A new species of heteromorph ammonoid Eubostrychoceras valdelaxum sp. nov. is described from the Platyceramus japonicus Zone (the lowermost Campanian) of the Haboro and Mikasa areas in Hokkaido, northern Japan. The most notable characteristic of E. valdelaxum sp. nov. is its extremely longitudinally elongated helical whorls. The new species is clearly distinguished from other species by this characteristic. The shell ornamentation and whorl expansion rate of the new species resemble most closely those of E. japonicum. Considering the stratigraphic relationships, it is the most reasonable conclusion that E. valdelaxum sp. nov. evolved from E. japonicum. The variation of body chamber size in E. valdelaxum sp. nov. might be dimorphism.
Introduction
The genus Eubostrychoceras Matsumoto, 1967 belongs to the family Nostoceratidae Hyatt, 1894 (Wright et al., 1996). Matsumoto (1967) established this genus for a group having helically coiled whorls that are in contact (but sometimes not in contact) with the preceding whorls, with simple ribs and no tubercles. The species with loosely coiled whorls (e.g. Heteroceras (?) japonicum Yabe, 1904) were not included in this genus in the first proposal. Subsequently, Matsumoto (1977) corrected the definition of this genus to include species with completely separated helically coiled whorls. As a result, the species with no tubercles in the genus Bostrychoceras Hyatt, 1894 were assigned to Eubostrychoceras, except for the species Bostrychoceras polyplocum (Roemer, 1841).
Phylogenetic relationships between Eubostrychoceras and other taxa were discussed in several studies (e.g. Matsumoto, 1967, 1977; Tanabe et al., 1981; Okamoto, 1988, 1989). From the perspective of similarity in whorl coiling in the earlier ontogenetic stage and/or shell surface ornamentation, the genera Nipponites, Scalarites, Muramotoceras, Ainoceras, and Horotateceras are considered to be offshoots from Eubostrychoceras (Matsumoto, 1967, 1977). The phylogenetic relationship from Eubostrychoceras japonicum (Yabe, 1904) to Nipponites mirabilis Yabe, 1904 is also suggested by studies of theoretical morphology (Okamoto, 1988, 1989).
Species of this genus are reported from the Turonian—Campanian strata all over the world (Canada; Whiteaves, 1903; Usher, 1952; Haggart, 1989; Haggart et al., 2005: USA; Young, 1963; Ward, 1976; Haggart, 1984; Cobban, 1987; Kennedy and Cobban, 2001; Ward et al., 2012: India; Kossmat, 1895: South Africa; Hoepen, 1921; Klinger, 1985; Klinger and Kennedy, 2003: Madagascar; Collignon, 1969; Klinger et al., 2007: Germany; Kaplan and Schmid, 1988: France; Kennedy et al., 2015: Denmark; Kennedy and Christensen, 1991: Antarctica; Kennedy et al., 2007). Several species belonging to Eubostrychoceras are reported from the Turonian—Santonian of Hokkaido and northeast Japan (e.g. Yabe, 1904; Matsumoto, 1967, 1977; Obata et al., 1991; Toshimitsu and Hirano, 2000). The worldwide species E. elongatum (Whiteaves, 1903) is reported from the upper lower Campanian of southwest Japan (Yabe, 1915; Matsumoto, 1977; Misaki and Maeda, 2009, 2010). However, species belonging to Eubostrychoceras have never been reported from the Campanian in the northwestern Pacific realm, including Hokkaido and Sakhalin, except for Wakayama, southwest Japan (Misaki and Maeda, 2010).
Figure 1.
Localities (A: Haboro area, B: Mikasa area) of Eubostrychoceras valdelaxum Aiba, Yamato, Kurihara and Karasawa sp. nov.
In this paper, we describe a new species belonging to Eubostrychoceras from the lowermost Campanian of Hokkaido and discuss the phylogenetic and paleobiological implications.
Notes on stratigraphy
The Aptian—Maastrichtian marine deposit, the Yezo Group, is widely distributed over 1000 km in a north to south direction from Hokkaido, northern Japan, to Sakhalin, Far East of Russia (Takashima et al., 2004; Maeda et al., 2005). The newly described species referable to Eubostrychoceras occurred from the upper part of the Yezo Group in the Haboro and the Mikasa areas (Figure 1).
The stratigraphy of the Yezo Group in the Haboro area was studied in detail (Toshimitsu, 1985,1988; Toshimitsu and Kikawa, 1997; Toshimitsu et al., 1998; Moriya and Hirano, 2001; Moriya et al., 2001; Okamoto et al., 2003; Kawabe and Okamoto, 2012). The Nagareya Formation mainly consists of mudstone and sandy mudstone and is the uppermost part of the Yezo Group in the Haboro area (Okamoto et al., 2003). Unit Ui-j, the lower part of the Nagareya Formation, mainly consists of mudstone, and its thickness is more than 350 m (Okamoto et al., 2003). Unit Ui-j contains Platyceramus japonicus, indicating the lowermost Campanian (Toshimitsu, 1988; Toshimitsu et al., 1995, 1998; Toshimitsu and Kikawa, 1997; Moriya and Hirano, 2001; Moriya et al., 2001; Okamoto et al., 2003; Toshimitsu et al., 2007). Platyceramus japonicus occurs only from this unit in the Haboro area (Okamoto et al., 2003). Three specimens of Eubostrychoceras described below were obtained from floated calcareous concretions that probably came from this unit (Locality 1 in Figure 1 A), judging from co-occurring P. japonicus.
The Yezo Group in the Mikasa area has been studied by many authors (Matsumoto et al., 1988; Ando, 1990a, b; Futakami, 1996; Takashima et al., 2004; Futakami et al., 2008). After Futakami et al. (2008), the Yezo Group was subdivided into the Shuparo, Hikagenosawa, Mikasa, Katsurazawa, and Kashima formations in the Mikasa area, in ascending order. The Kashima Formation mainly consists of monotonous mudstone, and its thickness is approximately 300 m (Futakami et al., 2008). Futakami et al. (2008) estimated the geological age of the Kashima Formation as Coniacian and Santonian. However, several studies reported Platyceramus japonicus from the Kashima Formation around the Kikumenzawa Creek (Matsumoto et al., 1988; Tajika and Wani, 2010). We also obtained P. japonicus from an outcrop along the lakefront of Katsurazawa Lake, near Kikumenzawa Creek (Locality 2 in Figures 1B, 2). Therefore, the top part of the Kashima Formation certainly extends to the lower Campanian. Two specimens of Eubostrychoceras described below were obtained from calcareous concretions from the P. japonicus Zone in situ in the Mikasa area (Locality 2 in Figure 1B).
Paleontological description
The systematic scheme follows the classification established by Wright et al. (1996), except for the treatment of Eubostrychoceras.
Institution abbreviation.—MCM = Mikasa City Museum, Hokkaido.
Order Ammonoidea Zittel, 1884
Figure 2.
Platyceramus japonicus from the outcrop along the lakefront of Katsurazawa Lake in the Mikasa area. This specimen occurred with Eubostrychoceras valdelaxum Aiba, Yamato, Kurihara and Karasawa sp. nov. The exact locality is Locality 2, as shown in Figure 1B.
Suborder Ancyloceratina Wiedmann, 1966
Superfamily Turrilitoidea Gill, 1871
Family Nostoceratidae Hyatt, 1894
Genus Eubostrychoceras Matsumoto, 1967
Type species.—Eubostrychoceras indopacificum Matsumoto, 1967.
Remarks.—Wright et al. (1996) treated Eubostrychoceras as a subgenus of the genus Nostoceras. However, since the publication of Wright et al. (1996) many authors have treated Eubostrychoceras as an independent genus because Nostoceras has tubercles in some ontogenetic stages while Eubostrychoceras has no tubercles throughout ontogeny (e.g. Kennedy and Cobban, 2001; Klinger and Kennedy, 2003; Klinger et al., 2007; Misaki and Maeda, 2009, 2010). Following these authors, we treat Eubostrychoceras as a genus in this study.
The earlier ontogenetic whorls of Hyphantoceras (?) heteromorphum Matsumoto (1977, p. 325, pl. 47, fig. 2) resemble the new species belonging to Eubostrychoceras described below. However, H. (?) heteromorphum has tubercles in the middle-later ontogenetic stages. Therefore, this species can be excluded from the genus Eubostrychoceras by its tubercles.
Eubostrychoceras valdelaxum sp. nov.
Figures 3–6
Hyphantoceras (?) heteromorphum Matsumoto. Matsumoto, 1977, p. 314 (pars), pl. 61, fig. 1 only.
Eubostrychoceras sp. Kawabe and Okamoto, 2012, pl. 1, fig. 8.
Type specimens.—The holotype MCM-A1783 (Figure 3A) and the paratype MCM-A1784 (Figure 3B), from Loc. 1, Haboro area (Figure 1A), both consist of the body chamber only. The paratype MCM-A1785 (Figure 3C), from Loc. 1, Haboro area (Figure 1A), consists of parts of the body chamber and phragmocone. The paratype MCM-A1476 (Figure 4A), from Loc. 2, Mikasa area (Figure 1B), consists of a secondarily deformed body chamber and a small portion of the phragmocone. The paratype MCM-A1477 (Figure 4B), from Loc. 2, Mikasa area (Figure 1B), consists of a part of the body chamber only.
Diagnosis.—Eubostrychoceras with completely separated coil encircling a straight axis, extremely longitudinally elongated, very narrow umbilicus, and slowly enlarging whorls; slightly oblique simple ribs.
Description.—The shell shape of the early stage is unknown because the most of the phragmocone is missing in all specimens. In the main stage, the whorl is elongated extremely longitudinally, but the whorl elongation is slightly variable. The paratypes MCM-A1477 and MCM-A1785 have more elongated whorls (Figures 3C, 4B), although the holotype MCM-A1783 and the paratypes MCM-A1784 and MCM-A1476 have less elongated whorls (Figures 3A, B, 4A). The cross sectional whorl shape is ellipse (Figure 3D, E). Ribs are normal (Figure 5A), slightly oblique, 40–60 per whorls. The tubercles and the constrictions are not observed in all specimens. In the later stage of paratype MCM-A1784, the body chamber slightly deviates from the coiling axis at the main stage (Figure 3B). Ribs are slightly serrated (Figure 5B). Tubercles and constrictions are not observed. The coiling is sinistral in all specimens.
A portion of a suture line is observed at the whorl-tube diameter of 10.1 mm of the paratype MCM-A1785 (Figure 6). The suture line is lytoceratid type, consisting of an external lobe, a lateral lobe, and an umbilical lobe. The lateral lobe and the umbilical lobe are incised bipartitely.
Measurements.—The measured shell morphology is shown in Figure 7. The measurement values are summarized in Table 1.
Comparison.—Eubostrychoceras valdelaxum sp. nov. is somewhat close to E. otsukai (Yabe, 1904, p. 14, pl. 3, fig. 9, pl. 4, figs. 1–3), E. Japonicum (Yabe, 1904, p. 17, pl. 3, fig. 8), and E. zulu Klinger and Kennedy (2003, p. 229, figs. 2A, 63F) in the density and depth of the ribs and the whorl expansion rate (non Raupian parameter W). However, the whorl elongation of E. valdelaxum sp. nov. is higher than in the above species. Kennedy et al. (2007, p. 525, figs. 15A, H) reported a fragment and mold specimens of Eubostrychoceras sp. from the Coniacian of Janies Ross Island, Antarctica. They pointed out that their specimens are similar to E. zulu Klinger and Kennedy (2003, p. 229, figs. 2A, 63F) from the Coniacian of Zululand, South Africa. Actually, the ribs and the estimated mode of coiling of the two illustrated specimens are similar to E. zulu. Therefore, it is likely that Eubostrychoceras sp. described by Kennedy et al. (2007) cannot be assigned at least to E. valdelaxum sp. nov.
The whorl expansion rate of Eubostrychoceras valdelaxum sp. nov. is close to that of E. protractum (Collignon, 1969, p. 31, pl. 29, figs. 2069,2070), but the density of the ribs of E. valdelaxum sp. nov. is lower than that of E. protractum. Similarly, the density of the ribs of E. valdelaxum sp. nov. is lower than that of E. densicostatum Matsumoto (1977, p. 332, pl. 52, fig. 2). The extremely high whorl elongation and normal ribs of E. valdelaxum sp. nov. are clearly different from the low whorl elongation and sharp coarse ribs of E. elongatum (Whiteaves, 1903, p. 331, pl. 44, fig. 2), E. matsumotoi Cobban (1987, p. A2, pl. 1, figs. 1–26), and E. amapondense (Hoepen, 1921, p. 17, pl. 4, figs. 1, 2). Eubostrychoceras valdelaxum sp. nov. is distinguished easily from E. indopacificum Matsumoto (1967, p. 333, pl. 18, fig. 1), E. muramotoi Matsumoto (1967, p. 335, pl. 19, figs. 1, 2), and E. nibelae Klinger and Kennedy (2003, p. 227, figs. 1H, I) by its high whorl elongation.
The fragmentary specimen identified as Hyphantoceras (?) heteromorphum by Matsumoto (1977, pl. 61, fig. 1) from Kikumenzawa Creek, Mikasa area is similar to E. valdelaxum sp. nov. in its normal ribs and low whorl expansion rate. That specimen can be assigned to the new species. Eubostrychoceras sp. described by Kawabe and Okamoto (2012, p. 781, pl. 1, fig. 8) has many similar characters with E. valdelaxum sp. nov. We also assign that specimen to the new species.
Occurrence.—The Mikasa specimens (MCM-A1476 and MCM-A1477) were collected from calcareous concretions in the Platyceramus Japonicus Zone (lowermost Campanian, Toshimitsu et al., 1995) together with P. Japonicus (Figure 2). The Haboro specimens (MCM-A1783, MCM-A1784, and MCM-A1785) were collected from floated calcareous concretions together with P. Japonicus. Kawabe and Okamoto (2012) reported Eubostrychoceras sp. which is assigned to E. valdelaxum sp. nov. from the Migi-no-sawa area, a tributary of the Haborogawa River. This specimen was also collected from the P.japonicus Zone.
Figure 3.
Eubostrychoceras valdelaxum Aiba, Yamato, Kurihara and Karasawa sp. nov. from the Haboro area, Hokkaido. A, MCM-A1783 (holotype); B, MCM-A1784 (paratype); C, MCM-A1785 (paratype); D, whorl cross section of MCM-A1783 (holotype); E, whorl cross section of MCM-A1784 (paratype). Black arrow indicates the position of the last septum. White arrow indicates the position of whorl cross sections.
Figure 4.
Eubostrychoceras valdelaxum Alba, Yamato, Kurihara and Karasawa sp. nov. from the Mikasa area, Hokkaido. A, MCM-A1476 (paratype); B, MCM-A1477 (paratype). The arrow indicates the position of the last septum.
Figure 5.
Schematic diagrams illustrating shell surface ornamentation of Eubostrychoceras valdelaxum Aiba, Yamato, Kurihara and Karasawa sp. nov. A, normal ribs in the main stage; B, slightly serrated ribs in the later stage of the paratype MCM-A1784.
Figure 6.
Suture line of Eubostrychoceras valdelaxum Aiba, Yamato, Kurihara and Karasawa sp. nov., MCM-A1785 (paratype), from the Haboro area in Hokkaido. E, external lobe; L, lateral lobe; U, umbilical lobe.
Figure 7.
Measured shell morphology of Eubostrychoceras valdelaxum Aiba, Yamato, Kurihara and Karasawa sp. nov. A, maximum height; B, minimum major axis diameter in cross-section; C, maximum major axis diameter in cross-section.
Table 1.
Measurements of the shell morphology of Eubostrychoceras valdelaxum Aiba, Yamato, Kurihara and Karasawa sp. nov.
Discussion
While Eubostrychoceras elongatum was reported from Wakayama prefecture (Misaki and Maeda, 2010), Eubostrychoceras valdelaxum sp. nov. is the first report of the genus occurring in the Campanian in Hokkaido. This species reveals that Eubostrychoceras survived until the Campanian in the northwestern Pacific realm as well as in other areas.
Phylogenetic relationships have often been estimated from the similarity of ornamentation in heteromorph ammonoids (e.g. Matsumoto, 1967, 1977; Okamoto, 1989). Three morphotypes are recognized in E. japonicum from the Turanian by differences in their shell surface ornamentation (Okamoto, 1989). The ornamentation and whorl expansion rate of E. valdelaxum sp. nov. are most similar to these of “Morphotype E” of E. japonicum (especially in the middle-later ontogenetic stage). The similarities suggest that the two species have a close phylogenetic relationship. In addition, the ribs and whorl expansion rate of E. valdelaxum sp. nov. also resemble these of E. otsukai and E. zulu in the main stage. The occurrence horizons of the type specimens of E. otsukai and E. japonicum are uncertain (Yabe, 1904). According to Matsumoto (1977), the many additional specimens from the same locality as the type specimen and from other areas in Hokkaido suggest that the occurrence horizon of E. japonicum is probably restricted to the Turanian. On the other hand, additional specimens referable to E. otsukai have not been reported from the northwestern Pacific realm, but are reported from the Campanian of Madagascar and South Africa (Collignon, 1969; Klinger and Kennedy, 2003). As the stratigraphic relationship between E. otsukai and E. valdelaxum sp. nov. is not certain, a phylogenetic relationship cannot be discussed. Klinger and Kennedy (2003) pointed out that the ornamentation and the coiling mode of E. zulu differ from these of E. japonicum in the early stage. As the characters of the early whorl are important for phylogeny (Tanabe et al., 1981), E. zulu seems to be not in the lineage of E. japonicum. Considering the above, it is most reasonable to conclude that E. valdelaxum sp. nov. evolved from E. japonicum.
The Santonian—Campanian species Eubostrychoceras elongatum and E. amapondense have lower elongated whorls with very sharp and coarse ribs. These characters of the two species are not similar to their expression in E. valdelaxum sp. nov. Therefore, E. elongatum and E. amapondense are considered to be not in a close phylogenetic relationship with E. valdelaxum sp. nov. According to Klinger and Kennedy (2003), there might be two or more lineages in the genus Eubostrychoceras. E. valdelaxum sp. nov. can be ascribed to “the Group of E. otsukai” in Klinger and Kennedy (2003).
In various Late Cretaceous heteromorph ammonoids, the coiling patterns and the shell surface ornamentation change at maturity (Eubostrychoceras; Matsumoto, 1967: Ainoceras; Matsumoto and Kanie, 1967: Otoscaphites and Scaphites; Tanabe, 1975, 1977: Nipponites and Nostoceras; Matsumoto, 1977: Pravitoceras; Matsunaga et al., 2008: Didymoceras; Misaki and Maeda, 2010). In Eubostrychoceras valdelaxum sp. nov., the mode of coiling and the shell surface ornamentation also change. One specimen (the paratype MCM-A1784) has a deviated body chamber with slightly serrated ribs (Figures 3B, 5B). The changes on the coiling patterns and shell surface ornamentation might indicate that the specimen is near the adult stage. The whorl size of the paratype, MCM-A1784 (Figure 3B), is smaller than those of the other two specimens with no deviated apertures (the holotype MCM-A1783, Figure 3A and the paratype MCM-A1476, Figure 4A), and that might indicate some variation in size at maturity.
On the other hand, dimorphism is commonly known in many ammonoid taxa (Makowski, 1962, 1971, 1991; Callomon, 1963; Guex, 1973; Tanabe, 1977; Futakami, 1990; Maeda, 1993; Davis et al., 1996; Neige et al., 1997; Zatoń, 2008; Landman et al., 2012; Klug et al., 2015; and references therein). Especially among the superfamily Turrilitoidea, dimorphism is often observed at the adult stage, with different shell diameters (Eubostrychoceras and Hyphantoceras; Kaplan and Schmid, 1988: Nostoceras and Solenoceras; Larson, 2012: Nipponites and Sciponoceras; Klug et al., 2015: and references therein). The variation of body chamber size in Eubostrychoceras valdelaxum sp. nov. might be dimorphism, but that should be investigated from the viewpoint of dimorphism by larger population samples including intact mature individuals.
Acknowledgments
We are grateful to Akihiro Misaki (Kitakyushu Museum of Natural History and Human History), Tomohiro Nishimura (Hobetsu Museum), Haruyoshi Maeda (Kyushu University), and Yasunari Shigeta (National Museum of Nature and Science) for their helpful comments to improve the manuscript. We thank Ryoji Wani (Yokohama National University) for providing critical comments on an early version of the manuscript. We also thank Naoaki Matsuda and Toshiaki Matsuda (both of Sanyo-ryokuka Co.) for helping with field work in collecting the Mikasa specimens; the late Sho-ichi Okabe, the late Tamotsu Yoshimatsu (both of the Palaeontological Society of Haboro), and Shin-ichi Mori (Furano City Ryokuho High School) for helping with field work in collecting the Haboro specimens; Kazushige Tanabe and Yusuke Takeda (both of the University of Tokyo) for collecting literature; and Kenji Ikuno (Yokohama National University) and Romain Jattiot (Universität Zürich) for providing their helpful advice.
