Middle Pleistocene Kamo Formation of the Kokubu Group in Aira City, Kagoshima Prefecture, southwestern Japan. These teeth are heavily worn and are identified as P2–M2 with missing M1. They are identified as an indeterminate genus and species of the Rhinocerotidae, although they were previously identified as Rhinoceros aff. sinensis. These dental fossil specimens and the rhinocerotid footprints from the lower to lower Middle Pleistocene of Japan indicate that rhinocerotids certainly existed in Japan during the early to early Middle Pleistocene.

A possible phylogenetic relationship of two species of Hyphantoceras (Ammonoidea, Nostoceratidae) was proposed, based on newly found specimens with precise stratigraphic occurrences in the Kotanbetsu and Obira areas, northwestern Hokkaido. Two closely related species, Hyphantoceras transitorium and H. orientale, were recognized in the examined specimens from the Kotanbetsu and Obira areas. Specimens of H. transitorium show wide intraspecific variation in the whorl shape. The stratigraphic occurrences of the two species indicate that they occur successively in the Santonian–lowermost Campanian, without stratigraphic overlapping. The similarity of their shell surface ornamentations and the stratigraphic relationships possibly suggest that H. orientale was derived from H. transitorium. The presumed lineage is likely indigenous to the northwestern Pacific realm in the Santonian–earliest Campanian. Hyphantoceras venustum and H. heteromorphum might stand outside a H. transitorium–H. orientale lineage, judging from differences of their shell surface ornamentation.

Two mandibles of fossil mysticetes from the early Pliocene, upper part of the Horokaoshirarika Formation at Numata Town, Hokkaido, Japan, belong to the archaic, extinct cetotheriid baleen whale, Herpetocetinae gen. et sp. indet. by having an elongated angular process projecting posteriorly beyond the mandibular condyle. The new materials of the Herpetocetinae represent their northernmost occurrence in the North Pacific.

The Upper Cretaceous Nakaminato Group, which contains the Chikko, Hiraiso, and Isoai formations in ascending order, crops out along the Pacific coast of Ibaraki Prefecture, central Japan. This group is composed mainly of sandstone, siltstone, and sandstone-siltstone alternations, with intercalated conglomerate layers at several levels. The siltstone of the Hiraiso and Isoai formations has yielded ammonites and inoceramid bivalves indicating a middle Campanian to Maastrichtian age. Some conglomerate layers in the Isoai Formation reach 1 m in thickness and mostly consist of pebbles and cobbles of rhyolite, dacite, chert, siliceous siltstone, siltstone, sandstone, and hornfels. We obtained late Paleozoic to Late Jurassic radiolarians from pebbles of argillaceous rock and chert from four levels of the conglomerate layers within the Isoai Formation. We describe the radiolarians systematically herein. The probable provenance of the radiolarian-bearing pebbles is interpreted as the Ashio and Yamizo terranes, which consist of Late Jurassic to Early Cretaceous accretionary complexes. We propose that there were two denudation stages of the accretionary complexes in the Kanto District, stages α (Barremian–) and β (Campanian–).

We studied the deep-sea benthic foraminiferal fauna from core NGC108 (36°36.85′N, 158°20.90′E; 645 cm long, recovered from 3,390 m water depth) collected from the Shatsky Rise in the central northwestern Pacific Ocean, to determine the relationship between benthic foraminifera and paleoproductivity in the transitional zone between the subtropical Kuroshio Extension and the subarctic current over the last 187 kyr. Cassidulina reniforme, Eilohedra levicula, and Epistominella exigua were the most abundant species. The relative abundance of C. reniforme was positively correlated with biogenic opal and organic carbon contents, which are proxies for paleoproductivity. The relative abundance of E. exigua was negatively correlated with biogenic opal and organic carbon contents, and the relative abundance of E. levicula had no correlation with biogenic opal and organic carbon contents. We conclude that C. reniforme and E. exigua are indicative of high and low productivity, respectively. The high relative abundances of C. reniforme reflect high paleoproductivity during glacial periods [oxygen isotope stages (OISs) 6, 3, and 2], indicating a southward advance of the Subarctic Front during global cooling. The high relative abundances of E. exigua reflect low paleoproductivity during the interglacial and postglacial periods (OISs 5 and 1). Faunal changes in the deep-sea benthic foraminifera on the Shatsky Rise suggest a direct response to orbital-scale paleoproductivity changes.

A fish fossil found from the Miocene Bessho Formation near Amikake, Sakaki Town, Hanishina County in Nagano Prefecture, central Japan is described as a new species of the genus Clupea, C. macrocephala of the family Clupeidae of Clupeiformes. This new species differs from other species of the genus in having a large head about 3.4 times in the standard length versus more than 4 times in other species. Clupea macrocephala sp. nov. is closer to Clupea pallasii than Clupea harengus in the number of vertebrae. Presence of the fossil herrings of the genus Clupea in the deposits of Pacific basin implies that the origin of this genus, probably, was in the Pacific Ocean in the Miocene.