Paleontological significance of late Triassic Bivalve Monotis : Part II : In the case of materials from the Southern Kitakami Mountains, Northeast Japan

DOI Web Site

Bibliographic Information

Other Title
  • 後期三畳紀二枚貝 Monotis の古生物学的意義 : その 2-南部北上山地の材料を例として
  • 後期三畳紀二枚貝Monotisの古生物学的意義-2-南部北上山地の材料を例として
  • コウキ サンジョウ キ2マイガイMonotis ノ コセイブツガクテキ イギ

Search this article

Abstract

Following a review of previous works in part I, some results of my observations on the Monotis samples from the Saragai Group of the Southern Kitakami Mountains, Northeast Japan are presented here. It is concerned with the mode of occurrence, zonation, growth pattern, morphological changes and evolution. The mode of fossil occurrence of Monotis observed in the Saragai Group is very different from that of ordinary bivalves. Though no positive evidence has been known in the field, Monotis seems to have had peculiar mode of life, probably epibenthic one attached to seaweeds on a considerably shallow sea floor. On the basis of the population samples from the typical Shirayama-zawa section of the Saragazaka Formation, the upper unit of the group, three species and a subspecies are recognized, indicating four Monotis zones. They are M. scutiformis, M. ochotica densistriata, M. ochotica ochotica, and M. zabaikalica in upward sequence. Nearly all specimens from one horizon belong to a single species. Individual relative growth in M. zabaikalica and average growth in all the species show that no abrupt change in shell form occurs throughout growth after younger stage than 5mm in length. The number of radial ribs of the genus Monotis decreases in upward sequence. Even within a single species, it decreases gradually in M. ochotica and rapidly in M. zabaikalica. This chronological change suggests an evolutional lineage from M. scutiformis to M. zabaikalica through M. ochotica.

Journal

  • Fossils

    Fossils 35 (0), 1-15, 1984

    Palaeontological Society of Japan

Details 詳細情報について

Report a problem

Back to top