A new pectinid species, Swiftopecten djoserus sp. nov. is described from the Pliocene Zukawa Formation in Takaoka City, Toyama Prefecture in Central Honshu. This species can be distinguished from S. swiftii, the type species of the genus, by its smaller size, the fewer fine radial riblets on both valves, the wider umbonal angle and the more uneven ledges. Swiftopecten djoserus sp. nov. is an extinct species of the Omma-Manganji fauna that occurred locally in the central part of the Sea of Japan borderland during the latest Pliocene.
Introduction
The pectinid genus Swiftopecten was proposed by Hertlein (1935), with Pecten swiftii Bernardi, 1858 as the type species. Swiftopecten swiftii is the only extant species in this genus (Masuda, 1959,1960, 1972). Swiftopecten swiftii is a cold-water dweller distributed from Choshi, Chiba Prefecture in Japan to Alaska on the Pacific coast and from Gangwon-do in Korea and northward areas in the northern Sea of Japan. It lives on rocky and gravel bottoms in depths shallower than 50 m (Sinelnikova, 1975; Volova and Scarlato, 1980; Okutani, 2000; Min et al., 2004).
Although the fossils of Swiftopecten in the North Pacific were studied taxonomically by Masuda (1959, 1960, 1962, 1972), variation of both fossil and recent specimens has not been studied in detail. In association with S. swiftii, a new species of Swiftopecten, S. djoserus sp. nov. has been found in the upper Pliocene Zukawa Formation (Mochiduki, 1930; Amano et al., 2012) in Takaoka City, Toyama Prefecture. For a comparison between the new species and S. swiftii, the variation of these species has been examined in detail. In this paper, I describe the new species from the Zukawa Formation, reveal the variation of both fossil and Recent specimens of S. swiftii, and also discuss the taxonomy of the genus Swiftopecten. The paleogeographic significance of the new species is also discussed.
Material and methods
The Zukawa Formation consists mainly of medium-to coarse-grained sandstone, and yields many molluscan fossils including Swiftopecten (Kiyu and Mizuki, 1983; Matsuura, 1985, 1992, 2009; Fujii and Shimizu, 1992; Goto et al., 1993; Amano et al., 2012). The molluscan fossils are mainly composed of cold-water species, with fewer warm-water species (Amano et al., 2012). Based on calcareous nannofossils from localities near the type locality of the Zukawa Formation, the age of the main part of this formation is late Pliocene while the lowermost part is early Pliocene (Amano et al., 2012).
In this study, 49 specimens assignable to Swiftopecten were collected from the Zukawa Formation at four localities: Ikarabe (Loc. 1 = Loc. Z-4 in Amano et al., 2012), Zukawa (Loc. 2), Nishiebizaka (Loc. 3) and Ishidutsumi (Loc. 4) in the western part of Takaoka City (Figure 1). Among them, 15 specimens belong to a new species: 13 specimens from Ikarabe, one from Zukawa and one from Nishiebizaka. The specimens from Ikarabe and Zukawa were recovered from the uppermost part of the formation. Judging from calcareous nannofossils, the uppermost part of the formation conesponds to the horizon just above Datum A (= 2.75 Ma after Sato and Kameo, 1996) (Amano et al., 2012).
In this study, 390 specimens of Swiftopecten swiftii were also examined in total for comparison with the new species. Among them, 179 were Recent specimens from 26 localities mainly in Hokkaido, Sakhalin and Primorye, and 192 were fossils from 11 formations represented at 18 localities in the Sea of Japan and the Pacific side of Honshu and Hokkaido (Figure 2, Appendix). Of these specimens, 102 of the recent ones were purchased at Yubetsu fishing port in Hokkaido and 49 of the fossil ones were collected from the Zukawa Formation by the author. Other Recent and fossil specimens stored at the following institutions were examined: JUE, Joetsu University of Education; NMNS, National Museum of Nature and Science, Tsukuba; TSM, Toyama Science Museum; NHMIC, Natural History Museum and Institute, Chiba; YFM, Yatsuo Fossil Museum, Kai-inkan; KSHS, Keio Senior High School.
Figure 1.
Index maps (A, B) showing the localities (C) and horizon (D) at Loc. 1 of the Swiftopecten djoserus sp. nov. specimens described from the Zukawa Formation. The geological map is modified from Sumi et al. (1989).
In addition to measuring shell length and height, measurements and counting have been done of the following three characters: (1) number of radial ribs and fine riblets, (2) umbonal angle, and (3) unevenness of the ledges. Radial ribs were counted on both valves while fine riblets were counted on the third ledge of all the specimens and only on the left valve because few well preserved right valves of the new species are available. Umbonal angle is the angle between the anterior and posterior margins on each specimen, measured by a goniometer (Figure 3). The ledge is a morphological term referring to developed growth ribs, in accordance with Waller (1991). In growing order, the first formed ledge is called the first ledge, and ledges are formed sequentially such as the second ledge and the third ledge for the vertical margin. Growth increments towards the ventral margin (3a, 4a, 5a in Figure 3) and towards the interior (3b, 4b, 5b in Figure 3) were measured with vernier calipers. However, immature specimens having fewer than three ledges were excluded as invalid data because the ledges are not fully developed and have not grown towards the interior.
Systematic descriptions
Family Pectinidae Rafinesque, 1815
Subfamily Chlamydinae von Teppner, 1922
Genus Swiftopecten Hertlein, 1935
Type species.—Swiftopecten swiftii Bernardi, 1858 by original designation.
Remarks.—The original description of the genus Swiftopecten has been referred to Hertlein (1935) (e.g. Hertlein and Grant, 1972; Masuda, 1972) or Hertlein (1936) (e.g. Del Rio, 1995; Matsubara, 2013). I accepted here the former opinion tentatively.
The genus Swiftopecten has been compared with three genera, Nanaochlamys, Manupecten, and Semipallium, in part. Swiftopecten can be separated from Nanaochlamys Hatai and Masuda, 1953 by having an inequivalve shell (nearly equivalve in Nanaochlamys; Yokoyama, 1929). These two genera are also distinguished by the differences in the allometric change of the umbonal angle versus shell height (Hayashida and Tanabe, 2006). Masuda (1960) noted that the character of the surface sculpture in the adult stage of S. swiftii is closely similar to the younger stage of N. notoensis and N. otutumiensis, but Swiftopecten can be distinguished by its nodular ribs in the adult stage, whereas Nanaochlamys has smooth ribs. Hertlein and Grant (1972) claimed that Manupecten Monterosato, 1872 lacks the ledges and hinge teeth present in Swiftopecten. Waller (1991) showed that Semipallium Jousseaume in Lamy, 1928 differs from Swiftopecten in having only incipient lodging, much less well developed than in Swiftopecten.
In conclusion, Swiftopecten is distinguished from other similar genera in an inequivalve shell, allometric change of the umbonal angle versus shell height, nodular ribs in the adult stage, developed ledges, hinge teeth and larger size.
Included species.—Although Kuroda (1931) ranked Pecten turpiculus Yokoyama, 1925 from the Pliocene Shigarami Formation as a subspecies of Swiftopecten swiftii, Masuda (1959) and Masuda and Noda (1976) considered it to be either a subspecies of Chlamys cosibensis (Yokoyama, 1911) or a separate species. Not including this one, five fossil species and one subspecies have been described in Swiftopecten: S. swiftii, S. swiftii kindlei (Dall, 1920), S. parmeleei (Dall, 1898), S. donmilleri (MacNeil, 1970) and S. merklini (Sinelnikova, 1975), all from the Asian and the North American side of the North Pacific, and S. iheringii Del Rio, 1995 from Argentina.
Hertlein and Grant (1972) and Moore (1984) indicated that the genus Swiftopecten from California is characterized by a small shell. Masuda (1972) claimed that S. parmeleei differs from the closely related S. swiftii in having a somewhat larger apical angle and a smaller shell that tends to become rounded with growth. MacNeil (1967) considered that Chlamys kimurai Kotaka, 1955 from the upper Oligocene Isomatsu Formation is the oldest species of the genus. After that, Del Rio (1995) described S. iheringii from upper Eocene deposits in Argentina as the oldest species of this genus. However, Matsubara (2013) mentioned that C. kimurai belongs to the genus Nanaochlamys and doubted whether S. iheringii is a species of Swiftopecten. If this is correct, the oldest record of this genus is S. swiftii from the middle Miocene deposits in northern Japan (Masuda, 1973, 1986).
Hertlein and Grant (1972) and Waller (1991) included Pecten cosibensis Yokoyama, 1911 in the genus Swiftopecten. However, Masuda (1972) argued that Chlamys cosibensis should not be allocated to Swiftopecten on the grounds that Swiftopecten can be distinguished from C. cosibensis by its large, higher, posteriorly contorted shell, smaller apical angle, triangular large anterior auricle and nearly fiat left valve in the younger stage. In my opinion, C. cosibensis should not be assigned to the genus Swiftopecten due to differences in the deep byssal notch, the small maximum shell height, the small number of fine riblets and the lesser unevenness of the ledges. I believe that prominent ledges have evolved independently in Swiftopecten and Nanaochlamys species and in Chlamys cosibensis, therefore C. cosibensis should be excluded from Swiftopecten.
Figure 2.
Map showing localities of the Swiftopecten djoserus sp. nov. (Pliocene) and S. swiftii (Plio-Pleistocene and Recent) specimens measured. 1, Takaoka; 2, Byellinszaueyena; 3, Okhotskoye; 4, Tret'ya Pad'; 5, Pochinka; 6, Due; 7, Cape Koinjo; 8, Syavyanka; 9, Veselyy Yar; 10, Mysovoy; 11, Vladivostok; 12, Wakkanai (JUE); 13, Yubetsu; 14, Notsukesaki; 15, Kunashiri; 16, Nemuro; 17, Akkeshi; 18, Kushiro; 19, Hakodate; 20, Okushiri; 21, Oshima; 22, Sapporo (?); 23, Hamamasu; 24, Mashike; 25, Rumoi; 26, Wakkanai (NMNS); 27. Mutsu; 28. Hachinohe; 29, Choshi; 30, Kamiiso; 31, Suttsu; 32, Minamitsugaru; 33, Oga; 34, Sado; 35, Kariwa; 36, Kisarazu; 37, Takaoka.
Figure 3.
Morphology and measurements of Swiftopecten. 1a, 2a, 3a, 4a, 5a, growth increment toward ventral margin; 3b, 4b, 5b, growth increment toward inside.
Swiftopecten djoserus Yoshimura, sp. nov.
[Japanese name: Zukawa-kinchakugai]
Figure 4
Swiftopecten swiftii (Bernardi, 1858). Kiyu and Mizuki., 1983, fig. 1.8; Amano et al., 2012, fig. 3.10.
Chlamys cosibensis (Yokoyama, 1911). Amano et al., 2012, fig. 3.2.
Etymology.—Named for the Pyramid of Djoser in Saqqara, Egypt because the commarginal constrictions of this species resemble the unique stepwise form of this pyramid. A noun in apposition.
Type material.—Holotype, NMNS PM27195 (Figure 4.1); Paratype, JUE No. 15938 (Figure 4.2).
Type locality. —About 1.5 kin northwest of the Kunisaki Bridge (crossing the Oyabe River), 7 in below the boundary of the Zukawa Formation and the Itaya Formation, Ikarabe area, Takaoka City, Toyama Prefecture, Central Japan (Loc. 1 in Figure 1 = Loc. Z-4 in Amano et al., 2012).
Dimensions.—See Table 1.
Diagnosis.—Medium -sized Swiftopecten, shell strongly convex; umbonal angle 85–90 degrees. Four radial ribs on right valve and five on left valve, covered with 42–44 fine riblets on the third ledge. Ledges very distinct. Adductor muscle scar slightly posterior and slightly dorsal from center of disc.
Description.—Shell medium-sized for genus, up to 75.4 mm in height, somewhat higher than long, fan-shaped, rather thick, very inflated; left valve slightly more inflated than right; umbonal angle 85–90 degrees, apical angle on first ledge 83–87 degrees. Right valve sculptured with four prominent radial ribs with interspaces each narrower than one rib, covered with 42–44 fine divaricated riblets on third ledge; 3–5 growth steps producing definite ledges. Left valve with five rather prominent ribs with broader interspaces than on right valve; divaricate riblets and ledges as on right valve. Anterior auricle large, triangular, with rounded margin, sculptured with 6–9 radial ribs. Posterior auricle much smaller than anterior, sculptured with 4–5 ribs. Byssal notch in right valve rather narrow; functional ctenolium consisting of six teeth. Resiliai pit large, nearly triangular in shape; resiliai teeth strong; cardinal crura distinct. Adductor muscle scar positioned slightly posterior and dorsal to center of valve, although aragonite area dissolved in all specimens. Interior surface of both valves distinctly or gently folded corresponding to external sculpture, with fine crenulations at ventral margin.
Comparison.—Swiftopecten djoserus sp. nov. is most similar to the modem species, S. swiftii. The anterior and posterior auricles are sculptured with 6–9 radial ribs, as in S. swiftii. Both auricles also are similar to those of S. swiftii in shape. The number of radial ribs also is the same in both species: four on the right valve and five on the left valve. However, the number of fine riblets differs, as S. djoserus sp. nov. has 42–44 radial riblets on the third ledge while S. swiftii has 55–57 (Table 2). As the result of a Mann-Whitney U test, a significant difference was found in median of the number of the fine riblets between the two species (U = 0, p = 2.2×10-16). Swiftopecten djoserus sp. nov. has an umbonal angle of 85–90 degrees versus a low of ca. 73 to a high of ca. 87 in S. swiftii (Figure 6). As the result of a Kolmogorov-Smirnov test, the null hypothesis of normal distribution was not rejected in either species (D = 0.13977, p = 0.9473). As a result of a t-test, a significant difference in mean was found between the two species (t = 19.253, df = 14.486, p = 1.005 × 10 -11). On the unevenness of ledges, S. djoserus sp. nov. is stronger than S. swiftii. Although both species have rather strongly uneven ledges, the mean of the slope of the regression line of the width of growth increment towards inside against that towards ventral margin is 0.660 (9 specimens) for S. djoserus sp. nov. compared with 0.329 (213 specimens) for S. swiftii (Figure 7). As results of Kolmogorov-Smimov test, the null hypothesis of the normal distribution was not rejected for the unevenness of the ledges in the samples of S. djoserus sp. nov. (D = 0.2201, p = 0.6609), as well as in the samples of S. swiftii (D = 0.060508, p = 0.6298). As the result of the t-test, a significant difference was found between the two species (t = 22.855, df= 14.341, p = 1.099 × 10-12).
Stratigraphic and geographic range.—Recorded only from the upper part of the Pliocene Zukawa Formation.
Figure 4.
Swiftopecten djoserus sp. nov. from the Pliocene Zukawa Formation. 1, holotype, NMNS PM27195, from Loc. 1; 1a, exterior view; 1b, interior view; 1c, lateral view; 1d, ventral view; 1e, oblique view from upper-right side; 1f, close-up of resilium; 2, paratype, JUE no.15938, from Loc. 1; 2a, exterior view; 2b, interior view; 2c, lateral view; 2d, ventral view; 2e, oblique view from upper-right side; 2f, close-up of ctenolium; 3, UMUT RM32338 from Loc. 2; 3a exterior view; 3b, lateral view; 3c, oblique view from upper-right side; 4, UMUT RM32343 from Loc. 3; 4a, exterior view; 4b, interior view; 5, UMUT RM32348 from Loc. 1; 5a, exterior view; 5b, interior view.
Table 1.
Measurements of Swiftopecten djoserus sp. nov. * broken.
Table 2.
Morphological comparison of Swiftopecten djoserus sp. nov. with the other species and subspecies of Swiftopecten.
Figure 5.
Recent and fossil specimens of Swiftopecten swiftii (Bernardi, 1858). 1, KSHS-F1-001, fossil specimen from Loc. 1 in the Zukawa Formation; 1a, exterior view; 1b, inner view; 1c, lateral view; 2, KSF1S-F2-001, fossil specimen from Loc. 1 in the Zukawa Formation; 2a, exterior view; 2b, inner view; 2c, lateral view; 3, KSF1S-R1-058, Recent specimen from Yubetsu Town in Hokkaido, depth 50 m; 3a, d, exterior view of right and left valves; 3b, e, inner view of the right and left valves; 3c, f, lateral view of right and left valves; 3g, lateral view of articulated valve.
Figure 6.
Relationship between shell height (mm) and umbonal angle (degrees) of Swiftopecten djoserus sp. nov. and S. swiftii (all symbols as in Figure 2).
Figure 7.
Scatter diagram showing the relationship between widths of growth increment towards the ventral margin (3a, 4a, 5a in Figure 3) and growth increment towards the inside (3b, 4b, 5b in Figure 3) for Swiftopecten djoserus sp. nov. and S. swiftii. Regression lines are shown for each species (all symbols as in Figure 2).
Discussion
Appearance and extinction of the new species and its causes
From its Pliocene occurrences in only the Sea of Japan borderland, Swiftopecten djoserus sp. nov. can be judged to be an endemic species of the Omma-Manganji fauna (Otuka, 1939) as well as the only extinct species of Swiftopecten in Japan. In this research, I examined 179 Recent and 192 fossil specimens of S. swiftii in the late Pliocene, Pleistocene and Recent. Significant difference was not observed among various ages or localities. Therefore, S. swiftii is characterized by a constant range of morphological variations.
The new species also was found only from the upper part of the Zukawa Formation. This new species was collected in association with S. swiftii and apparently lived sympatrically with it. As mentioned above, S. swiftii appeared during the middle Miocene. From these occurrences, S. djoserus sp. nov. is judged to have originated from S. swiftii in the central part of the Sea of Japan borderland during or before the latest Pliocene, because almost all specimens (a total of 15 specimens; 14 specimens and one specimen which is impossible to discriminate) of the new species were recovered from just above Datum A at Ikarabe and Zukawa, corresponding with the Pliocene cooling event in the Northern Hemisphere (Sato and Kameo, 1996). As a result of this cooling, the percentage of cold-water mollusks became higher than in the horizons below Datum A. Moreover, species now living in Hokkaido and northwards appeared above Datum A in the Zukawa and Sasaoka formations in Akita Prefecture (Amano et al., 2011, 2012). What can be assumed here is the possibility that the cooling event accelerated the speciation of the new species. However, we cannot judge whether the new species was alive before Datum A because S. swiftii was not present below the Datum A. In regard to the question of extinction, the new species has not been recorded from other formations in this research. Hence, the new species is likely to have become extinct in the latest Pliocene or later. It is considered that the new species became extinct as a result of the environmental deterioration caused by a fall in sea level during a glacial epoch in the Middle Pleistocene or later, as previously reported (Amano, 2001), but as yet I have no satisfactory evidence that would explain why only the new species became extinct.
Causes of the formation of ledges in the genus Swiftopecten
In this study, detailed measurements were carried out on the ledges on both valves. Generally, the formation of ledges along growth lines was caused by pauses in shell growth (Habe, 1977). In pectinids, Takenaka and Hay ami (1998) considered that ledges reflect a reproductive cycle. As mentioned above, the average value of the ratio of growth increment towards inside to growth increment towards ventral margin of S. djoserus sp. nov. (0.660) is significantly different from that of S. swiftii (0.329). This means that in S. swiftii, the growth ratio of increments towards the ventral margin to increments towards the inside is 3 to 1. On the other hand, this ratio in S. djoserus sp. nov. is 3 to 2 (Figure 7). This result suggests that S. djoserus sp. nov. and S. swiftii had different breeding seasons, explaining how the two species were able to live sympatrically. In order to support this conclusion, further research is necessary to observe the growth lines microscopically and to measure the stable oxygen and carbon isotope ratios.
Acknowledgements
This research was greatly supported by Kazutaka Amano (Joetsu University of Education) who provided insight and expertise. I am grateful to Alan G. Beu (GNS Science, New Zealand) for reviewing the manuscript. I am also grateful to Takenori Sasaki (The University Museum, The University of Tokyo) and Taiji Kurozumi (Natural History Museum and Institute, Chiba) for providing a lot of information and discussing the subject minutely. I also thank Takehiro Sato (Kanagawa Prefectural Museum of Natural History) for cooperating with the measurement data verification. My thanks are also due to Hiroshi Saito (National Museum of Nature and Science, Tsukuba), Tasuku Yoshioka (Toyama Science Museum) and Yatsuo Fossil Museum, Kai-inkan for their help in examining specimens. Furthermore, Yoko Kumanoya (Keio University) cooperated by translating Russian literature. Additionally, Koya Yamada (The Sea of Japan Mining Corporation Limited) kindly provided access to the sampling locality. I would also like sincerely to thank Masahiro Kishima (Keio Senior High School). Without his persistent support, this research would not have been completed.
References
Appendices
Appendix
List of material of 371 specimens of Swiftopecten swiftii (Bernardi, 1858) for morphologic comparison with S. djoserus sp. nov.
Fossil specimens
Zukawa Formation, upper Pliocene: Ishidutumi (N = 19) and Iwatsubo (N = 7), Takaoka City, Yoyama Prefecture, TSM. Ikarabe (N = 44) and Zukawa (N = 1), Takaoka City, Toyama Prefecture, JUE. Ikarabe (N = 3), Ishidutsmni (N = 2) and Zukawa (N = 4), Takaoka City, Toyama Prefecture, YFM. Nishiebizaka (N = 4), Ikarabe (N = 33), Iwatsubo (N = 6) and Zukawa (N = 6) Takaoka City, Toyama Prefecture, KSHS.
Mita Formation, lower Pliocene: Fuchu, Toyama City, Toyama Prefecture (N = 1), TSM. Fuchu, Toyama City, Toyama Prefecture (N = 1), JUE.
Daishaka Formation, upper Pliocene : Daishakazawa, Minamitsugaru District, Aomori Prefecture (N = 1), JUE.
Shitoka Formation, upper Pliocene: Kamakurazawa River (N = 1) and Shitoka River (N = 1), Minamiuonuma City, Niigata Prefecture, JUE.
Haizmne Formation, lower Pleistocene: Izumozakimachi (N = 1), Santo District, and Nishiyamacho (N = 9), Kariwa District, Niigata Prefecture, JUE.
Kamiizumi Formation, lower Pleistocene: Nagayoshi, Sodegaura City, Chiba Prefecture (N = 1), NHMIC.
Omma Formation, lower Pleistocene: Oyabe City, Toyama Prefecture (N = 3); Kanazawa City, Ishikawa Prefecture (N = 1), TSM.
Sawane Formation, lower Pleistocene: Sado City, Niigata Prefecture (N = 14), JUE.
Setana Formation, lower Pleistocene: Soibetsu River, Suttsu District, Hokkaido (N = 14), JUE.
Shibikawa Formation, middle Pleistocene: Yasuda, Oga City, Akita Prefecture (N = 11), JUE.
Tomikawa Formation, lower Pleistocene: Kamiisomachi, Kamiiso District, Hokkaido (N = 2), JUE.
Recent specimens
12. Sea of Japan: Hamamasu (N = 8), Mashike (N = 2) , Okushiri (N = 1), Oshima (N = 2), Rumoi (N = 9), Sapporo (?) (N = 2) and Wakkanai (N = 1), Hokkaido, NMNS. Wakkanai (N = 1), Hokkaido; Vladivostok (N = 1) and Syavyanka (N = 1), Primorye, NHMIC. Cape Koinjo (N = 2) and Due (N = 1), Sakhalin, JUE.
13. Pacific Ocean: Nemuro (N = 1), Kunashiri, Kuril (N = 2) and Shirikishinai (N = 2), Hokkaido; Shiroganemachi, Hachinohe City, Aomori Prefecture (N = 2), NMNS. Kushiro, Hokkaido (N = 1); Choshi, Chiba Prefecture (N = 2), NHMIC. Akkeshi (N = 3), Notsukesaki (N = 1) and Nemuro (N = 4), Hokkaido, JUE.
14. Sea of Okhotsk: Yubetsu, Hokkaido (N = 102), KSHS. Okhotskoye (N = 6), Pochinka (N = 2) and Tret'ya Pad' (N = 7), Sakhalin, JUE.
15. Tsugaru Strait: Hakodate, Hokkaido (N = 2), NMNS.
16. Mutsu Bay: Mutsu City, Aomori Prefecture (N = 1), NMNS.
