Fortipecten kenyoshiensis (Chinzei, 1960)
CHINZEI, K. 1960. A New Fortipecten from the Pliocene Sannohe Group in Aomori Prefecture, Northeast Japan. Japanese Journal of Geology and Geography, 31 (1): 63-70, pI. 7, text-figs. 1-3 [p. 64, pl. 7, figs. 1-7, text-figs. 1, 2]
1960 Pecten (Fortipecten) kenyoshiensis Chinzei, 1960
K. Chinzei, 1960, plate 7.
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«Diagnosis: Fortipecten characterized by broad and rounded radial ribs. 75 to 17 in number on the right valve, with every second or third of the rib-interspaces broader and deeper than the others, and the corresponding ribs of the left valve are much more prominent than the others. The surface with fine radial striations on the both valves and fine obtuse networks on the left valve.
Description: Shell large attaining about 120 mm in length, very thick, well inflated, nearly circular in outline; inequivalve, the right valve more convex than the left; equilateral except for auricles; hinge line straight, as long as about 2/3 to 3/4 of the shell length; in anterior and posterior view, contact margin of both valves nearly straight. Both valves ornamented with strong radial ribs, with fine radial striations and with concentric growth lines. Apical angie 100º to 110º. In the right valve, thickness about 1/3 to 1/5 of shell length; surface ornamented with strongly elevated, round-topped radial ribs, 16 or 17 in number, which are more densely spaced but tend to obsolete towards the anterior and posterior ends of the shell; 3 or 4 fine radial striations separated by shallow furrows are seen on ribs; interspaces equal to or narrower than rib-width, sculptured by 3 or 4 very weak striations; every third (or second, near the anterior and posterior margins) interspaces are deeper and wider than the others. Some of the growth lines are deeply sculptured and imbricated. Both auricules nearly equal in size; anterior auricle with shallow but broad byssal notch, byssal area weakly depressed; surface ornamented with weakly imbricated growth lines and several fine radial striations, the latter is absent on byssal area; posterior auricle simple triangular in shape, posterior sinus almost absent, ornamented with imbricated growth lines and about 10 radial striations. In inside view, hinge area broad; resilifer pit nearly round to triangular, large and deep, bordered on anterior and posterior sides by a conspicuous ridge; an indistinct ridge runs close to and nearly parallel to the hinge line. Left valve less convex than the right, thickness about 1/4 to 1/6 of length, the dorsal half of the shell almost flat and then rather abruptly bent towards the ventral margin. Surface sculptured with 15 to 17 radial ribs and many fine radial striations which are more prominent on the rib-interspaces; ribs narrower than their interspaces; every second or third ribs are broad and prominent corresponding to deep and wide interspaces of the right valve; concentric growth lines distinct; the dorsal half of the shell ornamented with weak and fine rhombic network similar to that of Patinopecten group. Ribs sometimes nodulose at the junctions between ribs and distinct growth lines which indicate the growth period of the shell. Both auricles are nearly equal in size and shape, except for shallow byssal sinus at the anterior. Hinge with deep resilifer pit, and a pair of low ridge and deep furrow along anterior and posterior margins of the pit. Internal surface of both valves bulgy, weakly undulated near the ventral margin; adductor muscle scar large, deep and semicircular; pallial line distinct, becomes disconnected irregular pits near the resilifer. Comparison and Remarks: The new species closely resembles Pecten (Fortipecten) takahashii (YOKOYAMA) (1930, pp. 416-417, pI. 78, figs. 1, 2; pl. 79, figs. 1, 2) in general features. The species, however is distinguishable from the latter by the shape of radial ribs in the right valve. That is, in this species ribs are broader than or equal to their interspaces and
sculptured with some fine striations, while in takahashii, ribs are much narrower than interspaces and almost triangular in profil. The number of ribs is 12 or 13 on the right and 9 to 11 on the left valves in takahashii, while the new species has 16 or 17 ribs on the right and 15 to 17 on the left. Further, the new species is different from takahashii having fine reticulated sculptures on the left valve, more convex left valve, and nearly straight contact margin between both valves.
The sculptures of the new species, especially weak secondary striationson ribs of the right valve and corresponding ones on rib-interspaces of the left valve, have a close resemblance to those of Pectem kagamianus YOKOYAMA, the species questionably referred to Patinopecten by MASUDA (1958). The new species, however, can easily be discriminated from P. kagamianus by its inflated valves and large auricles. Pecten (Lyropecten) crassicardo CONRAD (ARNOLD, 1906, p. 71, pl.16) from the Miocene of central and southern California has similar convex right valve and surface ornamentation, but it differs from Fortipecten in its hinge character.
Fortipecten was established by YABE and HATAI 1940, pp. 149-150 as the monotypic subgenus of Pecten, and was subsequently included by HABE (1951) in genus Patinopecten. Certainly, it resembles Patinopecten in having rounded radial ribs on the right valve, shallow but broad byssal notch, large muscle scars and flat hinge with deep resilifer pit bordered by elevated ridges. However, as defined originally by DALL (1898) and subsequently by GRANT and GALE (1931), Patinopecten has nearly equivalved and essentially compressed shell. In general, irregularity of ribs of the left valve cannot be observed in Patinopecten. No Patinopecten having such an inflated right valve has so far been reported. Vertipecten of GRANT and GALE (1931), which is considered by them an ancestral group of Patinopecten, resembles Fortipecten in having rough and inflated shells, but it has convex left and flat right valves contrary to Fortipecten. Fortipecten has many common characters to Patinopecten; nevertheless, the writer hesitates to include it in the latter because of some remarkable difference between the two as stated. Thus, he places Fortipecten tentatively in a subgenus of Pecten (s. l.), and points out merely the fact that it is most closely related to Patinopecten. Some characters of the new species are fairly variable; for example, the convexity of right valve shown by the Thickness/Lenght ratio, ranges from 0.19 to 0.35 (Holotype falls in 0.28); and shape of ribs varies also from low and broad ones to narrow but prominent ones. Some right valves (CM 8596, CM 8597, etc.) of the paratypes have fairly low convexity (ratios of T /L fall in 0.22 and 0.19) and have low but broadly rounded radial ribs, similar to those of Patinopecten group. They fall almost in the category of Patinopecten of DALL (1898) in many characters except their large auricles and the pattern of ribs which have several deeper and wider interspaces. Thus, the writer inclines to regard this new form as an intermediate one between Fortipecten takahashii and Patinopecten. It must be emphasized, however, that a series of Patinopecten-Fortipecten kenyoshiensis n. sp. -F. takahashii does not exhibit a phylogenetic lineage because the stratigraphic horison of kenyoshiensis is youngest among them. The series may be explained by the ecological peculiarity. Repository: All type specimen are reposited in the Geological Institute, University of Tokyo. Reg. No. CM 8594 (Holotype), CM 8595-CM 8614 (Paratypes). Locality, Geologic Horizon and Associated Fauna
The materials were collected from the granular conglomerate bed exposed at road-side cutting, about 500 m west of Ken'yoshi, Nagawa-macho (type locality)*. They were also found in a sandstone bed of small cliff about 500 m southeast of Kitamuki, Gonohe-machi, about 4 km northwest of the type locality both in Sannohe-gun, Aomori Prefecture.
The fossil-bearing bed belongs to the lower part of the Toga alternation of sand, mud and tuff; in the environs of the fossil locality the alternation represents the entire Togawa formation (CHENZEI, 1958, 1959). The Togawa formation belongs to the uppermost horizon of the Pliocene Sannohe group. The fossil bed of about 1 meter in thickness is compossed of andesite granules and white pumice grains, and shows remarkable cross-lamination. Text-fig. 3.map showing the geographical position of the type locality.
A large number of separate valves and fragments of this new species were found in the conglomerate, associated with the following molluscs: Anadara tatuno-kutiensis nagawensis CHINZEI (MS), Glycymeris cf. yessoensis (SOWERBY), Limopsis cumingi A. ADAMS, Venericardia crebricostata (KRAUSE), Venericardia sp., Mercenaria stimpsoni (GOULD), Spisula (Pseudocardium) cf. kurikoma (NOMURA), and Polytropa freycinetii (DESHAYES). The new Fortipecten is the dominant species, and Anadara, Mercenaria and Spisula are subordinate characteristic species. No organisms other than the above-mentioned megafossiis have so far been found in the conglomerate. The other species, i. e., Protothaca adamsi (REEVE), Dosinia kaneharai YOKOYAMA, Dosinia japonica (REEVE), Callista brevisiphonata (SCHRENK), Spisula voyi (GABB), Solen krusensterni (SCHRENK), Mya japonica (JAY), Turritella fortilirata SOWERBY, etc., were found in the lower part of the Toga alternation, in which the Fortipecten-bearing conglomerate bed is intercalated.
In consideration of the rock facies, the abundance of fragmental specimens, and the influence of water erosion on shell surface, the fossils might have been transported for some distance, or deposited under the influence of current and wave actions. Since the Toga alternation is the uppermost horizon of the Pliocene Sannohe group and is unconformably overlain by the Pleistoncee Kamikita group, the Fortipecten-bearing bed can be considered as the upper Pliocene, and is younger than the horizons of Fortipecten tahahashii which occurs limitedly in middle to lower Pliocene deposits.» * Lat. 40° 26' 43'' N, Long. 141° 20' 20'' E. KIYOTAKA CHINZEI, 1960
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«This species is clearly distinguished from Fortipecten takahashii which was described by Yokoyama (1930) from the Pliocene Maruyama Formation of south Sakhalin by its shape and the number of radial ribs on both valves. Fortipecten takahashii was recently discovered by the junior author in another fossil locality of the Taga Group in the Tateishi area of Naraha-machi. This takahashii-bearing sandstone is stratigraphically separated from the Futatsu-numa Sand and is correlated with the lower part of the Hirono Formation.
Fortipecten kenyoshiensis was originally described by Chinzei (1960) from the Pliocene Togawa Formation of Aomori Prefecture. This record may be one of the southernmost occurrences of the present species in the late Pliocene.» O'HARA, S. & N. NEMOTO. 1988. Pectinids from the Taga Group of the Joban Coalfield. Saito Ho-on Kai Special Publication, Professor Tamio Kotaka Commemorative Volume: 481-496, pls. 1-4. [p. 491]
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Fortipecten kenyoshiensis
(Chinzei, 1960); S. O'Hara & N. Nemoto, 1988, Pectinids from the Taga Group of the Joban Coalfield, plate 3, figures 1, 4a, 4b. |