A new genus and species of the gastropod family Capulidae, Vermeijia japonica, is described from the upper Pliocene Kuwae Formation in Niigata Prefecture and the lowermost part of the Sasaoka Formation in Akita Prefecture, Japan. This genus is similar to the boreal genus Ariadnaria although it was collected in association with several warm-water species. In another gastropod family, Nassariidae, Cyllene satoi is a new species from the upper Pliocene Tentokuji Formation in Akita Prefecture. Nine warm-water taxa in the Omma-Manganji fauna, including Cyllene, no longer live in the Japan Sea, except for its westernmost part. They suggest that the Tsushima Current had a higher SST during the late Pliocene than at present. Vermeijia is the fourth extinct genus of the Omma-Manganji fauna, but it disappeared in the Japan Sea by Datum A (2.75 Ma), earlier than the other three genera, which became extinct by the end of the early Pleistocene.
Introduction
During the early Pliocene to early Pleistocene, many endemic species of mollusks originated in the semi-closed Japan Sea (Chinzei, 1978). The fauna is known as the Omma-Manganji fauna (Otuka, 1939) and the taxonomy of its mollusks has been studied by many authors (Kaseno and Matsuura, 1965; Ogasawara, 1977, 1986; Kitamura and Kondo, 1990; Amano, 2001, 2004, 2007). The Omma-Manganji fauna mainly consists of shallow cold-water species as well as endemic extinct species. However, it contains some warm-water species, indicating inflow of the Tsushima warm current (e.g. Ogasawara, 1986). During the late Pliocene, many kinds of shallow warm-water taxa were recovered from the upper Pliocene deposits along the Japan Sea, from the Tentokuji Formation and the lowermost part of the Sasaoka Formation in Akita Prefecture, the Kuwae and Shitoka formations in Niigata Prefecture, and the Mita and Zuwaka formations in Toyama Prefecture (Amano et al., 2000a, b, 2008, 2009, 2011, 2012). Thus, in the late Pliocene, the warm Tsushima current flowed into the Japan Sea and reached as far north as Akita Prefecture, northern Honshu.
Two new gastropods have been collected from the Tentokuji Formation, the Kuwae Formation and the lowermost part of the Sasaoka Formation. One of them is the capulid Vermeijia japonica gen. and sp. nov. and the other is Cyllene satoi sp. nov. Both genera were collected from assemblages that included warm-water species. Among them, Cyllene is now widely distributed in the Indo-Pacific region, but is confined to the westernmost part of the Japan Sea (Higo et al., 1999). In this paper, I describe these gastropods and discuss their biogeographical significance.
Material and methods
One specimen of Vermeijia japonica gen. and sp. nov. was recovered from Loc. 2 by Amano et al. (2000a) (Loc. 3 in Figure 1) in the upper Pliocene Kuwae Formation in Niigata Prefecture and another one from Loc. TH1 by Amano et al. (2011) (Loc. 1 in Figure 1) in the upper Pliocene part of the Sasaoka Formation in Akita Prefecture. Based on the associated molluscan fossils, both formations were deposited in lower sublittoral depths. However, in the case of the Kuwae Formation, the fossils reveal that the occurrence is allochthonous, in contrast with their autochthonous occurrence in the Sasaoka Formation (see Amano et al., 2000a, 2011). The new genus possibly lived in water shallower than the lower sublittoral zone.
Six specimens of Cyllene satoi sp. nov. were collected from Loc. 21 by Amano et al. (2000b) (Loc. 2 in Figure 1) in the upper Pliocene Tentokuji Formation in Akita Prefecture. The occurrence at this locality is allochthonous. Many shallow-water gastropods were carried down to upper bathyal or lower sublittoral depths (Amano et al., 2000b).
For proposing these new taxa, the following specimens were also examined; Trichotropis planicostata Yokoyama, UMUT CM 20192 (holotype; Loc. Koshiba, Kanagawa Prefecture; the lower Pleistocene Koshiba Formation), NMNS PM25264 (Loc. Kamataki, Kimitsu City, Chiba Prefecture; the middle Pleistocene Ichijuku Formation), NMNS PM28601 (collected by Baba, 1990 from Higashi-Hikasa, Kimitsu City, Chiba Prefecture; the lower Pleistocene Kiwada Formation); Cyllene pulchella Adams and Reeve, IGPS no. 55136 (Loc. Ananai, Kochi Prefecture; the lowest Pleistocene Ananai Formation); Cyllene lugubris Adams and Reeve, IGPS nos. 53912, 53913, 53914, 53915, 53916, 53917, 53918, 53919 (the lower Pleistocene Byoritsu Beds in Taiwan); Cyllene rubulolineata Sowerby, JUE no. 16068 (Loc. off Minabe, Wakayama Prefecture, 80–100 m in depth).
I measured and counted the following characters of the fossils: shell height, height of the spire, maximum diameter, and the number of axial ribs and spiral cords on the penultimate and last whorls. All specimens representing the new taxa are stored at the University Museum of the University of Tokyo. Abbreviations of institutions: IGPS, Institute of Geology and Paleontology, Faculty of Science, Tohoku University; JUE, Joetsu University of Education; NMNS, National Museum of Nature and Science, Tsukuba; UMUT, University Museum of the University of Tokyo.
Systematic descriptions
Family Capulidae Fleming, 1822
Genus Vermeijia gen. nov.
Etymology.—The present new genus is named for Geerat J. Vermeij (University of California at Davis) who has contributed extensively to the taxonomy of gastropods.
Type species.—Vermeijia japonica sp. nov.
Diagnosis.—Shell moderate-sized, rather thin, globose; spire very low; surface of spire whorls cancellate with a few spiral cords and fine axial ribs; surface of last whorl sculptured with spiral costae; aperture very wide, posterior end located above deep suture between last and penultimate whorls; inner lip covered by callus with one weak fold in its anterior part; outer lip thin; umbilicus narrow, with weak fasciole; anterior canal very short and narrow.
Included species.—Other than the type species, Trichotropis planicostata Yokoyama, 1920 is included in this genus. Trichotropis planicostata was originally described by Yokoyama (1920) from the lower Pleistocene Koshiba Formation in Kanagawa Prefecture, based on one young specimen. Baba (1990) described and illustrated the adult specimen having a rapidly expanding last whorl from the lower Pleistocene Umegase Formation in Chiba Prefecture (Figure 2.3). Nemoto and O'Hara (2005) illustrated one specimen which has a rapidly expanding last whorl as T. insignis (Middendorff) from the lower Pleistocene Tomioka Formation in Fukushima Prefecture. Adding one specimen (Figure 2.4) from the middle Pleistocene Ichijuku Formation in Chiba Prefecture and stored at the National Museum of Nature and Science, Tsukuba, this species has been recorded from lower to middle Pleistocene deposits on the Pacific side of central Honshu.
Remarks.—The present genus has a unique and enigmatic form. However, despite lacking the protoconch, the close similarity to species of Ariadnaria Habe, 1961, Neoiphinoe Habe, 1978, and Trichotropis Broderip and Sowerby, 1829 demonstrates that it should be classified in Capulidae Fleming, 1822.
Ariadnaria is the most similar to Vermeijia gen. nov. It is similar in having a low spire, a large last whorl, distinct cancellate sculpture which is observed in the early whorls of the new genus, some strong spiral costae, a fold on the inner lip, a narrow umbilicus, a narrow fasciole and a short siphonal canal. However, the thin shell, rapidly expanding last whorl, and deep suture between the penultimate and last whorls of the present new genus easily enable us to separate the two genera.
Neoiphinoe is another similar genus to the present new genus in having a rather low spire, a large last whorl and some strong spiral costae in some species such as N. kryoyeri (Philippi, 1849) illustrated by Kantor and Sysoev (2006). However, it differs from the new genus by having a wide and distinct fasciole, a lower spire, a larger body whorl, and a very weak fold at the inner lip.
Trichotropis is another similar genus to the present new genus. It shares its strong spiral costae, rather deep suture, one weak fold on the inner lip, narrow fasciole and short siphonal canal. However, Trichotropis has a higher spire and a smaller aperture than in Vermeijia gen. nov.
Figure 2.
Vermeijia japonica gen. et sp. nov. and V. planicostata (Yokoyama, 1920). 1, 2, Vermeijia japonica gen. et sp. nov.; 1a, b, paratype, UMUT CM 32796; 1a, internal mold; 1b, outer cast (silicone rubber), Loc. 1; 2a–e, holotype, UMUT CM 32795; 2a, apertural view; 2b, apical view; 2c, enlargement of apical view; 2d, enlargement of early whorls from abapertural side; 2e, abapertural view, Loc. 3; 3, 4, Vermeijia planicostata (Yokoyama, 1920); 3, NMNS PM28601 abapertural view; 4, NMNS PM25264, apertural view. All scale bars show 5 mm.
Vermeijia japonica sp. nov.
Figures 2.1, 2.2
Ariadnaria insignis (Middendorff). Ogasawara et al., 1986, pl. 37, fig. 3a, b.
Etymology.—The present new species is named for the country of Japan.
Type specimens.—Holotype, UMUT CM 32795; paratype, UMUT CM 32796.
Type Locality.—River bank at 1.3 km upstream along the Koide River, Shibata City, Niigata Prefecture (Loc. 3 in Figure 1 = Loc. 2 of Amano et al., 2000a); Kuwae Formation.
Material.—Two specimens consisting of the holotype and the paratype. The paratype specimen was collected from the Sasaoka Formation at 2 km upstream along the Ogurosawa River, Akita City in Akita Prefecture (Loc. 1 in Figure 1 = Loc. TH1 of Amano et al., 2011).
Dimensions.—Holotype, UMUT CM 32795, shell height = 27.1 mm, maximum diameter = 26.3 mm, height of spire = 3.6 mm; paratype, UMUT CM 32796, shell height = 19.2 mm, maximum diameter = 17.3 mm, height of spire = 4.0 mm (a natural internal mold, and a silicone rubber cast of the exterior).
Diagnosis.—Shell moderate in size for Capulidae, rather thin and globose; spire very low, three spire whorls coiled at slight angle to last whorl; surface of penultimate whorl sculptured with three spiral cords and fourteen fine axial ribs forming nodes at their intersections; surface of last whorl sculptured with six sharp spiral costae, each narrower than an interspace; aperture very wide, posterior end located highly above suture between last and penultimate whorls.
Description.—Shell moderate in size for Capulidae, up to 27.1 mm in height, rather thin, fragile, globose (maximum diameter = 26.3 mm). Teleoconch whorls four, with large last whorl and very low spire (spire height/shell height = 0.13); spire whorls with coiling axis at slight angle to that of last whorl; protoconch not preserved. First whorl sculptured with two strong spiral cords with distinct growth lines; second whorl sculptured with one weak and two strong spiral cords, uppermost weakest, and 19 fine axial ribs forming cancellate sculpture; penultimate whorl sculptured with three spiral cords with fourteen fine axial ribs forming nodes at their intersections, lowermost cord buried below deep suture between penultimate and last whorls; last whorl sculptured with six sharp, narrow spiral costae with interspaces each crenulated wider and flatter than one costa, interspaces crossed by very weak, fine axial ribs and a few very low secondary spiral cords. Suture between last and penultimate whorls very deep and wide, but other sutures shallower. Aperture very wide, semi-lunular; posterior end forming vertical angle with inner lip and located above suture between last and penultimate whorls; most basal part of inner lip slightly broken but mostly covered by thin callus, with one weak fold on its anterior part; outer lip thin between triangular cross-sections of exterior spiral costae; false umbilicus very narrow, moderately deep, with weak fasciole; siphonal canal weakly defined, very short and narrow.
Remarks.—Ogasawara et al. (1986) illustrated a small specimen of Ariadnaria insignis (Middendorff) from the Sasaoka Formation at Loc. S15, about 3 km southeast from Loc. TH1 by Amano et al. (2011) (Loc. 1 in Figure 1). However, the illustrated specimen has a rapidly expanding last whorl, three spiral cords with fine axial ribs on the penultimate whorl, and six spiral costae on the last whorl. These characteristics are shared with Vermeijia japonica sp. nov.
Vermeijia planicostata (Yokoyama, 1920) differs from V. japonica sp. nov. by having a slightly smaller shell (maximum height = 21.2 mm +), spire whorls coiled at the same angle to the last whorl, more numerous (4) spiral cords on the penultimate whorl, lower and more numerous spiral cords (8 to 13) on the last whorl which become obscure above the shoulder.
Ariadnaria hirsuta (Golikov and Gulbin, 1978) living in the middle Kurile Islands (5–285 m in depth; Golikov et al., 2001; Kantor and Sysoev, 2006) is also similar to the present new species. It has a large last whorl, spire whorls coiled at a slight angle to the last whorl, and seven (12 by the original description) strong spiral costae with growth lines on the last whorl. Ariadnaria hirsuta differs from Vermeijia japonica sp. nov. in its much smaller size (9.8 mm in shell height) and smaller aperture.
Ariadnaria insignis (Middendorff, 1848) is another similar species living in the northern Japan Sea, the Okhotsk Sea, the Kurile Islands and the Bering Sea (0–100 m in depth; Golikov et al., 2001). It has a low spire, a large last whorl and a large aperture. However, the surface of the last whorl of A. insignis is covered with cancellate sculpture, which is observed in younger whorls in Vermeijia japonica sp. nov. Moreover, the more numerous and lower spiral cords of A. insignis enable us to separate it from the present new species.
Neoiphinoe kroyeri (Philippi, 1849) is yet another similar species living in the southern Okhotsk Sea, Bering Sea, Barents, Kara and Laptev seas (22–150 m in depth; Kantor and Sysoev, 2006). It has a low spire, a large body whorl and some strong costae crossed by distinct growth lines. However, N. kroyeri has a smaller body whorl with a lower spire, a wider and distinct fasciole, a narrow posterior sinus of aperture, a shallower suture between the body and the penultimate whorls and a weaker fold at the inner lip.
Torellia pacifica Okutani, 1980 also has a low spire and strong costae on the last whorl. However, T. pacifica has regularly coiled whorls, a smaller aperture and a wider siphonal canal than the present new species.
Stratigraphic and geographic distribution.—Upper Pliocene: Kuwae Formation in Niigata Prefecture and lowermost part of Sasaoka Formation in Akita Prefecture.
Family Nassariidae Iredale, 1916
Subfamily Cylleninae Bellardi, 1882
Genus Cyllene Gray in Griffith and Pidgeon, 1834
Type species.—Cyllene owenii Gray in Griffith and Pidgeon, 1834.
Cyllene satoi sp. nov.
Figures 2.1–2.3
Cyllene aff. pulchella Adams and Reeve. Amano et al., 2000b, pl. 1, figs. 4, 9.
non Cyllene aff. pulchella Adams and Reeve. Amano et al., 2000b, pl. 1, figs. 10, 11.
Etymology.—The present new species is named for Tokiyuki Sato (Akita University), who contributed to the age assignment of the Plio-Pleistocene formations in the Japan Sea area by calcareous nannofossils.
Type specimens.—Holotype, UMUT CM 32797; paratype, UMUT CM 32798.
Type Locality.—Riverside cliff at 0.5 km downstream along the Yodo River, Kyowa-Funaoka-Awasegai, Daisen City, Akita Prefecture (Loc. 2 in Figure 1 = Loc. 21 of Amano et al., 2000b).
Material.—Six specimens treated here were collected only from the type locality.
Dimensions.—Holotype, UMUT CM 32797, shell height = 12.8 mm, maximum diameter = 7.5 mm, height of spire = 2.1 mm; paratype, UMUT CM 32798, shell height = 12.6 mm, maximum diameter = 7.9 mm, height of spire = 1.8 mm.
Diagnosis.—Small species of Cyllene with low spire, well inflated last whorl sculptured with 14–18 low spiral cords and 20–22 low axial ribs. Outer lip sculptured with 11–12 crenulations. Fasciole thick, with 5–11 wrinkles.
Description.—Shell small, up to 12.8 mm in height, consisting of four teleoconch whorls, ovately conical, well inflated (maximum diameter/shell height = 0.59–0.63), spire low, last whorl large (height of spire/shell height = 0.14–0.16); protoconch not preserved. Suture deep; subsutural cord relatively wide, almost flat, bounded by moderately deep groove, then by three shallower grooves and ridges. Penultimate whorl ornamented with six to eight fine spiral cords including flat subsutural cord divided by deep grooves, and with 20–22 rounded, straight axial ribs forming nodes at intersections with spiral cords. Spiral cords on last whorl two above deep subsutural groove, five below this groove and 11 on basal part. Last whorl also sculptured with 23–32 weak axial ribs near suture between penultimate and last whorls. Aperture lenticular; anal sinus slit-like; inner lip covered with thin callus on posterior half but becoming thick towards basal part, with five to 11 folds; outer lip rather thick, with 11–12 short transverse ridges on interior; siphonal canal short, twisted; fasciole rather large, separated by a sharp, narrow ridge and deep groove from base of last whorl.
Remarks.—Amano et al. (2000b) illustrated this new species as Cyllene aff. pulchella with the earliest Pleistocene species from the Ananai Formation in Kochi Prefecture. This species was listed as C. pulchella Adams and Reeve by Nomura (1937) and described as Strigitella cf. decurtata Reeve by Okumura and Takei (1993). However, the Ananai species (Amano et al., 2000b, pl. 1, fig. 10, 11; IGPS 55136) can be identified as C. gracilenta (Yokoyama, 1928) because it has a smooth surface of the last whorl without any spiral grooves near the suture or on the basal part. Exceptionally one small specimen from the Ananai Formation (Figure 2.3) has ten axial ribs, which is similar to the holotype of C. gracilenta.
Cyllene gracilenta was proposed for the specimen from the Pliocene? Lower Byoritsu Bed in Taiwan as a species of Cassis by Yokoyama (1928). Later, it was described from the Pliocene Yonabaru Clay in Okinawa Prefecture by MacNeil (1960). He also synonymized C. lugubris Adams and Reeve, 1850 described by Nomura (1935) from the Byoritsu Beds in Taiwan with C. gracilenta. Hu (1992) described C. lugubris from the Pleistocene Tongxiao Formation in Miaoli (= Byoritsu) Province, Taiwan. This species can be classified into C. glacilenta, judging from its shell surface sculpture.
Cyllene satoi sp. nov. is most similar to this extinct species C. gracilenta in having a similar shell outline. C. satoi sp. nov. can be distinguished from C. gracilenta by having numerous axial ribs near the suture between the penultimate and the last whorls (23–32 in C. satoi; 0–15 in C. gracilenta; Figures 3.2c, 3.4c). Moreover, C. satoi sp. nov. has eighteen distinct spiral cords on the whole surface of the last whorl while C. gracilenta has a smooth and shiny surface in the middle part of the last whorl.
The Recent species Cyllene rubrolineata Sowerby, 1870 resembles the present new species. Cernohorsky (1984) illustrated the holotype of this species and described it as distributed only in Taiwan. However, Tsuchiya (2017) recorded this species from Sagami Bay, central Honshu and southward. Fifteen specimens of C. rubrolineata at hand from off Minabe, Wakayama Prefecture, southwest Honshu (80–100 m in depth) have seven, less inflated whorls (maximum diameter/shell height = 0.48–0.59, rarely to 0.61 in C. rubrolineata), a higher spire (height of spire/shell height = 0.16–0.24), and less numerous axial ribs on the last whorl (commonly 17).
Cyllene pulchella Adams and Reeve, 1850 is now living in the Red Sea to India, Japan and the New Hebrides (Cernohorsky, 1984). This species is different from C. satoi sp. nov. by having a smooth and shiny last whorl.
Stratigraphic and geographic distribution.—Upper Pliocene: Tentokuji Formation in Akita Prefecture.
Figure 3.
Cyllene satoi sp. nov. and C. glacilenta (Yokoyama, 1928). 1, 2, Cyllene satoi sp. nov.; 1a, b, holotype, UMUT CM 32797; 1a, apertural view; 1b, abapertural view; 2a–c, paratype, UMUT CM 32798; 2a, apertural view; 2b, abapertural view; 2c, enlargement of spire and upper part of the last whorl, Loc. 2; 3, 4, Cyllene glacilenta (Yokoyama, 1928); 3a, b, IGPS no. 55136-1; 3a, apertural view; 3b, abapertural view; 4a–c, IGPS no. 55136-2; 4a, apertural view; 4b, abapertural view; 4c, enlargement of spire and upper part of the last whorl, Loc. Ananai, Kochi Prefecture. All scale bars show 5 mm.
Table 1.
Distribution of the gastropod genera, subgenera or species not now living in the main part of the Japan Sea. *, extinct species; °, extant species; 1), northern limit of the distribution of modern species in the Japan Sea (NLDJ). Yamaguchi is the westernmost part of the Japan Sea side of Honshu. Parentheses mean the northernmost region of each taxon. 2), shallowest depth of each taxon. The data on the distribution of the extant species are based on Higo et al. (1999). Locality numbers in this table correspond to those in Figure 1.
Discussion
During the mid-Pliocene (3.256–3.025 Ma), the surface temperature was 2.7–4.0°C higher than today and sea level was 10–30 m higher than today during interglacial periods (Dwyer and Chandler, 2009; Haywood et al., 2013, 2016). The shallow warm-water Tsushima Current flowed into the Japan Sea during interglacial periods from 3.5 to 1.7 Ma (Kitamura and Kimoto, 2006; Gallagher et al., 2015). Based on calcareous nannofossils and tephrochronology, it has been elucidated that the warm Tsushima Current began to flow into the Japan Sea since about 4 Ma (Amano et al., 2008). As noted above, this current reached as far north as Akita Prefecture, northern Honshu, during the late Pliocene, while it reaches northern Hokkaido today. The reason why the influence of the current was weaker than now during the warmer period of the late Pliocene is possibly because the current reached a lesser depth than at present (Kitamura and Kimoto, 2006).
Some warm-water gastropod genera and species not living in the main part of the Japan Sea have been recorded (Amano, 2007) from the Omma-Manganji fauna. Table 1 summarizes these taxa, including Cyllene satoi sp. nov. Other than Mammilla spp. (10 m depth) and Calyptraea yokoyamai (20 m depth), the shallowest depth inhabited by these taxa is the intertidal zone (Higo et al., 1999). Among them, Cyllene herein treated is one of the typical species now living in the tropical Indo-Pacific, West Africa and S.E. Australia (Cernohorsky, 1984). Based on Higo et al. (1999), Conus, Mammilla, Cyllene and Scalptia are now distributed as far north as Yamaguchi Prefecture, in the westernmost part of Honshu. Fossils of these genera have been recorded from Akita Prefecture, northernmost Honshu. A relatively high winter SST is essential for the warm-water species' survival. The average SST in January (1981–2010) is about 4.0°C higher in Yamaguchi than in Akita (Japan Meteorological Agency; http://www.data.jma.go.jp/kaiyou/data/db/kaikyo/jun/sst_HQ.html?areano=3). A brackish-water dweller, Neritina (Vittina), collected from the Kuwae Formation in Niigata Prefecture, central Honshu, is now living southward from Amami-Ohshima. However, this genus is not suitable for estimating the paleotemperature because its habitat is affected also by salinity, not only by temperature. Calyptraea yokoyamai, Chicoreus (Triplex) and Mitra (Mitra) also have been collected from the Kuwae and Shitoka formations in Niigata. These are now not living in the Japan Sea, but do still live in the East China Sea off western Kyushu. The average SST in January (1981–2010) is about 4°C lower in Niigata than in northwestern Kyushu (Japan Meteorological Agency; http://www.data.jma.go.jp/kaiyou/data/db/kaikyo/jun/sst_HQ.html?areano=3). Punctacteon kajiyamai, also collected from the Shitoka Formation, is not suitable for estimating the paleotemperature because it is a modern endemic species in southeastern Shikoku. These estimations of SST, about 4.0°C higher than the present day, are nearly concordant with the global warming during the late Pliocene. As noted above, the Tsushima Current at the time was thinner but warmer. If global warming continues, it will allow these taxa to enter the Japan Sea again.
Vermeijia most closely resembles the boreal genus Ariadnaria. As described above, the young shell of Vermeijia looks like Ariadnaria insignis. Thus, Ogasawara et al. (1986) and Nemoto and O'Hara (2005) respectively misidentified V. japonica and V. planicostata with A. insignis. In the late Pliocene, V. japonica was alive on the Japan Sea side of Honshu. In the early to middle Pleistocene, V. planicostata existed on the Pacific side of central Honshu. Both species were associated with some warm-water molluscs (Baba, 1990; Amano et al., 2000a, 2011; Nemoto and O'Hara, 2005). Each species of this genus possibly evolved by adapting to a higher temperature, but did not develop a large population or wide distribution. On the Japan Sea side of Honshu, V. japonica suffered extinction at least by the cooling event at Datum A (2.75 Ma; Sato and Kameo, 1996). Up to this time, three genera of the Omma-Manganji fauna became extinct; Yabepecten Masuda, 1963, Profulvia Kafanov, 1976 and Pseudamiantis Kuroda, 1933. All these genera were monotypic during the Pliocene to early Pleistocene and originated during the Oligocene and Miocene (Iwasaki, 1963; Kafanov, 1980; Matsubara, 2003). Vermeijia is the fourth record of an extinct genus among the Omma-Manganji fauna. Some characteristic species of the Omma-Manganji fauna have been recorded from the Pacific side of central Honshu where Vermeijia planicostata occurred with cold- and warm-water species (Noda and Amano, 1977). Horikoshi and Kosuge (1971) listed Trichotropis planicostata from the upper Pleistocene Miyata Formation, but did not illustrate it. V. planicostata probably evolved from V. japonica and survived into the late Pleistocene on the Pacific side of central Honshu like Mizuhopecten tokyoensis (Tokunaga, 1906) (see Amano, 2007).
Acknowledgments
I am very grateful to Alan Beu (GNS Science) for his critical reading of this manuscript. I thank two anonymous reviewers for their critical comments to improve the manuscript. I also thank T. Sasaki (University Museum, The University of Tokyo), H. Nishi and J. Nemoto (Tohoku University, Sendai), T. Haga (National Museum of Nature and Science, Tsukuba), and N. Nemoto (Hirono Town) for their help in examining the fossil specimens. This study was partly supported by a Grant-in-aid for Scientific Research of the Japan Society for the Promotion of Science (C, 17K05691, 2017–2019).
