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.
Introduction
The genus Hyphantoceras Hyatt, 1900, belonging to the family Nostoceratidae Hyatt, 1894, has helically coiled (or sometimes irregularly coiled) whorls with slightly oblique, dense ribs and 2–4 tubercle rows (Wright et al., 1996). More than 10 species assigned to this genus have been reported from the Turonian–Campanian in various regions of the world (Europe: d'Orbigny, 1850; Schlüter, 1876; Kaplan and Schmidt, 1988; Santamaría-Zabala, 1992; Tarkowski, 1996; Wiese, 2000; Metzdorf and Sowiak, 2003; Wiese et al., 2004; Wilmsen and Wiese, 2004; Kennedy and Gale, 2015; New Zealand: Wright, 1957; Japan and Russian Far East: Yabe, 1904; Shimizu, 1935; Matsumoto, 1942, 1943, 1959, 1977b; Zonova and Yazykova, 1998, Madagascar: Collignon, 1966; Walaszczyk et al., 2004, 2014; USA: Anderson, 1902, 1958). Five species assigned to Hyphantoceras have been described from Hokkaido, northern Japan and Sakhalin, Russian Far East (e.g. Yabe, 1904; Shimizu, 1935; Matsumoto, 1942, 1943, 1959, 1977b; Wright and Matsumoto, 1954; Kodama et al, 2002; Yazykova, 2004; Jagt-Yazykova, 2011). However, the phylogenetic relationships among these species remained obscure. The main reason is the uncertainty of the stratigraphic horizons of the type specimens of each species, because most of them were collected from river floats in various areas. Additionally, almost all species of Hyphantoceras were described from small numbers of fragmentary specimens. Therefore, the intraspecific variation of each species is not understood adequately.
During the field survey, more than 30 specimens of Hyphantoceras orientale and H. transitorium were collected from the Santonian–lowermost Campanian outcrops in the Kotanbetsu area, northwestern Hokkaido (Figures 1, 2). Based on these specimens with the exact stratigraphic horizon from the Kotanbetsu area and some specimens collected in the Obira area by Oizumi et al. (2005), the intraspecific variation and the phylogeny of the two species were discussed.
Note on Hyphantoceras in the Yezo Group
Five species of Hyphantoceras have been described and/or reported from the Yezo Group (Yabe, 1904; Shimizu, 1935; Matsumoto, 1942, 1943, 1959, 1977b; Wright and Matsumoto, 1954; Ohya et al., 2012). Hyphantoceras orientale, H. oshimai, and H. venustum were originally described by Yabe (1904) as “Heteroceras(?) orientale”, “Heteroceras(?) oshimai”, and “Helicoceras(?) venustum” respectively. Shimizu (1935) assigned the latter two species to genus Hyphantoceras. Subsequently Wright and Matsumoto (1954) regarded “Heteroceras(?) orientale” as a species of Hyphantoceras. These identifications were accepted in later studies (e.g. Matsumoto, 1977b; Okamoto, 1988a; Okamoto et al., 2003). Hyphantoceras transitorium and “H.(?) heteromorphum” were described by Matsumoto (1977b). Ohya et al. (2012) regarded “H.(?) heteromorphum” as a species of genus Hyphantoceras. Following the above previous studies, H. orientale, H. oshimai, H. venustum, H. transitorium and H. heteromorphum are available as species of Hyphantoceras in this study. Geographical and chronological distributions of these species are compiled in Table 1.
Figure 1.
Index map of Hokkaido showing the distribution of the Yezo Group (black areas) and location of the Kotanbetsu and Obira areas. Modified from Takashima et al. (2004).
Figure 2.
Geological map and cross section of the Kotanbetsu area, northwestern Hokkaido, northern Japan.
Table 1.
The geographical and chronological distributions of five Hyphantoceras species. These data are compiled from previous studies (Yabe, 1904; Shimizu, 1935; Matsumoto, 1942, 1943, 1959, 1977a, b; Igi et al., 1958; Tsushima et al., 1958; Haggart, 1984; Toshimitsu, 1988; Zonova and Yazykova, 1998; Wani and Hirano, 2000; Moriya and Hirano, 2001; Kodama et al., 2002; Okamoto et al., 2003; Yazykova, 2004; Oizumi et al., 2005; Jagt-Yazykova, 2011).
Geological setting and material
The marine deposits of the Yezo Group are distributed over 1000 km in a north to south direction, from Hokkaido, northern Japan, to Sakhalin, Russian Far East (Figure 1; Takashima et al., 2004; Maeda et al., 2005). The sediments were deposited in the Yezo forearc basin during the Aptian–Maastrichtian of the Cretaceous (Takashima et al., 2004).
The Yezo Group is widely distributed in the Kotanbetsu area, northwestern Hokkaido (Figures 1,2; Igi et al., 1958; Tsushima et al., 1958; Okada and Matsumoto, 1969; Toshimitsu, 1988; Nishida et al., 1992, 1993, 1996, 1997; Wani and Hirano, 1999, 2000; Wani, 2003; Takashima et al., 2004; Tsujino, 2009), and subdivided into the Maruyama, Hikagenosawa, Saku, Haborogawa and Hakobuchi formations, in ascending order (Takashima et al., 2004; Tsujino, 2009). The Hakobuchi Formation is exposed only around the Nakanosawa River (Tsujino, 2009), but this area was not investigated in this study (Figure 2).
The Haborogawa Formation in the Kotanbetsu area is subdivided into two parts (Figures 2–4; Takashima et al., 2010; Ikeda and Wani, 2012). The lower part of the Haborogawa Formation in the Kotanbetsu area is composed of siltstone, siltstone with interbedded sandstone, sandy siltstone, and sandstone (Figures 3, 4; Toshimitsu, 1988; Wani and Hirano, 2000; Wani, 2003). The upper part of the Haborogawa Formation in the Kotanbetsu area consists of three sequences that coarsen upwards from sandy siltstone to sandstone (Figures 3, 4; Toshimitsu, 1988; Wani and Hirano, 1999, 2000; Wani, 2003). The thickness of the Haborogawa Formation in the Kotanbetsu area is approximately 2100 m (Figure 4). Based on ageindex ammonoids and inoceramids, the geological age of the Haborogawa Formation in the Kotanbetsu area is estimated to be Turonian-early Campanian (Figure 4; Wani and Hirano, 1999, 2000; Wani, 2003; Ikeda and Wani, 2012).
The stratigraphy of the Yezo Group in the Obira area has been studied in detail (Igi et al., 1958; Tsushima et al., 1958; Takayanagi, 1960; Tanaka, 1963; Tanabe et al., 1977; Taketani, 1982; Sekine et al., 1985; Toshimitsu, 1988; Asai and Hirano, 1990; Hasegawa and Saito, 1993; Nishi et al., 2003; Funaki and Hirano, 2004; Oizumi et al., 2005; Hayakawa and Hirano, 2013; Honda and Hirano, 2014). The Yezo Group is subdivided into the Takimibashi, Tenkaritoge, Saku, and Haborogawa formations in the Obira area, in ascending order (Funaki and Hirano, 2004). The Haborogawa Formation in the Obira area consists of siltstone with interbedded sandstone, alternating beds of sandstone and siltstone, and sandstone (Funaki and Hirano, 2004; Oizumi et al., 2005). The thickness of the Haborogawa Formation in the Obira area is approximately 2200 m (Funaki and Hirano, 2004; Oizumi et al., 2005). The geological age of the Haborogawa Formation in the Obira area is estimated to be Turonian–early Campanian based on age-index ammonoids and inoceramids (Funaki and Hirano, 2004; Oizumi et al., 2005; Hayakawa and Hirano, 2013).
The Haborogawa Formation in these areas is subdivided into twelve units (Ua–U1) by Tsushima et al. (1958) and Tanaka (1963). After that, the uppermost Unit U1 is identified as the Hakobuchi Formation in the Kotanbetsu area by Tsujino (2009). In the northwestern area, including the Kotanbetsu and Obira areas, several thick sandstone layers can be traced as key beds of the Haborogawa Formation (eg. MHs0, MHs2–MHs5, UHs1–UHs4, named by Toshimitsu, 1985, 1988).
Figure 3.
Columnar section of the Yezo Group in the Kotanbetsu area. 1–3, the Kotanbetsu River; 4, 5, the Horotate Creek; 6, the Kaminosawa Creek. The occurrences of Hyphantoceras transitoriam (solid black circles) and H. orientale (white circles) are shown.
Figure 4.
Generalized columnar section of the Yezo Group in the Kotanbetsu area, with the stratigraphic occurrences of Hyphantoceras transitorium and H. orientale, and the stage-diagnostic species of ammonoids and inoceramids. The numbers on the right side of the columnar section reflect the locality numbers in Appendix 1.
Figure 5.
Hyphantoceras transitorium from the Kotanbetsu area. A, KYC86 from Kotanbetsu area; B, MCM-W1624 from locality HR 109 of the Horotatesawa Creek; C, MCM-W1623 from locality HR051 of the Horotatesawa Creek; D, MCM-W1625 from locality KT133 of the Kotanbetsu River; E, MCM-W1622 from locality KT141 of the Kotanbetsu River; F, KYCH36 from the Kotanbetsu area.
Thirty-three specimens assigned to Hyphantoceras orientale and H. transitorium obtained from the units Ud–f, Ug and Uh in the Kotanbetsu and Obira areas were examined in this study (Figures 5B–E, 6; Table 2). These specimens are housed in the Mikasa City Museum. In addition to these specimens, two specimens assigned to H. transitorium obtained from the Kotanbetsu area without precise locality records, housed in the Nakagawa Museum of Natural History were also examined (Figure 5A, F; Table 2). All specimens have been recovered from spherical or mushroom-shaped calcareous concretions. The type specimens of H. orientale, H. transitorium, and H. oshimai, housed in the University Museum, the University of Tokyo and the National Museum of Nature and Science, Tsukuba were also examined (Figure 7; Table 2).
Correspondence between abbreviations prefixed to registered number and institutions is as follows: MCM-W, Mikasa City Museum; KYC and KYCH, Nakagawa Museum of Natural History; UMUT MM, the University Museum, the University of Tokyo; NMNS PM, National Museum of Nature and Science, Tsukuba.
Table 2.
List and measurements of specimens. The locality numbers of the specimens from the Kotanbetsu area correspond to these in Appendix 1. MCM-W0269-1 and MCM-W0335-1 are specimens listed in Oizumi et al. (2005). The localities of the specimens are cited in Oizumi et al. (2005). See Aiba et al. (2017, fig. 7) for the measuring method.
Morphological characteristics of Hyphantoceras transitorium
Hyphantoceras transitorium Matsumoto, 1977b, described originally as a monotypic species, is characterized by a turreted shell with whorls slightly separated from each other (Matsumoto, 1977b; Figure 7E). Ten newly found additional specimens examined in the present study are regarded to this species (MCM-W0269-1, MCM-W1620–W1626, KYC86, KYCH36; Figure 5; Table 2). The shell sizes of the present specimens fall within the rangeof 2.4–13.9 mm in whorl tube diameter (Table 2). The present specimens show a wide range of variation for the degree of separation of each whorl. Specifically, some have simple tightly coiled whorls (Figure 5A–C), while others have tightly coiled whorls followed by loosely coiled whorls (Figure 5D–F). Furthermore, there is intraspecific variation on timing of the change of whorl shapes among the specimens (Figure 5D–F). For instance, the whorl shape transition occurs on 5.0 mm in whorl tube diameter in MCM-W1625 (Figure 5D), and on 8.0 mm in whorl tube diameter in MCM-W1622 (Figure 5E) and KYCH36 (Figure 5F), respectively. A retroversally hooked body chamber is not observed in the present specimens. The ribs are coarse (19–24 ribs per half whorl), and slightly prorsiradiate (Matsumoto, 1977b; Figures 5, 7E, 8A–C). Ribs with three or four tubercles occur on every other or every third rib (Matsumoto, 1977b; Figures 5, 7E, 8A–C). Two ribs looped at a tubercle are observed in some ribs of some specimens (Matsumoto, 1977b; Figure 8B). The siphuncle is positioned at the middle of the external side (Matsumoto, 1977b). The coiling is sinistrai in all examined specimens (Figures 5, 7E; Table 2).
Figure 7.
Type specimens of Hyphantoceras orientale (Yabe, 1904), H. oshimai (Yabe, 1904), and H. transitorium Matsumoto, 1977b. A, B, H. orientale; A, holotype (UMUT MM7572a); B, paratype (UMUT MM7572b); C, H. orientale (paratype of H. oshimai in the original description, UMUT MM7554); D, H. oshimai, holotype (UMUT MM7553); E, H. transitorium, holotype (NMNS PM7261).
Figure 8.
Photographs of shell surface ornamentation. A–C, Hyphantoceras transitorium; A, MCM-W1624; B, MCM-W1621; C, NMNS PM7261, holotype; D, E, Hyphantoceras orientale; D, MCM-W1638; E, UMUT MM7572a, holotype; F, Hyphantoceras oshimai (Yabe, 1904), UMUT MM7553, holotype.
Morphological characteristics of Hyphantoceras orientale
Hyphantoceras orientale (Yabe, 1904) is characterized by extremely elongated and slowly enlarging whorls (Yabe, 1904; Matsumoto, 1977b; Figure 7A, B). Twentyfive specimens from the Kotanbetsu and Obira areas are assigned to this species (MCM-W0335-1, MCM-W1627–W1650; Figure 6). The shell sizes of the present specimens range within 3.0–11.8 mm in whorl tube diameter (Table 2). The whorl shapes do not change drastically in the range of the preserved part of the present specimens (Figure 6). Okamoto (1988a) described the initial straight whorl of H. orientale, but that part is not preserved in the present specimens. The ribs are not dense (11–20 ribs per half whorl; Figures 6, 7A, B, 8D, E), much coarser in some specimens (Figures 6D, 7A, 8E), and rather prorsiradiate (Matsumoto, 1977b; Figures 6, 7A, B, 8D, E). Ribs with three or four tubercles occur on every other rib or in higher frequency (Figures 6, 7A, B, 8D, E). The coiling is mostly sinistrai (Matsumoto, 1977b). The paratype of H. oshimai (Yabe, 1904) can be assigned to this species (Shimizu, 1935; Wright and Matsumoto, 1954; Matsumoto, 1977b; Figure 7C).
Stratigraphic occurrences of Hyphantoceras transitorium and H. orientale
The stratigraphic occurrences of Hyphantoceras transitorium and H. orientale in the Kotanbetsu area are shown in Figures 3 and 4. The stratigraphic ranges of two species do not overlap. Hyphantoceras transitorium occurs in the upper part of units Ud-f and the lower part of Unit Ug of the Haborogawa Formation, which is in the Inoceramus amakusensis Zone (= Santonian; Toshimitsu et al., 1995, 2007), and H. orientale appears in the upper part of Unit Ug and Unit Uh of the Haborogawa Formation, which is in the Inoceramus amakusensis Zone (= Santonian; Toshimitsu et al., 1995, 2007).
In the Obira area, one specimen (MCM-W0269-1) assigned to Hyphantoceras transitorium occurs in the horizon between MHs 3 or MHs 4 and MHs 2, and the other specimen (MCM-W0335-1) assigned to H. orientale occurs in the horizon a few meters below UHs1 (Oizumi et al., 2005). The horizons of two species in the Obira area are concordant with those in the Kotanbetsu area. Matsumoto (1977b) reported the specimens assigned to H. orientale probably derived from units Ui–j (Platyceramus Japonicus Zone; = lowermost Campanian; Toshimitsu et al., 1995, 2007) in the Obira area.
Okamoto et al. (2003) listed Hyphantoceras orientale from the upper part of Unit Ug and Unit Uh in Haboro area. The results of this study in Kotanbetsu and Obira areas are concordant with the stratigraphic occurrence of H. orientale in the Haboro area.
Discussion
Variation of Hyphantoceras transitorium
Hyphantoceras transitorium was erected based on a single specimen from the Mikasa area, Hokkaido by monotypy (Matsumoto, 1977b; Figure 7E). However, ten newly found additional specimens of H. transitorium show that this species has wide intraspecific variation in the whorl shape and the timing of whorl shape change (Figure 5). Wide intraspecific variations for whorl shape are observed in other nostoceratid ammonoids (e.g. Ainoceras: Matsumoto and Kanie, 1967; Yezoceras: Matsumoto, 1977b; Eubostrychoceras: Matsumoto, 1977b; Okamoto, 1989; Misaki and Maeda, 2010). Switching timing of the shell ornament phase is also variable even in single species of some heteromorph ammonoids (Polyptychoceras: Okamoto and Shibata, 1997; Baculites: Tsujino et al., 2003). Several previous studies presume that the ontogenetic change of whorl shape is related to change in the mode of life (Okamoto, 1988a, b).
Matsumoto (1977b) pointed out that the deviated part of the later whorl in the holotype of Hyphantoceras transitorium could be a partially preserved retroversally hooked body chamber. On the other hand, more loosely coiled parts following tightly coiled whorls are recognized in some of the present specimens (Figure 5A–C). Considering this, it is not certain that the deviation of the later whorl of the holotype is an indication of the end of growth.
Phylogenetic lineage of two Hyphantoceras species
Two species, Hyphantoceras transitorium, and H. orientale share many characters of shell ornamentation characteristics, such as prorsiradiate and relatively frequent tuberculate ribs, no flared ribs, and three or four tubercles on each tuberculate rib (Figures 5, 6, 7A–C, E, 8A–E; Table 2). The similarity of the shell surface ornamentation of the two species suggests that they are phylogenetically close. In addition, the two species occur successively (Figures 3, 4), and the specimen (MCM-W1625) of H. transitorium with the loosely coiled whorl for the most part occurs in the topmost horizon of the stratigraphic range (Figure 5D). The specimen might be the intermediate form between H. transitorium (e.g. holotype; Figure 7E) and H. orientale. Several lines of evidence indicate a high possibility that H. orientale was derived from H. transitorium during the Santonian. This lineage was probably indigenous to the northwestern Pacific realm.
Relationships with the other species of Hyphantoceras in the Yezo Group
Hyphantoceras oshimai (Yabe, 1904, p. 12, pl. 3, fig. 5; Figure 7D) shares the loosely coiled whorls and many features of shell surface ornamentation such as coarse, slightly prorsiradiate, and sometimes ribs looped at tubercles with some specimens of H. transitorium (Figures 5D–F, 8A–C, F). However, H. oshimai is much larger than H. transitorium, and the whorl sizes of the two species do not overlap (Figures 5, 7D; Table 2). Hyphantoceras oshimai is listed and reported from the same stratigraphic level (Unit Ug) in the Kotanbetsu area (Wani and Hirano, 2000) and the stratigraphic levels just below the occurrences of H. transitorium (Unit Uf) in the Haboro and Obira areas (Matsumoto, 1977b; Okamoto et al., 2003). Considering the similarities of the two species, there is a possibility that two species are closely related. However, the phylogenetic relationship between the two species cannot be concluded in this paper, because the whorl sizes of two species differ considerably and stratigraphic data is insufficient. Careful comparison throughout ontogeny and investigation of FAD of H. transitorium and LAD of H. oshimai are necessary for clarifying the relationship between them.
Hyphantoceras venustum (Yabe, 1904, p. 11, pl. 3, fig. 4, pl. 5, figs. 1 and 2) differs from H. transitorium, H. orientale, and H. oshimai by its periodic tuberculate flared ribs, numerous intervening fine ribs, and the wide umbilicus. Hyphantoceras heteromorphum (Matsumoto, 1977b, p. 314, pl. 47, fig. 2) also differs from H. transitorium, H. orientale, and H. oshimai, by its periodic tuberculate flared ribs, numerous intervening fine ribs, and irregularly coiled whorls. These comparisons suggest that H. venustum and H. heteromorphum stand outside of the lineage of H. transitorium–H. orientale.
Acknowledgments
I sincerely thank Ryoji Wani (Yokohama National University) for his encouragement and critical reading of the manuscript. I also thank Ryuichi Majima, Masahiro Ishikawa, Tomohiko Kikuchi, and Shinji Yamamoto (all, Yokohama National University) for their critical comments; Kenji Ikuno (Museum of Nature and Human Activities, Hyogo) for fruitful discussions and suggestions, and assistance during fieldwork; Yasunari Shigeta (National Museum of Nature and Science) for fruitful discussions and suggestions, and access to the type specimens; Tomoki Karasawa (Mikasa City Museum) for fruitful discussions and assistance during fieldwork; Ken'ichi Kurihara (Hokkaido Museum) for encouragement and fruitful discussions; Kazushige Tanabe and Yusuke Takeda (both, the University of Tokyo) for fruitful discussions and suggestions, and assistance in collecting the literature; Yasuhiro Ito and Takenori Sasaki (both, the University Museum, the University of Tokyo) for access to the type specimens; Yoshinari Hikida (Nakagawa Museum of Natural History) for access to the significant specimens; Kan'ichi Fujiwara, Toshiaki Matsuda, Keichi Mori (all, Volunteer's Association of Mikasa City Museum), and Yasuji Kera (Sapporo City) for donating or letting me observe referable specimens; Yasuhiro Iba (Hokkaido University) for fruitful discussion; Yasuyuki Tsujino (Tokushima Prefectural Museum), Tomohiro Nishimura (Hobetsu Museum) and Haruyoshi Maeda (Kyushu University) for their valuable comments to improve the manuscript on peer review; Fumiko Murakami and Mamoru Murakami (both, Murakami Lodging House, Haboro Town), and Junya Yonemaru (Yonemaru Automobile, Iwamizawa City) for their kind assistance during fieldwork. Thanks are extended to Kotanbetsu and Tappu forestry offices. This study was partly supported by the Sasakawa Scientific Research Grant from the Japan Science Society and the Fukada Research Grant from the Fukada Geological Institute.
