In this study, a new late Campanian (Late Cretaceous) heteromorph ammonoid, Didymoceras morozumii Misaki and Tsujino sp. nov., from the Hiketa Formation, Izumi Group, southwestern Japan, is described. It is characterized by a loose helical post-embryonic early whorl and relatively tight helical middle whorls, followed by a retroversal hook with a highly elevated apertural end, and two rows of irregular tubercles. Didymoceras morozumii sp. nov. occurs in the upper part of the Didymoceras sp. Zone, which is overlain by the Didymoceras awajiense Zone. Didymoceras awajiense-bearing beds of the Hiketa Formation in the study area is correlated to the lower part of that of the Toyajo Formation in the Kii Peninsula on the basis of the shell forms of D. awajiense. Didymoceras morozumii sp. nov. and D. awajiense have similar characteristics, and it is thought that D. awajiense evolved from D. morozumii sp. nov. As indicated in the descendants, some specimens of D. morozumii sp. nov. are also encrusted by anomiid bivalves.
Introduction
The Upper Cretaceous Izumi Group in southwestern Japan contains abundant molluscan fossils, and many paleontological studies have been carried out on them (Yokoyama, 1891; Yabe, 1901, 1902, 1915; Noda and Tashiro, 1973; Matsumoto and Morozumi, 1980, 1988; Matsumoto et al., 1980, 1981; Bando and Hashimoto, 1984; Morozumi, 1985; Tashiro et al., 1986, 1993; Iwaki and Maeda, 1989; Tsujino, 2004; Misaki et al., 2014; Tanabe et al., 2015, 2019; Yoshino and Matsuoka, 2016). The Izumi Group is characterized by very thick deposits, reflecting an extremely high rate of sedimentation, which provide detailed biostratigraphic information. The ammonoid and inoceramid zonation of this group by Morozumi (1985) is used as a standard biostratigraphic framework of the uppermost Cretaceous strata in the northwest Pacific region (e.g. Toshimitsu et al., 1995; Shigeta et al., 2010, 2019).
Although many nostoceratid ammonoids have been identified from the Izumi Group in the Sanuki Mountains (Asan Mountains) of northeastern Shikoku (Morozumi, 1985, 2007), not all have been the subject of detailed systematic study. Recently, biostratigraphic studies of the contemporaneous deposits of the Yezo Group in Hokkaido, northern Japan, have been performed (Shigeta et al., 2016, 2019), and ammonoids from the Sanuki Mountains are expected to be the key for more detailed stratigraphic correlation among the uppermost Cretaceous strata in the northwestern Pacific region. Phylogenetic and paleoecological information of nostoceratid ammonoids has also recently been obtained on the basis of materials from the Izumi Group and contemporaneous deposits in Japan (Misaki and Maeda, 2010; Misaki et al., 2014; Tanabe et al., 2015; Shigeta in Shigeta et al., 2019).
Many ammonoid specimens collected from the Sanuki Mountains by Ichiro Bando, Yoshihiro Kanazawa, Takeshi Kuroda, Isamu Shinohara, and Kenji Shirai are housed in the Osaka Museum of Natural History, Osaka, and the Tokushima Prefectural Museum, Tokushima. In the course of our geological survey, several nostoceratid ammonoid specimens were collected from the southeastern Kagawa Prefecture to the northeastern Tokushima Prefecture. Some of them described under Bostrychoceras sp. by Morozumi (2007) are described herein as a new species of Didymoceras, D. morozumii Misaki and Tsujino sp. nov., and the systematic relationship between this new species and D. awajiense (Yabe, 1901) from the higher horizons and the biostratigraphic and paleoecological significance of the two species are discussed.
Figure 1.
Index map (A) and route map (B) of the study area. The stratigraphic division of the Izumi Group modified after Yamasaki (1986) and the radiolarian zonation of this group reported by Hashimoto et al. (2015) are also shown in B.

Geologic notes
The Campanian–Maastrichtian Izumi Group is distributed in Shikoku, Awaji Island, and the Kii Peninsula, southwestern Japan along the Median Tectonic Line (MTL). This group is composed of fossiliferous neritic and deep-marine turbiditic deposits forming a synclinal structure with an eastward plunging axis and is thought to have been deposited within a strike-slip basin that was formed by left-lateral strike-slip movement along the MTL (Miyata, 1990; Taira et al., 1983; Yamasaki, 1986; Tanaka, 1989; Hashimoto et al., 2015).
The stratigraphy of the Izumi Group in northeastern Shikoku has been the subject of numerous studies (Nakano, 1953; Nakagawa, 1960, 1961; Suyari, 1966, 1973; Suyari et al., 1968; Yamasaki, 1986; Morinaga and Okumura, 1988; Kikuchi and Kotake, 2013; Hashimoto et al., 2015; Noda et al., 2017). The Izumi Group is generally divided into three parts: the Northern Marginal, Main, and Southern facies (Ichikawa et al., 1979; Itihara et al., 1986; Matsuura et al., 2002; Makimoto et al., 2004; Noda et al., 2017). The Northern Marginal and Main facies are distributed in the study area in the eastern part of the Sanuki Mountains (Yamasaki, 1986; Makimoto et al., 1995) (Figures 1, 2). The Northern Marginal and Main facies are interpreted as contemporaneous inter-fingering deposits (Suyari, 1973; Noda et al., 2017). Yamasaki (1986) divided the Northern Marginal Facies in northeastern Shikoku into two formations: Shiroyama and Hiketa formations, and indicated that these formations are contemporaneous heterotopic facies that consist of sandstone- and conglomerate-dominated strata and fossiliferous mudstone-dominated strata, respectively. Yamasaki (1986) also divided the Main Facies in northeastern Shikoku into the Takikubo, Horita, Higaidani, and Bandodani formations toward the upward sequence. The Main Facies mainly consists of alternating beds of sandstone and mudstone, and the Horita, Higaidani, and Bandodani formations are distributed around the study area (Figures 1, 2).
Figure 2.
Stratigraphic distribution of nostoceratid ammonoids from each locality in the study area and the correlation among the upper Campanian strata of the Izumi, Sotoizumi, and Yezo groups.

In the present study, mudstones of the Hiketa Formation were investigated along the Higaidanigawa, Minatogawa, Omigawa, and Umayadogawa rivers and the coast of Sakamoto and Nagahama in the northern foot of the Sanuki Mountains (Figure 1B). Some specimens of Didymoceras morozumii sp. nov. (Figures 3–9) were found in mudstone outcrops at Locs. S1, S6, and S9–11 (Figure 1B), and several specimens of this species were also collected from float rocks at and near these localities derived from the Hiketa Formation (Figures 3A–F, 6E, F, 9). Specimens of this new species deposited in the Osaka Museum of Natural History and the Tokushima Prefectural Museum (Figures 3G–L, 4, 5, 6A–D, 7, 8) were also collected around these localities in Nagano, Onara, Kurokawa, Omatsu, Suekuni, and Tenno (Figure 1B) by Ichiro Bando, Yoshihiro Kanazawa, Takeshi Kuroda, and the late Kenji Shirai. Some specimens of D. awajiense were collected from the higher horizons of the Hiketa Formation in Kawamata and Nagahama (Locs. S14, S15; Figure 1B). A number of D. awajiense specimens from Senzoku, Minamino, and Sakamoto, in addition to Kawamata and Nagahama (Figure 1B), are housed in the Osaka Museum of Natural History and the Tokushima Prefectural Museum (Figure 10A–E).
The Izumi Group in the study area has been correlated to the upper Campanian on the basis of ammonoids (Bando and Hashimoto, 1984; Morozumi, 1985). Didymoceras morozumii sp. nov.-bearing mudstones in Nagano, Onara, Kurokawa, Omatsu, and Suekuni and D. awajiense-bearing mudstones in Senzoku, Kawamata, Minamino, Sakamoto, and Nagahama are contained in the Didymoceras sp. and D. awajiense zones of Morozumi (1985), respectively (Figure 2). They are also contained in two different radiolarian zones: the Archaeodictyomitra lamellicostata and Clathrocyclas tintinnaeformis zones (Hashimoto et al., 2015; Figure 1B). Didymoceras sp. of Bando and Hashimoto (1984) and Morozumi (1985) (= Didymoceras sp. A of Morozumi, 2007) has been known to occur in the lower part of the Didymoceras sp. Zone around Tawakanewari (left of Figure 1B), and D. morozumii sp. nov. (= Bostrychoceras sp. of Morozumi, 2007) occurs in the upper part of this zone (Morozumi, 2007; Shigeta et al., 2016).
Figure 3.
Didymoceras morozumii Misaki and Tsujino sp. nov. from the Hiketa Formation. A–F, KMNH IvP 900071 (paratype) from a float rock at Loc. S11; A–D, lateral view; E, apical view; F, basal view; G, TKPM.GFI1625 (paratype) from Kurokawa, lateral view; H, I, OMNH MI-3162 (paratype) from Omatsu; H, lateral view; I, close-up of the fragmented shell of early growth stage; J, TKPM.GFI1626 (paratype) from Kurokawa, lateral view; K, L, TKPM.GFI1612 (paratype) from Kurokawa; K, apical view; L, lateral view. Scale bars are 20 mm.

Figure 4.
Didymoceras morozumii Misaki and Tsujino sp. nov., TKPM.GFI7123 (holotype), from the Hiketa Formation at Loc. S4. A–D, lateral view. Scale bar is 20 mm.

Figure 5.
Didymoceras morozumii Misaki and Tsujino sp. nov. from the Hiketa Formation. A–C, OMNH MI-3151 (paratype) from Kurokawa, lateral view; D, TKPM.GFI7088 (paratype) from Onara, lateral view; E, TKPM.GFI1639 (paratype), originally illustrated as Bostrychoceras sp. by Morozumi (2007), from Kurokawa, lateral view. Scale bars are 20 mm.

Figure 6.
Didymoceras morozumii Misaki and Tsujino sp. nov. from the Hiketa Formation. A, B, OMNH MI-3158 (paratype) from Kurokawa, lateral view; C, D, OMNH MI-3160 (paratype) from Loc. S3, lateral view; E, F, KMNH IvP 900072 (paratype) from a float rock at Loc. S11; E, lateral view; F, basal view. Scale bars are 20 mm.

Figure 7.
Didymoceras morozumii Misaki and Tsujino sp. nov. from the Hiketa Formation. A–C, TKPM.GFI7119 (paratype) from Loc. S11, lateral view; D, OMNH MI-3157 (paratype) from Kurokawa, dorsal side of the body chamber; E, F, OMNH MI-3164 (paratype) from Loc. S9, lateral view. A white arrow in F indicates the position of the last septum. The position of the last septa of other specimens cannot be determined due to the mode of preservation. Scale bars are 20 mm.

Figure 8.
Didymoceras morozumii Misaki and Tsujino sp. nov. with anomiid bivalves from the Hiketa Formation. A–C, OMNH MI-3152 (paratype) from Loc. S5; A, B, lateral view; C, close-up of anomiids; D–F, OMNH MI-3155 (paratype) from Loc. S3; D, E, lateral view; F, close-up of anomiids (ano3–5). Two and three anomiids (ano1, 2 of C and ano3–5 of E, F) are attached to the shell surface of these two ammonoids, and some disarticulated left valves (e.g. ano6 of E) with xenomorphic ornament presumably derived from ribs of D. morozumii sp. nov. are also preserved in OMNH MI-3155. Scale bars are 20 mm.

Figure 9.
Didymoceras morozumii Misaki and Tsujino sp. nov., KMNH IvP 900074 (paratype), from a float rock at Loc. S13 probably derived from the Hiketa Formation. A, lateral view; B, suture line of the last septum indicated by a white arrow in A; E, external (ventral) lobe; L, lateral lobe (= adventive lobe, A, of Korn et al., 2003). The straight line in B indicates the middle of the venter, although the siphuncle is not visible in this specimen. Scale bars are 20 mm.

Systematic paleontology
Institution abbreviations.—OMNH, Osaka Museum of Natural History, Osaka, Japan; TKPM, Tokushima Prefectural Museum, Tokushima, Japan; KMNH, Kitakyushu Museum of Natural History and Human History, Kitakyushu, Japan.
Other abbreviations.—Hp, total height of the preserved whorls; ap, estimated apical angle of middle whorls; h, height of a preserved late whorl; d, diameter of the same whorl; R, number of ribs on the same whorl.
Superfamily Turrilitoidea Gill, 1871
Family Nostoceratidae Hyatt, 1894
Genus Didymoceras Hyatt, 1894
Type species.—Ancyloceras? nebrascense Meek and Hayden, 1856.
Diagnosis (modified after Hyatt, 1894; Kennedy et al., 2000a; Klinger and Kennedy, 2003).—Early whorls loose and/or irregular, followed by helical middle whorls that may or may not be in contact and a retroversal body chamber. Ornamentation consists of numerous ribs and two rows of tubercles which may be irregularly developed.
Discussion.—The classification of the genus Didymoceras Hyatt, 1894 and related genera has been discussed by numerous authors (Spath, 1953; Arkell et al., 1957; Anderson, 1958; Matsumoto, 1959, 1967, 1977; Wiedmann, 1962; Jones, 1963; Howarth, 1965; Schmid and Ernst, 1975; Błaszkiewicz, 1980; Kennedy and Summesberger, 1984; Wright et al., 1996; Küchler and Odin, 2001; Klinger and Kennedy, 2003). Wright et al. (1996) treated Didymoceras as a subgenus of Nostoceras Hyatt, 1894, and some authors have agreed with this designation (e.g. Küchler and Odin, 2001; Niebuhr, 2004; Niebuhr and Jagt, 2016). However, a number of other authors treated Didymoceras as an independent genus (Kennedy and Cobban, 1999; Kennedy et al., 2000a; Klinger et al., 2007; Kruta et al., 2010; Landman et al., 2012; Shigeta in Shigeta et al., 2016, 2019). In this study, we treated Didymoceras as an independent genus. Nostoceras and Didymoceras can be distinguished, because the former has more tightly coiled helical whorls and the more abruptly turned retroversal hook with the apertural end located just below the helical part (Matsumoto, 1967; Klinger and Kennedy, 2003).
The difficulty in distinguishing Didymoceras and Bostrychoceras Hyatt, 1900 has been highlighted (Howarth, 1965; Matsumoto, 1967; Niebuhr, 2004), although the generic positions of some non-tuberculate species that had once been included in Bostrychoceras were changed to Eubostrychoceras (Matsumoto, 1967, 1977). Błaszkiewicz (1980) noted that Bostrychoceras (s.s.) has considerably irregular tubercles, in contrast to Didymoceras, which has two consistent rows of tubercles, and Klinger and Kennedy (2003), Niebuhr (2004), and Niebuhr and Jagt (2016) considered that characteristics of Didymoceras are loose and/or irregular early whorls and a distinct large retroversal hook. However, the tubercles of D. nebrascense (Meek and Hayden, 1856), the type species of Didymoceras, usually disappear on the last half of the final helical whorl (Kennedy et al., 2000a), and the tubercles of other Didymoceras species, such as D. platycostatum (Kennedy and Cobban, 1993a); D. postremum Błaszkiewicz, 1980; D. wernickei (Wollemann, 1902); and D. hidakense Shigeta in Shigeta et al., 2016, are also diminished at the part of the shell (Kennedy and Cobban, 1997; Niebuhr and Jagt, 2016; Shigeta in Shigeta et al., 2016, 2019). Additionally, B. polyplocum (Roemer, 1841), the type species of Bostrychoceras, also has loose early whorls before a relatively tight main helical growth stage (Kennedy, 1986; Kennedy and Kaplan, 1997). Conversely, the irregular early whorls of D. nebrascense are occasionally limited to quite small sizes (Kennedy et al., 2000a). The loose early whorls of D. hidakense and D. awajiense (Yabe, 1901) are also very small or almost unidentifiable, respectively (Shigeta in Shigeta et al., 2019; Morozumi, 1985), although the tiny loosely coiled early part of the latter is shown in this study (Figure 10D). Furthermore, D. binodosum (Kennedy and Cobban, 1993b) and D. wrighti Kennedy, Cobban and Scott, 2000b have relatively small retroversal hooks located near the helical whorls (Kennedy and Cobban, 1999; Kennedy et al., 2000b). The characteristics of D. morozumii sp. nov. described in this study make distinguishing Didymoceras and Bostrychoceras more difficult; D. morozumii sp. nov. has irregular tubercles and a distinct loose early whorl and contains various specimens with retroversal hooks situated closely along the helical whorls, or with large pendent retroversal hooks. Consequently, these two genera can no longer be distinguished. Therefore, it is highly probable that Bostrychoceras is a junior synonym of Didymoceras.
Figure 10.
Didymoceras awajiense (Yabe) from the Izumi Group in the study area. A–C, TKPM.GFI1678 from the Hiketa Formation in Sakamoto; A, B, lateral view; C, close-up of the body chamber encrusted by an anomiid bivalve (ano of B); D, TKPM.GFI1831 from the Hiketa Formation in Sakamoto, lateral view; E, OMNH MI-3159 from the Hiketa Formation near the Senzoku Dam site in Senzoku, lateral view; F, G, TKPM.GFI1733, originally illustrated as Didymoceras sp. B by Morozumi (2007), from the Higaidani Formation in Kanyake, lateral view. Scale bars are 20 mm.

Didymoceras morozumii Misaki and Tsujino sp. nov.
Figures 3–9
Bostrychoceras sp. Morozumi, 2007, p. 3.
Type specimens.—Holotype, TKPM.GFI7123, from Loc. S4; paratypes, two specimens, OMNH MI-3155, 3160, from Loc. S3; one specimen, OMNH MI-3152, from Loc. S5; one specimen, OMNH MI-3164, from Loc. S9; three specimens, TKPM.GFI7119, KMNH IvP 900071, 900072, from Loc. S11; one specimen, KMNH IvP 900074, from Loc. S13; one specimen, TKPM.GFI7088, from Onara; seven specimens, OMNH MI-3151, 3157, 3158, TKPM.GFI1612, 1625, 1626, 1639, from Kurokawa; one specimen, OMNH MI-3162, from Omatsu; eight specimens, OMNH MI-3151, 3152, 3155, 3157, 3158, 3160, 3162, 3164, collected by Yoshihiro Kanazawa; four specimens, TKPM.GFI1612, 1625, 1626, 1639, collected by Takeshi Kuroda; three specimens, TKPM.GFI7088, 7119, 7123, collected by the late Kenji Shirai. The holotype, TKPM.GFI7123, from an outcrop at Loc. S4; two paratypes, KMNH IvP 900071, 900072, from a float rock at Loc. S11; one paratype, KMNH IvP 900074, from a float rock at Loc. S13. It is not clear whether the other paratypes were collected from outcrops or float rocks.
Material examined.—In addition to the type specimens, the following specimens were examined. One specimen, TKPM.GFI7117, from Loc. S1; one specimen, TKPM.GFI7092, from Loc. S2; one specimen, OMNH MI-3153, from Loc. S3; two specimens, TKPM. GFI7103, 7122, from Loc. S4; one specimen, TKPM. GFI7110, from Loc. S7; one specimen, TKPM.GFI7063, from Loc. S8; three specimens, OMNH MI-3154, TKPM. GFI7070, KMNH IvP 900073, from Loc. S9; one specimen, TKPM.GFI7073, from Loc. S11; one specimen, TKPM.GFI7120, from Loc. S12; one specimen, TKPM. GFI7047, from Loc. S13; 23 specimens, OMNH MI-3156, 3161, TKPM.GFI1610, 1611, 1614, 1618, 1620–1623, 1627, 1722, 1723, 1727, 1829, 5002–5007, 5495, 7116 from Kurokawa; one specimen, OMNH MI-3163, from Omatsu; one specimen, TKPM.GFI5495, collected by Ichiro Bando; five specimens, OMNH MI-3153, 3154, 3156, 3161, 3163, collected by Yoshihiro Kanazawa; 13 specimens, TKPM.GFI1610, 1611, 1614, 1618, 1620–1623, 1627, 1722, 1723, 1727, 1829, collected by Takeshi Kuroda; 16 specimens, TKPM.GFI5002–5007, 7063, 7070, 7073, 7092, 7103, 7110, 7116, 7117, 7120, 7122, collected by the late Kenji Shirai. Two specimens, TKPM.GFI7103, 7122, from an outcrop at Loc. S4; one specimen, KMNH IvP 900073, from an outcrop at Loc. S9. It is not clear whether the other specimens were collected from outcrops or float rocks.
Diagnosis.—Didymoceras with a loose helical early whorl and relatively tight helical middle whorls just touching each other, followed by a retroversal hook, which is located right around the helical part or considerably pendent from it. Apertural end reaches the level of the last or penultimate helical whorl or higher. Ornamentation consists of numerous relatively simple ribs and two rows of irregular tubercles, which are often obscure except for the very early growth stage.
Etymology.—Named after the paleontologist Yoshiro Morozumi in recognition of his significant contribution to the studies of Cretaceous ammonoids from the Izumi Group.
Description.—The holotype, TKPM.GFI7123 (Figure 4) consists of about two volutions of relatively tight helical middle whorls and a retroversal hook, which begins at about 6 cm in diameter. The retroversal hook is located right around the helical part, and the aperture reaches the level of the next to last helical whorl. The shell is ornamented with numerous simple ribs, and they become slightly coarser on the retroversal hook. Two rows of weak and sparse tubercles are developed on helical middle whorls, and they become frequent on the retroversal hook.
The paratypes (Figures 3, 5–9) and other material examined contain many specimens of different growth stages, and show considerable intraspecific variation. Embryonic shell (ammonitella) and earliest post-embryonic whorls less than 2 mm in whorl height are not preserved in the present material. In the early growth stage (less than 2–4 cm in diameter), the preserved whorl is loosely helically coiled and does not touch the succeeding whorl. In the middle growth stage, the whorls just touch each other and form a relatively tightly coiled helical shell with an apical angle of around 30–50°. There is large variation in the timing of the transition from the early to the middle growth stage. For example, one of the paratypes, TKPM. GFI1625 is tightly coiled at about 2 cm in diameter (Figure 3G), and the other paratype, TKPM.GFI1612 is loosely coiled at about 4 cm in diameter (Figure 3K, L). In the later growth stage (over 5–10 cm in diameter), the later part of the body chamber protrudes from the helix and strongly curves upward around it, forming a retroversal hook. In many cases, the retroversal hook does not depart from the helix significantly (Figures 4, 5, 6A, B, 8, 9); however, the retroversal hooks of some specimens are somewhat pendent (Figure 6C, D) and occasionally form distinct U-shaped hooks (Figure 7A–C, E, F). The aperture of the retroversal hook reaches the level of the last or next to last helical whorl of the middle growth stage or higher.
The ornamentation consists mainly of numerous relatively simple ribs and irregular tubercles. In the early growth stage, ribs are almost straight and are nearly at right angles to the growth direction. In the middle growth stage, ribs are rursiradiate on the lower whorl face, are generally slightly sinuous (concave at the upper side and convex at the lower side), and are generally inclined adaperturally from the upper side toward the lower side on the outer whorl face. On the inner side of the helical whorls, ribs become weaker. In the later growth stage, ribs are often inclined adapically from the dorsal side toward the ventral side on the retroversal hook. At the beginning of the retroversal hook, the succeeding ribs obliquely cut the previous ribs in some specimens (e.g. TKPM.GFI1639, 5005; Figure 5E). The second or third rib from the apertural end is generally stronger than other ribs. On the dorsal side of the retroversal hook, ribs become weaker and narrower, and convex adaperturally (Figure 7D), and also frequently branch. Although the branching and intercalation of ribs are rare throughout all growth stages, except on the dorsal side of the retroversal hook, these features are observed in some specimens (e.g. TKPM.GFI1620, 7117, OMNH MI-3157, 3160, 3161). For example, ribs rarely branch or intercalate at the lower part of the outer side of the helical whorls toward the upper side and at the flank of the retroversal hook toward the ventral side. In some cases, branching of ribs occurs at the tubercle. Tubercles present on all ribs on the upper and lower two rows in the middle of the outer whorl face in the very early growth stage (less than several millimeters in whorl height). Then, upper and lower rows become located near the mid-line and lower shoulder of the outer side of the helical whorls, respectively, and tubercles become obscure and sparse in many cases. In some specimens (TKPM.GFI1622, 7119, 7123, OMNH MI-3152, 3154, 3157, 3164), two rows of tubercles become stronger again on the ventral side of the retroversal hook. Constrictions are rarely observed (Figure 6E, F).
The whorl section is circular to elliptical. Both dextral and sinistral specimens exist. Among the 55 examined specimens, 27 are dextral and 25 are sinistral, except for three fragmentary specimens.
The position of the last septum is located at about three-fourths volutions before the beginning of the retroversal hooks in OMNH MI-3164 (Figure 7E, F) and KMNH IvP 900074 (Figure 9), although it cannot be determined for most specimens because of the insufficient mode of preservation. The suture is finely and deeply incised (KMNH IvP 900074; Figure 9). The first lateral saddle is deeply bifid. The lateral lobe is expanded and deeper than the external lobe, and has a narrow base.
Measurements.—TKPM.GFI7123 (holotype): Hp = 90.3 mm, ap = 42°, h = 28.6 mm, d = 59.2 mm, R = ca. 75 (number of ribs at the area covered by the host rock was estimated); TKPM.GFI1639 (paratype): Hp = 96.6 mm, ap = 40°; TKPM.GFI7119 (paratype): Hp = 98.7 mm; OMNH MI-3152 (paratype): Hp = 87.6 mm, ap = 45°; OMNH MI-3158 (paratype): Hp = 132.3 mm; KMNH IvP 900071 (paratype): Hp = 47.9 mm, h = 20.8 mm, d = 52.6 mm, R = 89; KMNH IvP 900072 (paratype): Hp = 48.4 mm, ap = 49°, h = 23.8 mm, d = 61.8 mm, R = 90.
Comparison.—Didymoceras morozumii sp. nov. is easily distinguished from many other species of Didymoceras by its relatively tight helical whorls that just touch each other in the middle growth stage and by the strongly recurved retroversal hook with the aperture reaching the level of the helical part. The present new species is similar to D. awajiense (Yabe, 1901) in having relatively tight helical whorls just touching each other in the middle growth stage, though the former has a much looser helical whorl in the early growth stage. Pendent U-shaped hooks of some specimens of D. morozumii sp. nov. (Figure 7A–C, E, F) are also similar to those of D. awajiense; however, the apertural ends of the former recurve higher, reaching the side of the helical part. In addition, D. awajiense has more distinct tubercles than the present new species throughout all growth stages.
D. binodosum (Kennedy and Cobban, 1993b) is similar in the overall shell shape to that of D. morozumii sp. nov. (Kennedy and Cobban, 1999; Kennedy et al., 2000b), though the former has coarser and more frequently branching or looped ribs and stronger tubercles. In addition, D. morozumii sp. nov. generally has a higher elevated apertural end than D. binodosum.
Didymoceras polyplocum (Roemer, 1841) (= Bostrychoceras polyplocum (Roemer, 1841)) shows a considerable range of variation (Kennedy, 1986; Kennedy and Kaplan, 1997), and some specimens of the species are similar to those of D. morozumii sp. nov.; however, the loose helical early whorl of D. morozumii sp. nov. departs more widely from the succeeding whorl, and more abruptly changes into the relatively tight helical middle whorls. In addition, the apertural part of D. morozumii sp. nov. protrudes more strongly laterally, and recurves higher.
Masukawa and Ando (2018) described a specimen from the mudstone underlying the D. awajiense-bearing beds of the Hiraiso Formation, Nakaminato Group, in central Japan as Didymoceras sp. This specimen differs from any examined specimen of D. morozumii sp. nov. by having much stronger tubercles and more frequently branching ribs.
Remarks.—Oblique crossings of ribs at the beginning of the retroversal hook are observed in some specimens of the present new species (Figure 5E). Because D. nebrascense (Meek and Hayden, 1856) and D. sanctaeluciense Klinger, 1976 also show similar characteristics (Kennedy et al., 2000a, fig. 9B; Klinger and Kennedy, 2003, figs. 13, 15), oblique crossings of ribs often seem to occur in connection with the change of the torsion of shell at the beginning of the retroversal hook in some species of Didymoceras.
Occurrence.—Described specimens were collected from the upper part of the Didymoceras sp. Zone (upper Campanian) and float rocks probably derived from the same biozone in the Hiketa Formation, Izumi Group in northeastern Shikoku.
Discussion
Biostratigraphic implications
This study recognized that Didymoceras morozumii sp. nov. and D. awajiense occurred in the western area around Nagano, Onara, Kurokawa, Omatsu, and Suekuni and the eastern area around Senzoku, Kawamata, Minamino, Sakamoto, and Nagahama (Figure 1B), respectively. Although the specimens of the latter species from the eastern area were treated as D. cf. awajiense by Morozumi (2007), they can be identified as D. awajiense showing a wide intraspecific variation, on the basis of the analysis of specimens deposited in the Osaka Museum of Natural History and the Tokushima Prefectural Museum, as well as some specimens collected in this study. Consequently, the boundary between the zones of Didymoceras sp. and D. awajiense in the Izumi Group is thought to be located between these western and eastern areas near the center of the study area and seems to correspond approximately to the boundary between the two radioralian zones, Archaeodictyomitra lamellicostata and Clathrocyclas tintinnaeformis zones (Hashimoto et al., 2015; Figure 1B).
Because Didymoceras sp. described by Bando and Hashimoto (1984) and Morozumi (1985) (= Didymoceras sp. A of Morozumi, 2007) from the lower part of the Didymoceras sp. Zone is similar to D. hidakense from the Baculites subanceps beds of the Chinomigawa Formation, Yezo Group, in Hokkaido, the lower part of the Didymoceras sp. Zone is correlated to the B. subanceps beds (Shigeta et al., 2016, 2019; Figure 2). The Baculites rex beds overlying the B. subanceps beds of the Chinomigawa Formation were correlated to the Didymoceras sp. Zone by Shigeta et al. (2019), but the detailed stratigraphic relationship between the D. morozumii sp. nov.-bearing beds of the upper part of the Didymoceras sp. Zone and B. rex beds has not yet been clarified.
The upper part of the D. awajiense-bearing beds of the Toyajo Formation, Sotoizumi Group, around Mt. Toyajo in the Kii Peninsula (Figure 1A) contains relatively planar specimens of this species. It was correlated to the lower part of the Seidan Formation, Izumi Group, in Awaji Island, from which some specimens of D. awajiense with relatively low helical whorls have been collected (Misaki and Maeda, 2009, 2010; Figure 2). Although more than 15 D. awajiense specimens have been collected around Senzoku, Kawamata, Minamino, Sakamoto, and Nagahama, specimens with relatively planar forms or relatively low helical whorls were not found there (Figure 10A, B, E). In addition, the strata exposed at these localities are thought to be the lowermost part of the D. awajiense Zone of the Izumi Group. Therefore, they are correlated to the lower part of the D. awajiensebearing beds of the Toyajo Formation. Shigeta et al. (2017) reported the radiometric age of the tuff in this part (= lowest part of the Hasegawa Muddy Sandstone Member) as 72.4 ± 0.8 Ma (95% conf.).
Specimens of D. awajiense collected by Ichiro Bando and the late Isamu Shinohara from the Higaidani Formation at Kanyake in the study area (bottom of Figure 1B) are also kept in the Tokushima Prefectural Museum (Figure 10F, G). These specimens were treated as D. cf. awajiense and Didymoceras sp. B by Morozumi (2007); however, this study clearly demonstrates that they are identified as D. awajiense showing a wide range of individual variation. Bando and Hashimoto (1984) reported D. awajiense from the Bandodani Formation at Bando in the study area (right of Figure 1B). Although D. awajiense specimens from Kanyake and Bando do not have relatively planar forms and relatively low helical whorls, a sufficient number of specimens have not been collected there. On the other hand, the D. awajiense-bearing strata in Kanyake and Bando seem to be above the Hiketa Formation around Senzoku, Kawamata, Minamino, Sakamoto, and Nagahama, in view of the geological structure and geographical position. In addition, Pravitoceras sigmoidale Yabe, 1902, which is a representative of the P. sigmoidale Zone, has been reported from the eastern part of Bando (Matsumoto et al., 1981). Therefore, the strata exposed at these localities may be correlated to the relatively upper part of the lower part of the D. awajiense-bearing beds of the Toyajo Formation or the upper part of them.
Masukawa and Ando (2018) reported some specimens of D. awajiense from the Units Hl2 and Hl3 of the Hiraiso Formation, Nakaminato Group in central Japan and documented that a specimen from the Unit Hl3 was somewhat similar to relatively planar specimens of this species from the Toyajo Formation. This fossil evidence may suggest that the Unit Hl2 of the Hiraiso Formation is correlated to the Hiketa Formation around Senzoku, Kawamata, Minamino, Sakamoto, and Nagahama and the lower part of D. awajiense-bearing beds of the Toyajo Formation, although more specimens from the Hiraiso Formation are needed to confirm this interpretation. On the other hand, specimens similar to those of Didymoceras sp. reported from the Unit Hl1 of the Hiraiso Formation by Masukawa and Ando (2018) have not been found from the Izumi Group, and the stratigraphic relationship between the Unit Hl1 of the Hiraiso Formation and the Izumi Group is to date not clarified.
Relationship between Didymoceras morozumii sp. nov. and Didymoceras awajiense
Didymoceras morozumii sp. nov. occurs in mudstones just below the horizons containing D. awajiense in the study area (Figure 2), and the former is similar to the latter in having relatively tight helical middle whorls that just touch each other. They also have fairly similar ornamentation. In addition, some specimens of D. morozumii sp. nov. with distinct pendent U-shaped hooks (Figure 7A–C, E, F) are very similar to specimens of D. awajiense. In contrast to D. morozumii sp. nov., with a loose helical early whorl, D. awajiense is thought to have tight helical whorls from very early growth stages (Morozumi, 1985). However, a well-preserved specimen of the early whorls of D. awajiense collected from Sakamoto by Takeshi Kuroda (Figure 10D) has a loosely coiled half whorl in the very early growth stage (less than 4 mm in diameter). These observations suggest a close systematic relationship between these two species; namely, D. awajiense possibly evolved from D. morozumii sp. nov.
Encrustation by anomiid bivalves
Attached anomiid bivalves were observed on both D. morozumii sp. nov. and D. awajiense from the study area (Figures 8, 10A–C). Many disarticulated left valves with xenomorphic ornament presumably derived from ribs of these two Didymoceras species were also observed in the field and in museum collections (Figure 8E), and it is thought that they were originally frequently encrusted by anomiids. It has been suggested that D. awajiense and its probable descendant, P. sigmoidale, were colonized by anomiids when these ammonoids were still alive (Misaki et al., 2014). This commensal relationship between ammonoids and anomiids might continue to persist in descendants during the course of evolution from D. morozumii sp. nov. to P. sigmoidale via D. awajiense.
The encrustation of anomiids has also been reported on D. hidakense from the lower upper Campanian strata of the Yezo Group in Hokkaido (Shigeta in Shigeta et al., 2019). Specimens of D. polyplocum from the upper Campanian strata in northern Germany and Diplomoceras cylindraceum (Defrance, 1816) from the upper Maastrichtian strata in the Bay of Biscay region of France, illustrated by Kennedy (1986, text-fig. 33A) and Ward and Kennedy (1993, fig. 43.10), respectively, seem to contain sessile organisms with similar characteristics to xenomorphic ornaments derived from ribs of these ammonoids. The encrustation of anomiids might occur on many species of Didymoceras and other heteromorph ammonoids in the Campanian–Maastrichtian age.
Acknowledgments
We wish to thank Kiyoshi Kawabata, Shoji Hayashi, Yoshihiro Tanaka, and Tomohiro Nishimura for their help in accessing museum collections; Haruyoshi Maeda for his help in accessing references; Ichiro Bando, Hisao Hashimoto, Yoshihiro Kanazawa, Takeshi Kuroda, the late Isamu Shinohara, and the late Kenji Shirai for collecting and donating specimens; and Yasunari Shigeta and two anonymous reviewers for their valuable comments on the first draft. This study was supported in part by the Sasakawa Scientific Research Grant of the Japan Science Society and JSPS KAKENHI Grant Numbers JP25800290, JP16K16346, JP17H02028, and JP19K04063.