Nanaochlamys notoensis notoensis (Yokoyama, 1929)
YOKOYAMA, M. 1929. Pliocene Shells from near Nanao, Noto. Imperial Geological Survey of Japan, Report, 104: 1-7, pIs. 1-6 [p. 4, pl. 3, figs. 1-4; pl. 4, figs. 1, 2; pl. 5, fig. 1]
1929 Pecten notoensis Yokoyama, 1929
1930 Pecten natoriensis Matsumoto, 1930
1930 Pecten natoriensis var. inequilateralis Matsumoto, 1930
1930 Pecten natoriensis var. subovalis Matsumoto, 1930
1930 Velopecten survivans Matsumoto, 1930
1955 Chlamys kitamurai Kotaka, 1955
1962 Nanaochlamys notoensis setanensis Kanno, 1962
1930 Pecten natoriensis Matsumoto, 1930
1930 Pecten natoriensis var. inequilateralis Matsumoto, 1930
1930 Pecten natoriensis var. subovalis Matsumoto, 1930
1930 Velopecten survivans Matsumoto, 1930
1955 Chlamys kitamurai Kotaka, 1955
1962 Nanaochlamys notoensis setanensis Kanno, 1962
M. Yokoyama, 1929, plates 3-5.
|
«Shell of medium size, thick, rather compressed, nearly equivalve, equilateral except for ears, radiately ribbed. Ribs usually five or six near the beak, more or less elevated though rounded, generally smooth, sooner or later dichotomizing either once or twice, with interspaces narrower, in which there is usually an intercalary rib either appearing only near the ventral border or higher up, sometimes more than half way up the shell; number of ribs at ventral border varying, but often more than thirty, counting both large and small, making inner border unequally serrate. Convexity: presumably nearly equal in both valves of the same individual, but somewhat varying in different ones; equal to from one-fourth to almost one-fifth of height. Ears unequal, anterior larger than byssal notch distinct, acute, shell-height.
In some specimens, the shell is bent inward as shown in fig. 1, pl. IV, making the convexity greater than when it is not. This species is closely akin to Pecten turpiculus Yok. (Tert. Moll. Shinano. a. Echigo, p. 18, pl. Il, fig 4), a species founded on an imperfect left (?) valve from Shinano. The latter, however, has the ribs about ten in number either split into two or three with two or three interstitial riblets. But the bending of the shell in the middle as well as its general appearance strongly reminds us of the species of Noto, particularly when we observe on one of the specimens of the latter (fig. 3, pI. III) a longitudinal striation of the ribs near' the beak resembling the trifurcation seen on the Shinano example. Consequently I am of opinion that it is not at all impossible in future to find specimens which will bridge over the gap now existing between the two.» MATAJIRO YOKOYAMA, 1929
|
«Nanaochlamys notoensis was first described by Yokoyama (1929) as Pecten notoensis from the Middle Miocene beds in the Nanao area, Noto Peninsula, central Japan. Hatai and Nishiyama (1952) noted that the type specimens of Pecten notoensis Yokoyama, 1929 were lost, but the eight specimens figured by Yokoyama (1929) are actually reposited at the University Museum of Tokyo, with registered numbers UMUT CM25506-CM25513. These specimens are all considered as syntypes by the International Code of Zoological Nomenclature, because Yokoyama (1929) did not designate a holotype and paratypes. Later, Sato (1982) designated CM25509 as the lectotype and CM25510 as a paralectotype.
As shown in Figure 2, this species has a circular shell outline. Radial ribs are rounded, solid and smooth, and increase in number ventrally by bifurcation on both valves. Prominent ribs occur on the shells; the number is six on the right valve and five on the left valve in the juvenile stage, but it increases as the shell grows by the insertion of the secondaries and tertiaries on both valves. This species is inequivalved; the left valve is slightly concave and abruptly bends at a distance from the beak so as to form a flat platform (Figure 2B, D, G), but such ontogenetic change of valve convexity does not occur in the right valve (Figure 2I, M).» HAYASHIDA, N. & K. TANABE. 2006. The mode of life and taxonomic relationship of a Japanese Miocene pectinid bivalve Nanaochlamys notoensis. Paleontological Research, 10 (1): 1-10, figs. 1-7. [p. 8, 9]
|
Nanaochlamys notoensis (Yokoyama, 1929); N. Hayashida & K. Tanabe, 2006, The mode of life and taxonomic relationship of a Japanese Miocene pectinid bivalve Nanaochlamys notoensis, figures 2A-2M.
|
«COMPARISON WITH FOSSIL SPECIES:— While Nanaochlamys Hatai and Masuda (1955) included the genus Nanaochlamys in the Subfamily Pectininae, Masuda (1962) assigned it to the Subfamily Chlamydinae. Later Vokes (1976) included the present genus in the Chlamydinae. In this paper present taxon is treated as a genus of the Subfamily Chlamydinae by the following reasons. (1) Ctenolium is clear even in adult stage. (2) Inner surface of hinge area is similar to Chlamys. (3) Convexity of both valves is nearly equal in adult stage. It is difficult to distinguish the present species from the subspecies otutumiensis in younger growth stage. Primary bifurcation of the primary ribs is fundamentally the same both on otutumiensis and notoensis. But the latter is distinguishable from the former by the following characteristics of ribs; (1) stouter primary ribs; (2) interspaces between primary ribs being equal to or a little broader than primary ribs; (3) each primary rib being divided into four to five riblets and intercalary ribs between bifurcated ribs being more than three on both valves; and (4) furthermore, otutumiensis being generally larger in size than notoensis.
Number of primary ribs of the present species is the same as that of Swiftpecten swiftii. The latter is distinguishable from notoensis by the following characters; (1) four riblets being radiate on primary ribs; (2) primary ribs being not bifurcated and provided with nodules at intersections with concentric growth lines; (3) anterior auricule being much larger than the posterior one; and (4) both valves being smaller and more convex and apical angle being smaller than the present species throughout growth stages. COMPARISON WITH RECENT SPECIES:— There is no close ally to the present species among the living pectinids. Present species has a similar inner surface of hinge area to that of Gloripallium pallium from Okinawa-jima, but the latter has not cardianal teeth on both sides of resilial pit (Waller, 1972, p. 238). Inner surface of Swiftpecten swiftii resembles the present species. But it is distinguishable from the latter by the obscure triangular plane, tapered cardinal crura reaching both posterior and anterior ends. PHYLOGENY:— Masuda (1960) inferred that Nanaochlamys kitamurai, N. notoensis and N. notoensis otutumiensis organize a phyletic line in this order. The present writer considers that N. kitamurai and N. notoensis setanaensis are synonymy of notoensis as pointed out by Masuda and Noda (1976). Therefore only Nanaochlamys notoensis and N. notoensis otutumiensis constitute a phyletic line. Masuda (1986b) pointed that the present species originated in the central southern Pacific.» SATO, Y. 1991. Paleontological study of molluscan assemblages of the Miocene Moniwa Formation, Northeast Japan and description of their pectinidae. Report, Geological Survey of Japan, 272: 1-249, pls. 1-34 [p. 67]
|
Nanaochlamys notoensis (Yokoyama); Y. Sato, 1991, Paleontological study of molluscan assemblages of the Miocene Moniwa Formation, plates 9, 10; plate 11, figures 1, 2.
|
«Geological significance — The Nanao formation at Iwaya from where abundant specimens of notoensis occur, consists of a richly calcareous sandstone containing abundant pectens, Brachiopoda, Bryozoa, Foraminifera, sponge spicules and balanid fragments. From this sandstone at Iwaya, M. YOKOYAMA (1929) reported Pecten (Patinopecten ?) kagamianus, Pecten notoensis, Pecten (Chlamys) hastatus Sow. var. ingeniosa, Pecten (Chlamys) crassivenius and Pecten sp.
Concerning this sandstone, K. ÔYAMA (1952) stated that it is easily induced that the Nanao formation consists of "the channel sediments" which were deposited when the Nanao Peninsula was an island or bank. However, whether the calcareous sandstone should be considered channel sediments is open to question, because the detail paleogeographical conditions at that time have not been worked out. It is evident that notoensis lived on a clean sea bottom remote from the influence muddy sediment drained from the land. The depth of the warm water environment is difficult to estimate, although it seems certain that it was beyond the littoral zone. The Moniwa formation in the southern border of Sendai where notoensis is common, consists of granule conglomerate to coarse grained sandstone in which abundant pectens, other pelecypods, some gastropods, echinids, balanids, brachiopods, bryozoan-fragments and rarely shark's teeth occur. This formation at the localities in the southern border of Sendai where notoensis occurs is lacking in muddy sediments and from stratigraphical evidence, there is no doubt that the formation represents a transgressive phase by which the total absence of muddy materials may be explained. Although calcareous, the Moniwa formation is not so significant as the Nanao in this respect, but the thermal conditions of the sea in which the two geographically isolated formations were deposited are remarkably similar. The Ôtsutsumi formation, in the northwest of Sendai, a correlative of the Moniwa, differs in its Pectinidae fauna and notoensis is very rare. The Ôtsutsumi consists of tuffaceous, very coarse sandstone or conglomeratic sandstone in which muddy material is embedded. Although the thermal conditions in which the Otsutsumi was deposited is similar to that of both the Moniwa and the Nanao, the ecological conditions seem to have been different as can be noticed from its paucity of calcareous sediments. Therefore, it is inferred that notoensis flourished only in environments consisting of a clear sea bottom influenced by warm thermal conditions, and that its absence or rarity in correlative formations may be explained, in part, by such reason. Nanaochlamys notoensis under the generic names of Pecten or Chlamys has been reported from the Nanao, Takasegawa, Daijima, Setana, Moniwa, Ôtsutsumi, Chiganoura, Yanagawa, Shioda and Hatatate formations, but not all of them have been figured thus leaving one in doubt as to their details. This species has also been recorded from Ennichi in South Korea. Of the named formations, the species is very rare in the Hatatate formation in the southern region of Sendai, and in the Chiganoura formation developed around Shiogama City, both in Miyagi Prefecture. It is also not common in the formations other than those of Nanao and Moniwa, probably owing to both bottom control and geological horizon. Its record from the Setana formation was not confirmed. From the geological distribution and range, it is inferred that this species attained its maximum flourishing, which seems to have been very short, during the Early Miocene, that is to say, in a two-fold division of the Miocene.» HATAI, K. & K. MASUDA. 1953. On the Pecten notoensis (Yokoyama). (On the Miocene Pectinidae from the environs of Sendai, Part 2). Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 11: 75-82, pl. 7, text-figs. 1-3 [p. 80, 81]
|
Nanaochlamys notoensis (Yokoama); Hatai, K. & K. Masuda, 1953, On the Pecten notoensis (Yokoyama), plate 7, figures 1-7.
|