Kuruma Group

This unit comprises Lower Jurassic continental shelf sediments and is widely exposed in the northeastern part of the circum–Hida terrane of eastern Toyama, western Niigata, and northern Nagano prefectures, central Japan (Fig. 1). This group's distribution is divided into the Inugatake and the Kotakigawa–Akahageyama–Kuruma areas because of the difficulty of their lithostratigraphic correlation (Takeuchi et al., Reference Takeuchi, Tokiwa, Kumazaki, Yokota and Yamamoto2017) (Fig. 1), and the former yields numerous Early Jurassic ammonoid fossils (Sato, Reference Sato1955; Kobayashi et al., Reference Kobayashi, Konishi, Sato, Hayami and Tokuyama1957). Most of the studied materials were obtained from the Inugatake area. Detailed localities are shown in Table 1.

Figure 1.

Locations of the Toyora and Kuruma groups with Amaltheus localities denoted by the ammonoid shape. The stratigraphies in the Sakai River 1, 2, and 3 localities are displayed in Figure 2; fossil locality numbers are shown in Table 1.

Table 1.

Occurrence and repository list of Amaltheus from Japan. All of the specimens included in this table are illustrated in Figures 4–7.

The lithostratigraphy of the Kuruma Group has been separately analyzed for each area. In the Inugatake area, the group was subdivided into seven formations by Kobayashi et al. (Reference Kobayashi, Konishi, Sato, Hayami and Tokuyama1957): the Jogodani, Kitamatadani, Negoya, Teradani, Shinatani, Otakidani, and Mizukamidani formations, in ascending order (Fig. 2). Later, Takizawa (Reference Takizawa1984) assigned the Mizukamidani Formation to the Tetori Group, which consists of epicontinental sediments of Early Cretaceous age. The Kuruma Group is composed mainly of sandstones, mudstones, and conglomerates with numerous fossils, including ammonoids, bivalves, gastropods, belemnites, crinoids, fish, turtles, and terrestrial plants and is characterized by alternation of marine and non-marine clastic sediments (Kobayashi et al., Reference Kobayashi, Konishi, Sato, Hayami and Tokuyama1957; Kimura and Tsujii, Reference Kimura and Tsujii1980–1984; Hayami and Akahane, Reference Hayami and Akahane1981; Goto, Reference Goto1983, Reference Goto1994; Kimura et al., Reference Kimura, Ohana and Tsujii1988; Sonoda et al., Reference Sonoda, Goto and Terada2015; Iba et al., Reference Iba, Sano and Goto2015). Ammonoid fossils have been collected from the Teradani Formation (Sato, Reference Sato1955), which is composed mainly of sandy siltstones, and the Otakidani Formation (Kobayashi et al., Reference Kobayashi, Konishi, Sato, Hayami and Tokuyama1957), which is composed mainly of black mudstones with thin sandstone and conglomerate intercalations. On the basis of these ammonoid faunas, the Teradani Formation has been roughly correlated to the upper Pliensbachian, and the Otakidani Formation is thought to correspond to the upper Toarcian to Aalenian (Sato, Reference Sato1955, Reference Sato and Westermann1992; Kobayashi et al., Reference Kobayashi, Konishi, Sato, Hayami and Tokuyama1957). In addition, Takeuchi et al. (Reference Takeuchi, Tokiwa, Kumazaki, Yokota and Yamamoto2017) reported that the U–Pb age of detrital zircon grains from four sandstone samples in the Jogodani and Kitamatadani formations in the Inugatake area were dated ca. 188–186 Ma and compared these data to Pliensbachian age.

Figure 2.

Columnar sections, biostratigraphic succession of ammonoids, and the biostratigraphic framework of the lowermost part of the Teradani Formation, Kuruma Group, exposed along the Sakai River. The horizon numbers beside the columnar sections correspond to the horizon numbers in Table 1. The lithostratigraphic division of the Kuruma Group with the thickness of each formation utilized in this figure follows Kumazaki and Kojima (Reference Kumazaki and Kojima1996).

Almost all the additional Amaltheus specimens in this study were obtained from the lowermost Teradani Formation exposed along the Sakai River, except for some specimens from Mt. Kikuishi, the Teradani Valley, and the Kanayamadani Valley (Fig. 2; Table 1).

Toyora Group

This unit outcropping in the western part of Yamaguchi Prefecture, southwest Japan, consists of epicontinental sediments deposited during the Early and Middle Jurassic (Fig. 1). This group crops out separately in two regions, the northern and the southern districts (Hirano, Reference Hirano1971) (Fig. 1). The northern district is characterized by abundant occurrences of ammonoids (Matsumoto and Ono, Reference Matsumoto and Ono1947; Hirano, Reference Hirano1971, Reference Hirano1973a, b; Nakada, Reference Nakada2008; Nakada and Matsuoka, Reference Nakada and Matsuoka2009, Reference Nakada and Matsuoka2011), including all the materials used in this study.

The Toyora Group is composed mainly of sandstones, mudstones, and conglomerates. It has been divided into three formations in ascending order: the Higashinagano, Nishinakayama, and Utano formations; each formation has been further subdivided into several members (Matsumoto and Ono, Reference Matsumoto and Ono1947; Hirano, Reference Hirano1971; Nakada and Matsuoka, Reference Nakada and Matsuoka2011; Fig. 3). Ammonoid fossils are especially abundant in the Sakuraguchidani Mudstone Member, the lower part of the Nishinakayama Formation that is composed dominantly of black mudstone, and thus the ammonoid biostratigraphy has been analyzed frequently in this member (Matsumoto and Ono, Reference Matsumoto and Ono1947; Hirano, Reference Hirano1973b; Tanabe, Reference Tanabe, Kotaka, Dickins, Mckenzie, Mori, Ogasawara and Stanley1991; Nakada and Matsuoka, Reference Nakada and Matsuoka2009, Reference Nakada and Matsuoka2011). Nakada and Matsuoka (Reference Nakada and Matsuoka2011) proposed four successive ammonoid zones: the Canavaria japonica, Paltarpites paltus, Dactylioceras helianthoides, and Harpoceras inouyei zones, in ascending order. Two additional Amaltheus specimens described in this study (NU-MM0077 and NU-MM0078) were obtained from the lowermost part of the Sakuraguchidani Mudstone Member, corresponding to the lowermost part of the japonica Zone, exposed along the Sakuraguchidani Valley (Figs. 1, 3; Table 1). An Amaltheus specimen described by Hirano (Reference Hirano1971) was obtained from the lowermost part of the Kido Sandstone Member, which is composed mainly of bedded sandstones and sandy mudstones with thin coal beds, along the Chuzankei Valley (Fig. 3; Table 1).

Figure 3.

Generalized geological column of the Toyora Group in the study area after Nakada and Matsuoka (Reference Nakada and Matsuoka2011) with Amaltheus-bearing horizons and the zonal framework proposed by Nakada and Matsuoka (Reference Nakada and Matsuoka2011) and this study.

Type species

Amaltheus margaritatus de Montfort, Reference de Montfort1808.

Remarks

The whorl section is compressed and subelliptical. Surface ornament is extremely varied; some species have smooth whorl flanks with sigmoidal ribs, and others are characterized by spiny or strong ribbing. Almost all species have a crenulated keel and a suboxycone coiling with moderately open umbilicus.

Amaltheus stokesi (Sowerby, Reference Sowerby1818)
Figure 4

Reference Sowerby1818

Ammonites stokesi Sowerby, p. 205, pl. 190.

Reference Imlay1955

Amaltheus cf. A. nudus (Quenstedt, Reference Quenstedt1858; Imlay, p. 87, pl. 10, fig. 5.

Reference Howarth1958

Amaltheus stokesi; Howarth, p. 3, text–figs. 4–5, pl. 1, figs. 5, 7, 12–14, pl. 2, figs. 1, 3, 10, with synonymy.

Reference Frebold1964

Amaltheus stokesi; Frebold, pl. 6, figs. 6, 13.

Reference Frebold, Mountjoy and Tempelman-Kluit1967

Amaltheus stokesi; Frebold, Mountjoy, and Tempelman–Kluit, pl. 1, figs. 1, 7.

Reference Hirano1971

Amaltheus cf. A. stokesi; Hirano, p. 101, pl. 17, fig. 11.

Reference Repin and Sachs1974

Amaltheus (Amaltheus) stokesi; Repin, pl. 1, figs. 4–9.

Reference Frebold1975

Amaltheus stokesi; Frebold, pl. 4, figs 3, 4.

Reference Dagis1976

Amaltheus (Proamaltheus) stokesi; Dagis, p. 21, pl. 10, figs. 5–7.

Reference Sey and Kalacheva1980

Amaltheus stokesi; Sey and Kalacheva, pl. 1, figs 4, 7, 8, 11, 13.

Reference Kimura, Ohana and Tsujii1988

Amaltheus (Amaltheus) stokesi; Krymholts, Mesezhnikov and Westermann, pl. 2, fig. 1.

Reference Poulton1991

Amaltheus stokesi; Poulton, p. 18, pl. 8, figs. 2, 7, 8.

Reference Westermann1992

Amaltheus stokesi; Westermann, pl. 3, fig. 11, pl. 5, fig. 9, pl. 6, fig. 4 (cf.).

Reference Pálfy and Hart1994

Amaltheus cf. A. stokesi; Pálfy and Hart, pl. 1, fig. 12.

Reference Jakobs and Pálfy1994

Amaltheus stokesi; Jakobs and Pálfy, pl. 1, fig. 9.

Reference Smith and Tipper1996

Amaltheus stokesi; Smith and Tipper, pl. 19, fig. 1, pl. 20, fig. 3.

Reference Johannson, Smith and Gordey1997

Amaltheus stokesi; Johannson, Smith, and Gordey, pl. 2, fig. 13.

Reference Géczy and Meister1998

Amaltheus stokesi; Géczy and Meister, p. 102, pl. 6, fig. 10.

Reference Rakus and Guex2002

Amaltheus stokesi; Rakus and Guex, pl. 27, fig. 8.

Reference Meister and Friebe2003

Amaltheus stokesi; Meister and Friebe, pl. 16, figs. 9, 14.

Reference Schubert2004

Amaltheus stokesi; Schubert, pl. 1, fig. 1.

Reference Callomon and Gradinaru2005

Amaltheus stokesi; Callomon and Gradinaru, pl. 2, fig. 2.

Reference Fauré2006

Amaltheus stokesi; Fauré, p. 43, fig. 8.

Reference Topchishvili, Lominadze, Tsereteli, Todria and Nadareishvili2006

Amaltheus stokesi; Topchishvili et al., pl. 11, fig. 4.

Reference Rulleau, Guiffray and Dommergues2007

Amaltheus stokesi; Rulleau, Guiffray, and Dommergues, pl. 34, fig. 5, pl. 35, fig. 1.

Reference Mouterde, Dommergues, Meister and Rocha2007

Amaltheus stokesi; Mouterde et al., p. 75, pl. 1, figs. 9, 14, with synonymy.

Reference Repin2016

Amaltheus (Amaltheus) stokesi; Repin, p. 37, figs 4, 5.

Reference Nützel, Schneider, Hülse, Kelly, Tilley and Veit2016

Amaltheus stokesi; Nützel et al., p.3, fig. 2.

Figure 4.

Amaltheus stokesi from Japan. The locality and the repository data are shown in Table 1. (1) NU-MM0001; (2) NU-MM0075; (3) GK.G.11293 (rubber cast); (4, 5) NU-MM0003: (4) lateral view; (5) ventral view; (6) NU-MM0002; (7) NMNS PM17559 (rubber cast); (8) FFM3051-4; (9) NMNS PM23447; (10) FMM3051-3; (11) TOYA-Fo-2992; (12) FMM3051-5.

Type

The holotype, OUM.J2248, was collected by J. Sowerby (Reference Sowerby1818) from Dorset.

Diagnosis (modified from Howarth, Reference Howarth1958)

Large oxycone with highly compressed elliptical whorl section. Ventral halves of the inside whorls are covered by the next outer whorls. The venter bears a barely developed keel with coarse crenulation. Each serration of crenulation is connected with ribs.

Occurrence

Amaltheus stokesi is the index species at the base of the late Pliensbachian in Northwest Europe.

This taxon has been recognized in the Northwest European province, the northern Tethys margin (Southern Alps, Austroalpine units, Carpathians), the Caucasus (Georgia), East Asia (Japan), North American Cordillera (British Columbia and northern Alaska), Arctic Canada, and Russia (the Northeast, the Siberian Platform, and the Far East regions).

Description

The whorls are moderately to extremely involute, with a high expansion rate; the umbilici are narrow. Detailed characteristics of the whorl section are unknown because of compression. The venter bears a broad and elevated keel with coarse crenulations (NU-MM0003; Fig. 4.4). The radial and sometimes irregular ribs are stronger on the inner two-thirds of the whorl flank and become slightly weak toward the venter. They are strongly projected forward in the ventrolateral part and pass on to the keel as a serration of the crenulation. Tubercles are not recognized on ribs.

Remarks

Amaltheus stokesi is distinguished from Amaltheus margaritatus de Montfort, Reference de Montfort1808, one of the most typical species of this genus, mainly by the characteristics of the keel and the connecting pattern between the ribs and crenulations. Amaltheus stokesi has a rounded keel and is characterized by ribs that are connected to each serration of the crenulated keel; a typical example of this is seen in NU-MM0075 (Fig. 4.2). The keel of Amaltheus margaritatus is more developed, the crenulation becomes finer, and the ribbing seems a little weaker. Moreover, the ribs of Amaltheus margaritatus are not connected individually with each serration of the crenulation. In Japan, the morphological variability of A. stokesi is rather high. If the classic morphology of Sowerby's species is represented in Figure 4.1–4.7, 4.10–4.12, a specimen illustrated in Figure 4.8 develops a more derived morphology with a “stokesi habitus” in the inner whorls and a “margaritatus habitus” in the outer whorls with a more differentiated keel. This kind of ontogeny was already illustrated by Poulton (Reference Poulton1991, pl. 8, fig. 5) and Frebold (Reference Frebold1964, pl. 6, fig. 7). For this specimen, we use the terminology A. stokesi (Sowerby, Reference Sowerby1818) transitional form to A. margaritatus de Montfort, Reference de Montfort1808.

Amaltheus sensibilis Dagis, Reference Dagis1976 and A. subbifurcus Repin, Reference Repin, Efimova, Kinasov, Paraketsov, Polubotko, Repin and Dagis1968 were both collected from the Northeast Russian region (Repin, Reference Repin and Sachs1974, Reference Repin and Zakharov2009, Reference Repin2016; Dagis, Reference Dagis1976) and are distinguished from A. stokesi by the larger umbilicus and stronger projection of the ribs in the ventrolateral part.

A. stokesi is quite similar to A. bifurcus Howarth, Reference Howarth1958. However, A. bifurcus is characterized by coarse ribbing and an undeveloped keel (see Howarth, Reference Howarth1958, pl. 1, fig. 6). NMNS PM23447 (Fig. 4.9) still evokes the ancestral morphology of A. bifurcus with a coarser and stronger ribbing, but the whorl section is already more compressed and the inner-to-middle whorls seem to develop a finer and more flexuous ribbing than does the true A. bifurcus. Due to the preservation, we prefer to keep this specimen in the variability domain of A. stokesi.

Amaltheus margaritatus de Montfort, Reference de Montfort1808
Figures 5, 6.1–6.6

Reference de Montfort1808

Amaltheus margaritatus de Montfort, p. 90, fig. 91.

Reference Sowerby1812

Ammonites acutus Sowerby, p. 51, pl. 17, fig. 1.

Reference Buckman1845

Ammonites sedgwickii Buckman, p. 40 (nomen nudum).

Reference Giebel1852

Ammonites foliaceus Giebel, p. 540.

Reference Quenstedt1885

Ammonites amaltheus nudus Quenstedt, p. 321, pl. 41, figs. 1, 2.

Reference Quenstedt1885

Ammonites amaltheus compressus Quenstedt, p. 327, pl. 41, fig. 17, pl. 42, fig. 8.

Reference Sato1955

Amaltheus sp. indet. Sato, p. 114, pl. 18, figs. 1, 2.

Reference Howarth1958

Amaltheus margaritatus; Howarth, p. 13, text–fig. 8 (neotype), 9 (lectotype), pl. 3, figs. 4–6, with synonymy.

Reference Frebold1964

Amaltheus stokesi (Sowerby, Reference Sowerby1818); Frebold, pl. 6, fig. 7.

Reference Elmi, Atrops and Mangold1974

Amaltheus margaritatus; Elmi, Atrops, and Mangold, pl. 1, fig. 1.

Reference Pourmotamed and Motamed1976

Amaltheus margaritatus; Pourmotamed and Motamed, p. 106, fig. 3.

Reference Sey and Kalacheva1980

Amaltheus margaritatus; Sey and Kalacheva, pl. 1, figs 5, 9, 10, 12.

Reference Meister1988

Amaltheus margaritatus form margaritatus; Meister, pl. 1, figs. 2–4, pl. 2, figs. 1–3, 5, 10, pl. 4, figs. 1, 3, 4.

Reference Meister1988

Amaltheus (Amaltheus) margaritatus; Krymholts, Mesezhnikov, and Westermann, pl. 2, fig. 5 only.

Reference Poulton1991

Amaltheus stokesi (Sowerby, Reference Sowerby1818); Poulton, p. 18, pl. 8, fig. 1.

Reference Westermann1992

Amaltheus (Amaltheus) margaritatus; Westermann, pl. 17, fig. 6, except for fig. 3 (A. (Amaltheus) nov. sp. in Repin, 2009).

Reference Smith and Tipper1996

Amaltheus stokesi (Sowerby, Reference Sowerby1818); Smith and Tipper, pl. 19, fig. 1, pl. 20, fig. 3.

Reference Johannson, Smith and Gordey1997

Amaltheus stokesi (Sowerby, Reference Sowerby1818); Johannson, Smith, and Gordey, pl. 2, fig. 12.

Reference Géczy and Meister1998

Amaltheus margaritatus; Géczy and Meister, p. 103, with synonymy.

Reference Fauré2002

Amaltheus margaritatus; Fauré, pl. 7, fig. 16.

Reference Topchishvili, Lominadze, Tsereteli, Todria and Nadareishvili2006

Amaltheus margaritatus; Topchishvili et al., pl. 11, figs. 1, 2.

Reference Fauré2006

Amaltheus margaritatus; Fauré, p. 43, figs. 1, 3, 4.

Reference Rulleau, Guiffray and Dommergues2007

Amaltheus margaritatus; Rulleau, Guiffray, and Dommergues, pl. 35, figs. 3–5.

Reference Mouterde, Dommergues, Meister and Rocha2007

Amaltheus margaritatus; Mouterde et al., p. 76, pl. 2, figs. 1, 8, with synonymy.

Reference Dommergues, Dugué, Gauthier, Meister, Neige, Raynaud, Savary and Trevisan2008

Amaltheus margaritatus form margaritatus; Dommergues et al., p. 293, pl. 10, fig. 4.

Reference Seyed-Emami, Fürsich, Wilmsen, Majidifard and Shekarifard2008

Amaltheus margaritatus; Seyed-Emami et al., p. 244, 246, fig. 4T.

Reference Bardin, Rouget and Cecca2013

Amaltheus aff. margaritatus; Bardin, Rouget, and Cecca, p. 321, fig. 4A, B, p. 323, fig. 5A, B, E, F, fig. 6F, G.

Reference Dommergues and Meister2017

Amaltheus gr. margaritatus; Dommergues and Meister, p. 302, fig. 175, with synonymy.

Reference van de Schootbrugge2018

Amaltheus margaritatus; van de Schootbrugge, Richoz, Pross, Luppold, Hunze, Wonik, Blau, Meister, van der Weijst, Suan, Fraguas, Fiebig, Herrle, Guex, Little, Wignall, Piittmann and Oschmann, 2018, p. 10, figs 27, 28, 34.

Reference Fauré and Teodori2019

Amaltheus margaritatus; Fauré and Teodori, pl. 23, fig. 8.

Figure 5.

Amaltheus margaritatus from Japan. The locality and the repository data are shown in Table 1. (1) NU-MM0076. (2) NU-MM0005. (3) NU-MM0006. (4) MM2791 (rubber cast). (5) MM2792 (rubber cast). (6) FMM2009. (7) NU-MM0007. (8) FMM2007. (9) FMM2008. (10) FMM3051-2 (rubber cast).

Figure 6.

Amaltheus from Japan. The locality and the repository data are shown in Table 1. (1–5) Amaltheus margaritatus: (1) NU-MM0011; (2) TOTA-Fo-3015; (3) FMT0001; (4) NU-MM0077 (rubber cast); (5) NU-MM0078 (rubber cast). (6) Amaltheus sp. cf. A. margaritatus, NU-MM0004. (7) Amaltheus sp. indet., NU-MM0008.

Type

The neotype, BM.37039, designated by Howarth (Reference Howarth1958), was obtained from Croisilles (France).

Diagnosis (modified from Howarth, Reference Howarth1958)

Large oxycone with a compressed triangular or elliptical whorl section. The venter bears a well-developed keel with fine crenulation, except for the large body chamber. Radial and sigmoidal ribs curve forward strongly in the ventrolateral part and are reduced to striae before reaching the keel.

Occurrence

This species is the index taxon of the Amaltheus margaritatus Zone for the standard zonation. It ranges from the Amaltheus subnodosus Subzone of the margaritatus Zone to the Pleuroceras apyrenum Subzone of the Pleuroceras spinatum Zone in the Northwest European zonation, late Pliensbachian.

Amaltheus margaritatus was distributed mainly in high-latitude areas of the Northern Hemisphere, approximating to the Boreal Realm in the late Pliensbachian, as follows. It has been documented from the Northwest European province, the Caucasus (Georgia), West Asia (Alborz Province in Iran), East Asia (Japan), the North American Cordillera (British Columbia), and Russia (the northeast, the Siberian Platform, and the Far East regions). In addition, a few specimens of this species have been recognized in the Mediterranean province (Sicily in Italy, and Algeria and Morocco in North Africa).

Description

The whorls are moderately involute and enlarge rapidly, and the umbilici are narrow. Detailed characteristics of the whorl section are unknown because of compression. The venter bears a well-developed and elevated keel with fine crenulation. The present species from Japan has characteristic ribbing. On the inner whorl, the radial and slightly sigmoidal ribs starting from the umbilical edge are well developed in the middle of the flank and tend to disappear near the venter, but do not completely fade out. They are projected forward strongly in the upper part of the flank until the keel, but each serration of the crenulated keel is not connected with the ribs. The ribs weaken and finally fade on the outer whorl. NU-MM0005 (Fig. 5.2) displays some longitudinal striae on the whorl's flank. Tubercles on the ribs are not visible in this species.

Remarks

On the basis of recognizable characteristics of surface ornament and keel, all of the materials identified as the present species are similar to the Amaltheus from Europe (defined as A. margaritatus form margaritatus in Meister, Reference Meister1988). The present species is distinguished from A. stokesi by characteristics of the ribbing and keel, as mentioned in the preceding.

The Japanese specimens of Amaltheus margaritatus are quite similar to those described by Dagis (Reference Dagis1976, pl. 1–3) from the Northeast Russian region. However, the Northeast Russian specimens, especially those shown in Dagis (Reference Dagis1976, pl. 2, fig. 2, pl. 3, fig. 1), have more evolute whorls and less-developed (thicker and round) keels. Thus, these specimens are excluded from the synonymy of A. margaritatus in this taxonomic analysis.

Amaltheus conspectus Dagis, Reference Dagis1976 is similar to the present species in terms of ribbing characteristics, but it differs in that it has quite involute coiling of the whorls (e.g., Dagis, Reference Dagis1976, pl. 12, figs. 4, 5) and connections between the serration of the crenulated keel and the ribs (Dagis, Reference Dagis1976, pl. 13, fig. 2).

Amaltheus margaritatus also has some affinities with A. talrosei Repin, Reference Repin, Efimova, Kinasov, Paraketsov, Polubotko, Repin and Dagis1968, which likely represents varieties of A. margaritatus. However, A. talrosei is distinguishable from the present species by the more evolute whorl and stronger projection of the ribs in the ventrolateral part (Dagis, Reference Dagis1976, pl. 7, fig. 1).

The specimens collected from the japonica Zone of the Nishinakayama Formation, Toyora Group (NU-MM0077, Fig. 6.4; NU-MM0078, Fig. 6.5), and some specimens from the Teradani Formation (e.g., NU-MM0076, Fig. 5.1; NU-MM0011, Fig. 6.1) are characterized by an elevated and individual keel with well-prorsiradiated serrations and the direct connection between some ribs and the serrations of the crenulated keel. These characteristics are limited to the specimens obtained from the apyrenum Subzone of the Northwest European province (e.g., Meister, Reference Meister1988, pl. 4, fig. 3). The japonica Zone proposed by Nakada and Matsuoka (Reference Nakada and Matsuoka2011) in the Toyora Group was correlated with the apyrenum Standard Subzone on the basis of the occurrence of some specimens of Amaltheus margaritatus exhibiting these morphological characters.

A specimen obtained from the Kuruma Group, NU-MM0004 (Fig. 6.6), has some affinities with the present species—for example, the relatively fine serration of the crenulated keel. However, this specimen is small with inadequate preservation of ventrolateral surface ornament. In this description, the specimen is treated as Amaltheus cf. A. margaritatus.

A specimen derived from the Teradani Formation, Kuruma Group, NU-MM0008 (Fig. 6.7; Table 1) is similar to the present species in terms of ribbing characteristics, but lacks a venter and an inner whorl. Thus, this specimen is described as Amaltheus sp. indet. in this study.

Amaltheus repressus Dagis, Reference Dagis1976

Figure 7.7, 7.8

Reference Dagis1976

Amaltheus (Proamaltheus) repressus Dagis, p. 24, pl. 11, figs. 1–5.

Figure 7.

Amaltheus from Japan. The locality and the repository data are shown in Table 1. (1–6) Amaltheus orientalis n. sp.: (1, 2) TOYA-Fo-2974: (1) lateral view; (2) ventral view of gypsum cast photographed from the direction of the arrow of (1); (3, 4) NU-MM0010: (3) lateral view; (4) ventral view photographed from the direction of the arrow of (3); (5) TOYA-Fo-2986; (6) NMNS PM23448. (7, 8) Amaltheus repressus: (7) NU-MM0079 (rubber cast); (8) NU-MM0009.

Type

The holotype, no. 517–50, from Omolon Massif, Northeast Russia, was illustrated by Dagis (Reference Dagis1976, pl. 11, fig. 2).

Diagnosis (modified from Dagis, Reference Dagis1976)

The whorls are slightly involute. The flat whorl flanks have strong ribs that are radial along the dorsal part and curve forward in the ventrolateral part. This species characteristically exhibits a large number of constrictions. The venter bears a slightly developed keel with crenulations independent of ribs.

Occurrence

Amaltheus repressus co-occurs with A. stokesi in the Northeast Russian region, and thus the range of A. repressus is correlated with the Amaltheus stokesi Standard Subzone of the margaritatus Standard Zone, earliest late Pliensbachian (Dagis, Reference Dagis1976).

The present species was distributed in the Northeast Russian region and East Asia (Japan).

Description

The whorls are moderately involute and expand rapidly; thus, the umbilicus is relatively narrow (Fig. 8). Details of the whorl section are unknown. The venter bears a weak and crenulated keel, and the crenulations are very fine and prorsiradiated. However, surface ornaments on the whorl flank are generally weak. The present species is characterized by the presence of constrictions. On the early whorl of specimen NU-MM0079 (Fig. 7.7), the radial and irregular ribs starting from the umbilical edge are well developed in the inner half of the whorl and weaken rapidly toward the venter. They project strongly forward in the ventrolateral part and are connected to the keel. A slightly sigmoidal constriction is developed on the whorl flank, and it also projects strongly onto the ventrolateral part. In the outer whorls on specimen NU-MM0009 (Fig. 7.8), the ribs become very weak, and two constrictions per 45° of a whorl are developed near the aperture.

Figure 8.

Variation in whorl height and umbilical width of Amaltheus specimens from Japan.

Remarks

In previous studies, characteristic Russian species of Amaltheus were recognized outside Russia only in the North American Cordillera. Imlay (Reference Imlay1955) and Smith et al. (Reference Smith, Tipper and Ham2001) described a Russian species, A. viligaensis (Tuchkov, Reference Tuchkov1954), from Alaska. The occurrence of the present species is the first recorded occurrence of the Russian Amaltheus outside of Russia.

The present species is similar to Amaltheus margaritatus and A. conspectus in surface ornament (e.g., the slightly sigmoidal, irregular, and weak ribbing and the strongly prorsiradiate crenulation of the keel). However, the former is distinguished from the latter by the presence of constrictions.

Type

The holotype, TOYA-Fo-2974, and two paratypes, TOYA-Fo-2986 and NU-MM0010, were collected from the fossil localities Sakai River 2 and 3 (Fig. 2; Table 1). Another paratype, NMNS PM23448, is from Loc. 5303 (shown in Sato, Reference Sato1955) along the Teradani Valley.

Diagnosis

The whorls are moderately evolute. The whorl flanks have quite irregular, rigid (strong and rather tense), and generally radial ribs that are characterized by a strong projection in the ventrolateral portion. The ribs are irregular with some intercalations. The venter bears a not developed (thick and round) keel with coarse crenulations.

Occurrence

The detailed biostratigraphic distribution of the present endemic species is difficult to discuss because no index taxa have been obtained from the Sakai River 2 section (Fig. 2). However, NMNS PM23448 (Fig. 7.6) co-occurs with Amaltheus margaritatus (MM2791, Fig. 5.4; MM2792, Fig. 5.5) and Canavaria sp. ex gr. geyeriana (Haas, Reference Haas1913) from Loc. 5303 (Sato, Reference Sato1955) along the Teradani Valley (Fig. 1). On the basis of the biostratigraphic range of these latter two taxa (e.g., Howarth, Reference Howarth1958; Repin, Reference Repin, Efimova, Kinasov, Paraketsov, Polubotko, Repin and Dagis1968; Dagis, Reference Dagis1976; Meister, Reference Meister1988), the range of the present species seems to be comparable to the subnodosus Standard Subzone of the margaritatus Standard Zone to the spinatum Standard Zone, equal to the margaritatus and the Canavaria assemblage zones of the Kuruma Group (see Figs. 2, 9).

Figure 9.

Biostratigraphic correlation for assemblage zones of the Teradani Formation recognized in this study with the upper Pliensbachian zonal schemes of the Northwest European province, the Northeast Russian region, and the Toyora Group area.

The present species is found only in East Asia (Kuruma Group, central Japan).

Description

The whorls are moderately evolute (Fig. 8). The umbilici are relatively large and shallow with steep umbilical edges, as seen in TOYA-Fo-2974 (Fig. 7.1). The whorl sections seem to be subelliptical with flat flanks, recognized in the holotype (TOYA-Fo-2974; Fig. 7.1), but details are unknown because of compression. The venter bears a broad and round keel.

This species’ surface ornaments show some intraspecific variations. First, the serrations of the crenulation are generally weak, fine, and strongly prorsiradiated (e.g., Fig. 7.1, 7.2, 7.6), but NU-MM0010 partly has coarse and strong serrations (Fig. 7.3, 7.4). Second, most ribs are radial, wide, strong, quite irregular, and projected toward the aperture on the dorsolateral edge, typically in the holotype (TOYA-Fo-2974; Fig. 7.1). However, the ribbing of NMNS PM23448 (Fig. 7.6) is partly rursiradiate near the umbilical edge and slightly projected backward at the dorsolateral part. Finally, the rib density also shows some variation. The holotype (Fig. 7.1) has a relatively high rib density with some intercalated ribs (19 ribs on half an outer whorl); however, NU-MM0010 (Fig. 7.3), TOYA-Fo-2986 (Fig. 7.5), and NMNS PM23448 (Fig. 7.6) are characterized by coarser and stronger ribbings without intercalations (16 ribs on half an outer whorl in TOYA-Fo-2986; Fig. 7.5).

The ribs start from the umbilical edge and become strong near the inner flank. They tend to disappear from the middle to the outer flank and are projected forward strongly in the ventrolateral part. Some of them pass through onto the crenulated keel and constitute a part of the serrations. The present species has no tubercles, spines, or constrictions on the surface ornament.

Etymology

The species name is derived from their distribution area, the oriental region (western Panthalassa area).

Remarks

The present species is quite similar to Amaltheus talrosei, described only from the Northeast Russian region by Repin (Reference Repin, Efimova, Kinasov, Paraketsov, Polubotko, Repin and Dagis1968) and Dagis (Reference Dagis1976). It displays evolute coiling, strongly prorsiradiate serration of the crenulated keel, the connection between the ribs and the serration of the keel, and the strong projection of ribs in the ventrolateral part. However, the present species has stronger and more irregular ribbing (see the holotype of the present species illustrated in Fig. 7.1) than A. talrosei, typical forms of which are illustrated by Dagis (Reference Dagis1976, pl. 6, figs. 2–6). Consequently, the present new species from East Asia is closely related to A. talrosei from the Northeast Russian region.

The present species, especially TOYA-Fo-2974 (Fig. 7.1), is very similar to Amaltheus sensibilis (illustrated by Dagis, Reference Dagis1976, pl. 4, fig. 1), another of the endemic species of the Northeast Russian region, in the rursiradiate, strong, and wide ribbing and the evolute coiling. Nevertheless, the present new species is different from A. sensibilis by its irregular ribbing, most typical in the holotype (TOYA-Fo-2974) and the larger umbilicus.

The present species is similar to some Russian species, Amaltheus (Nordamaltheus?) molodoensis Repin, Reference Repin2017 (pl. 1, figs. 2, 12) and Amaltheus striatus asiaticus Repin, Reference Repin and Sachs1974 (see Repin, Reference Repin and Zakharov2009), in terms of ribbing characteristics. However, the present new species is distinguishable from these species by the more evolute whorl.

Some specimens of the present species, TOYA-Fo-2974 (Fig. 7.1) and NU-MM0010 (Fig. 7.3), also have some affinities with A. margaritatus and A. viligaensis in terms of the characteristics of the ribbing; both of the latter species are characterized by weak and irregular sinuous ribs. In addition, A. viligaensis (e.g., Tuchkov, Reference Tuchkov1954, pl. 37, fig. 3; Repin, Reference Repin, Efimova, Kinasov, Paraketsov, Polubotko, Repin and Dagis1968) is similar to present species in the irregular ribbings. However, the present species is distinguishable from these two species by its more evolute whorl and the stronger projection of its ribs in the ventrolateral part.

The present specimens, especially NU-MM0010 (Fig. 7.3), are similar to Amaltheus (Nordamaltheus?) borealis Dagis, Reference Dagis1976 and Amaltheus (Nordamaltheus?) bulunensis Repin, Reference Repin, Efimova, Kinasov, Paraketsov, Polubotko, Repin and Dagis1968 (pl. 36, fig. 1) in terms of ribbing and crenulation characteristics. However, the present species differs from those Russian species in that it has more evolute whorl.