Shuji Niko

Actinocerid Cephalopods from the Ordovician of Myanmar, and their Paleobiogeographic Implications for Northern Gondwana

Abstract. This paper describes a new actinocerid cephalopod fauna from the western part of the Shan Plateau in Myanmar (Sibumasu Block), and discusses its significance. The cephalopod fossils are preserved in shallow marine limestones of the Wunbye Formation (Pindaya Group) and its equivalent strata. The fauna consists of Ordosoceras theini sp. nov. (Floian or Dapingian, late Early or early Middle Ordovician age), Armenoceras myanmarense sp. nov. (Darriwilian, late Middle Ordovician), Paratunkuskoceras sp. (Darriwilian), and Wutinoceras moeseini (Floian, late Early Ordovician). The presence of Ordosoceras, which was previously known only in the North China Block, and specific characters in the actinocerid fauna of Sibumasu indicate a strong linkage with that of North China during Early—Middle Ordovician time; in contrast, Sibumasus affinity to the coeval Australian fauna is less definable. It is possible that an actinocerid faunal exchange took place between North China and Sibumasu over the shelf sea environment open to the Prototethys Ocean but this did not happen to the inland seas of Australian Gondwana during the period.

The first diplobelid coleoid from the Late Cretaceous (Turonian) of Hokkaido (Japan)

Abstract. Outcrops of the Turonian Saku Formation in the Obira region of Hokkaido (northern Japan) have yielded two breviconic phragmocones that represent a previously unknown species of Conoteuthis, a comparatively well known genus of diplobelid coleoids. The selected holotype of Conoteuthis hayakawai sp. nov. preserves a spherical protoconch that deviates slightly from the longitudinal axis of the phragmocone and is the first record of a protoconch in Conoteuthis. Conoteuthis hayakawai sp. nov. differs only slightly from the type species Conoteuthis dupiniana from the Hauterivian-Aptian of Europe and Conoteuthis azizi Fuchs et al. from the Cenomanian of southeastern India through its weakly developed dorsal saddles. Conoteuthis hayakawai sp. nov. is the youngest report as well as the first record of the Diplobelida from the Paleopacific. This new find considerably extends the stratigraphical and geographical occurrence of the Diplobelida. Phylogenetically, the genus Conoteuthis is considered to be an isolated group within the Diplobelida. With respect to the controversial origin of the Sepiida and Spirulida, morphological comparisons have shown that the genus Conoteuthis cannot be a potential stem-group of the Sepiida and/or Spirulida.

Trigonoceratid nautilids from the Early Carboniferous Imo Formation of Arkansas, Midcontinent North America

ABSTRACT Four species of trigonoceratid nautilids from the late Chesterian (Early Carboniferous) strata in the Imo Formation of Arkansas, Midcontinent North America are described. A specimen of Aphelaeceras arkansanum Gordon, 1964 reveals the external juvenile whorl morphology. Based on the discontinuous points of its surface lirae, we suggest the possibility that the hatching shell of A. arkansanum is cyrtoconic with two septa and a length of approximately 4.9 mm. Species previously described or assigned to Aphelaeceras are reexamined. Epistroboceras lesliense sp. nov. differs from E. caneyense Niko and Mapes, 2004 and E. pitkinense Niko and Mapes, 2005 in its lower ratios of umbilical zone height per whorl height and lower siphuncular position ratios. Epistroboceras? sp. indet. is also identified in this fauna. A new genus, Imonautilus, is erected on the basis of I. meeki sp. nov. The most characteristic features of this genus are the Stroboceras-like juvenile whorl, the obsolescence of the adoral ventrolateral grooves at or near maturity, and the roundly inflated adoral venter. Based on early ontogenetic similarities, a close evolutionary relationship between Aphelaeceras and Imonautilus is suggested.

YOUNGEST RECORD OF TENTACULITOIDS: HIDAGAIENITES NEW GENUS FROM NEAR THE CARBONIFEROUS–PERMIAN BOUNDARY IN CENTRAL JAPAN

Abstract A new genus and species of tentaculitid, Hidagaienites arcuatus, is described from calcareous beds near the Carboniferous–Permian boundary in the Hida-Gaien Terrane, Central Japan. Its mode of occurrence excludes the possibility that the specimens are derived from older strata. Thus, this tentaculitid represents the youngest record of the class Tentaculitoidea, which was previously thought to have become extinct in the Late Devonian.

Redescription and New Information on the Carboniferous Cephalopod Brachycycloceras normale Miller, Dunbar and Condra, 1933

Abstract. The concept of the Carboniferous nautiloid Brachycycloceras normale Miller, Dunbar and Condra, 1933, is emended on the basis of new material from the Desmoinesian to Missourian sequences in Oklahoma. Because the species represents the type of Brachycycloceras and the family Brachycycloceratidae is monogeneric, this emendation requires modification of both the generic and familial diagnoses. This cephalopod genus has a longiconic conch with an annulated immature shell with a slight endogastric curvature, a rapidly inflated, straight, mature conch without annulations, and a siphuncle in a supracentral position. Contrary to earlier interpretations, new evidence from yet undescribed specimens shows that the living orientation of the convex side of the conch was dorsal. These discoveries suggest that assignment of the Brachycycloceratidae to the order Orthocerida is questionable.

Isorthoceras wahlenbergi, a new Late Ordovician cephalopod from the Boda Limestone of Dalarna, Sweden

A new species of Isorthoceras (Proteoceratidae: Cephalopoda), I. wahlenbergi, is described from the Upper Ordovician Boda Limestone in Dalarna, central Sweden. The Boda Limestone consists of up to 20 isolated carbonate mounds ranging in size from a few meters to over 400 m in diameter and attaining 70 m in thickness (Jaanusson, 1982; Cocks, 2005). Each mound is divided into a micritic core facies, a pelmatozoan flank facies, and a bioclastic terminal facies (Suzuki and Bergstrom, 1999). The diverse faunas of the Boda Limestone include well documented taxonomic groups such as trilobites (e.g., Warburg, 1925; Suzuki, 2002), brachiopods (e.g., Wright, 1974, 1993; Cocks, 2005) and gastropods (e.g., Koken and Perner, 1925; Ebbestad and Peel, 1997). These faunas are suggestive of a middle Ashgillian age for these limestones. In contrast to these, taxonomic studies of cephalopods from the Boda Limestone are limited to Frye (1982, 1987), who described 18 species belonging to the orders Ascocerida, Oncocerida, Tarphycerida and Barrandeocerida.

A NEWLY HATCHED COILED NAUTILOID FROM THE PERMIAN OF ITALY

Well-preserved, newly hatched coiled nautiloids from the late Paleozoic are relatively unusual. Thus, the recovery of a hatchling specimen from the Sosio Limestone (Permian, probably Wordian) of Italy that can be assigned to Sholakoceras cf. S. pleuronautiloides (Gemmellaro, 1890) is notable. The embryonic and posthatchling shell of S. cf. S. pleuronautiloides shows some morphological similarities to Recent Nautilus Linnaeus, 1758; however, some dissimilarities are significant, and these features have implications on the overall phylogenetic pattern exhibited by these organisms through time and the evolutionary processes that led to that pattern. The specimen comes from the extremely fossiliferous Pietra di Salomone megablock southwest of Palazzo Adriano, Sosio Valley (Monti Sicani, western Sicily). This and a couple of similar occurrences (blocks) are informally known as the “Sosio Limestone.” The geographic situation and geological setting were given in detail by Flugel et al. (1991), who also discussed the facies and age of the Pietra di Salomone megablock. These authors recognized carbonate breccias and calcarenites deposited by debris flows and turbidity currents in a base-of-slope position. The calcareous material was originally deposited on carbonate platform margins prior to gravity-induced transport. According to Flugel et al. (1991), most of the Pietra di Salomone megablock has a Wordian (Murghabian) age. However, for some of the pebbles derived from reef facies, a Capitanian (Midian) or even Wuchiapingian (Dzhulfian) age is also possible. Jenny-Deshusses et al. (2000) gave a Capitanian to Wuchiapingian and probably even a Changhsingian age for the Pietra di Salomone megablock. Therefore, the studied nautiloid specimen has a Wordian to latest Permian age. The Sosio Limestone is famous for its well-preserved invertebrate fauna. Overall, more than 300 species of invertebrates have been described from the Pietra di Salomone, and more than 520 species have been reported from all Sosio blocks (Mascle, …

A new Early Carboniferous nautilid from the Caney Formation of Oklahoma, Central North America

The Caney Formation, named and defined by Taff (1901), comprises a relatively thin (approximately 115 m thick in the type section) marine sedimentary unit consisting mostly of shale with limestone concretions. The formation is divided into the Ahloso, Delaware Creek, Sand Branch, and Rhoda Creek Members in ascending order (Elias, 1956; Elias and Branson, 1959, and others). We describe below a new trigonoceratid nautilid species, Epistroboceras caneyense, recovered from limestone concretions in the dark gray shale of the Sand Branch Member at the type section for this member. The material examined was collected at an outcrop in the stream bed of Sandy Branch Creek, approximately 1.6 km downstream from the junction of this stream and Highway 48 south of the town of Wapanucka, Johnston Co., Oklahoma. The locality is identical to loc. 2082 (Girty, 1909) and loc. M-15 (Mapes, 1979). Ammonoids, including Cravenoceras richardsonianum and Eumorphoceras bisulcatum, from this locality indicate a middle Chesterian (late Early Carboniferous) age in Gordon’s (1964) biostratigraphic scheme.

Kobayashiceras gifuense, a new actinocerid cephalopod from the Lower Devonian of Japan

A new actinocerid genus, Kobayashiceras, is defined on the basis of Kobayashiceras gifuense new species from the Lochkovian (Early Devonian) of the Fukuji Formation, central Japan. This genus shares with known actinocerids in the apical shell morphology, but is diagnosed by its orthocerid like stenosiphonate form in the adoral shell. Its taxonomic and paleobiogeographic implications are discussed.

Plicatoceras , a new lamellorthoceratid cephalopod genus from the Gedinnian (Early Devonian) of central Japan

A new lamellorthoceratid genus, Plicatoceras , is defined on the basis of Plicatoceras nishidai n. sp. from the early Gedinnian shale of the Fukuji Formation in central Japan. The new genus is the earliest known member of the Lamellorthoceratidae and is the first record of a lamellorthoceratid cephalopod from East Asia.