Mizuhopecten Masuda, 1963
MASUDA, K. 1963. The so-called Patinopecten of Japan. Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 52: 145-153, pls. 22, 23. [p. 151]
«Genus Mizuhopecten MASUDA. n. gen.
Type species:— Pecten yessoensis JAY, 1857. Recent. Northern Japan.
Geological and geographical distributions:— Oligocene to Recent. Northern Japan. Diagnosis:— Shell large, rather thick, suborbicular, inequivalve, right valve more inflated than nearly flat or slightly inflated left valve; right valve usually with rather distinct, broad, rounded radial ribs, rarely dichotomous; left valve with distinct but low, rounded, narrow radial ribs and fine network; anterior auricle nearly equal to posterior one, with wide and shallow byssal notch; hinge with distinct but simple cardinal crura, wide and shallow resilial pit with distinct lateral ridges: interior surface somewhat folded corresponding to external sculpture. Remarks:— This new genus is named based upon the old name of Japan. Mizuhopecten can be distinguished from Patinopecten by its having no auricular crurae with distal denticle, rounded, broad radial ribs and large auricles with wide and shallow byssal notch. From Yabepecten it differs by its having no auricular crurae and broad, rather distinct radial ribs. Fortipecten also can be distinguished from the present one by its greatly inflated right valve, nearly flat left valve, very large auricles and narrow radial ribs. Judged from the figures and description of Pecten (Aequipecten) pseuduloa EAMES and COX (1956) from the Upper Miocene to Pliocene of Persia, it can be distinguished from the present one by its left valve being slightly more inflated than the right valve. Whether their specimen can be referred to Aequipecten and Pecten may be in need of a re-examination. Remarks
With regard to the development of the so-called Patinopecten of Japan the writer previously noticed that Patinopecten (s. s.) which is newly proposed in the present article as Mizuhopecten, is very abundant in specific and individual numbers during the Tertiary period, but only two species are known in the Pleistocene and only one species. Patinopecten (s. s.) yessoensis (JAY) is known from the Recent sea of Northern Japan.
Mizuhopecten chichibuensis (KANNO) from the Nenokami sandstone of the Hikokubo group, Chichibu Basin, Saitama Prefecture (KANNO, 1957) and also questionably from the Taishu formation, Tsushima Island, Nagasaki Prefecture (KANNO, 1955) is the earliest known species of the genus Mizuhopecten in Japan. Judging from its associated fauna the Nenokami sandstone is considered to have been deposited under the influence of warm thermal conditions. Therefore, The ancestral stock of Mizuhopecten may be found in the Early Tertiary formations of a Southern region. In general, the changes in the marine environmental conditions from the Early Neogene to the Recent are well reflected in the mega fauna. The water temperature generally became gradually lower from the Early to latest Neogene and showed fluctuation during the Pleistocene. The decreasing in the number of species of Mizuhopecten besides other pectinids from during the Tertiary to the Recent may coincide with the gradual lowering of the water temperature associated with physical and nutrient changes in japan, especially in Northern Japan. The decreasing of pectinids in general with the advance of geological time can be explained by the changes of environmental conditions. The earliest occurrence of the common Recent species of Northern japan, Mizuhopecten yessoensis (JAY), is from the Early Pliocene formations of the Japan Sea borderland province (MASUDA, 1962b) in association with several species of Mizuhopecten and Yabepecten tokunagai (YOKOYAMA). Although Mizuhopecten yessoensis (JAY) survived to the present, almost all other species of Mizuhopecten and Yabepecten tokunagai (YOKOYAMA) became extinct at the end of the Early Pliocene. Mizuhopecteu tokyoensis hokurikuensis (AKIYAMA) appeared in the Early Pliocene but it became extinct at the end of the Early Pliocene and Mizuhopecten tokyoensis (s. s.) (TOKUNAGA) which may be derived from tokyoensis hokurikuensis, appeared in the Late PIiocene and survived to the Pleistocene. Mizuhopecten tokyoensis sematensis (AKIYAMA) appeared in the Pleistocene and became extinct before the close of the age. The development and extinction of the species Mizuhopecten may have had intimate relation with the changes of the environmental conditions. The genus Patinopecten distributed in the Northern Pacific region may be closely related to Pecten or Amusiinae considering from the presence of the auricular crurae with distal denticle. The occurrence of the genus Patinopecten is unknown from the japanese Miocene or from the Miocene of Southeastern Asia. Therefore, its ancestral stock may not be a migrant from Asia but from some other regions, probably from the Mediterranean region. Yabepecten tokunagai (YOKOYAMA) which appeared in the Early Pliocene of Northern Japan probably branched off from the Patinopecten group of the Northern Pacific region of North America as a result of its westward migration from North America in the Early Pliocene. It became extinct during that time.» |
Mizuhopecten yessoensis (Jay); K. Masuda, 1963, The so-called Patinopecten of Japan, plate 23, figures 4, 5.
Pecten yessoënsis; C. E. Lischke, 1871, Japanische Meeres-Conchylien, volume II, plate 13.
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KÔICHIRÔ MASUDA, 1963